Beacon Roofing Supply (BECN) 10-K 20 Nov 20 2020 FY Annual report Financial data

Document And Entity Information

Document And Entity Information - USD ($) $ in Billions	12 Months Ended
	Sep. 30, 2020	Oct. 31, 2020	Mar. 31, 2020
Cover [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Sep. 30, 2020
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Trading Symbol	BECN
Entity Registrant Name	BEACON ROOFING SUPPLY, INC.
Entity Central Index Key	0001124941
Current Fiscal Year End Date	--09-30
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Public Float			$ 906.3
Entity Common Stock, Shares Outstanding		68,990,505
Title of 12(b) Security	Common Stock, $0.01 par value
Security Exchange Name	NASDAQ
Entity File Number	000-50924
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	36-4173371
Entity Address, Address Line One	505 Huntmar Park Drive
Entity Address, Address Line Two	Suite 300
Entity Address, City or Town	Herndon
Entity Address, State or Province	VA
Entity Address, Postal Zip Code	20170
City Area Code	571
Local Phone Number	323-3939
Document Annual Report	true
Document Transition Report	false
Documents Incorporated by Reference	The information required by Part III (Items 10, 11, 12, 13 and 14) will be incorporated by reference from the Registrant’s definitive proxy statement for its 2021 Annual Meeting of Shareholders, which will be filed pursuant to Regulation 14A with the United States Securities and Exchange Commission (“SEC”) within 120 days after the end of the fiscal year to which this report relates.

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Millions		Sep. 30, 2020	Sep. 30, 2019
Current assets:
Cash and cash equivalents		$ 624.6	$ 72.3
Accounts receivable, less allowance of $19.2 and $13.1 as of September 30, 2020 and 2019, respectively		1,029.3	1,108.1
Inventories, net		944.6	1,018.2
Prepaid expenses and other current assets		378.3	315.6
Total current assets		2,976.8	2,514.2
Property and equipment, net		243.7	260.4
Goodwill		2,490.4	2,490.6
Intangibles, net		801.2	1,125.5
Operating lease assets		443.3
Other assets, net		2.1	2.1
Total assets		6,957.5	6,392.8
Current liabilities:
Accounts payable		954.6	822.9
Accrued expenses		563.8	599.2
Current operating lease liabilities		100.5
Current portions of long-term debt/obligations		12.3	18.7
Total current liabilities		1,631.2	1,440.8
Borrowings under revolving lines of credit, net		251.1	81
Long-term debt, net		2,494.2	2,494.6
Deferred income taxes, net		74	103.9
Non-current operating lease liabilities		340.4
Long-term obligations under equipment financing, net			4.6
Other long-term liabilities		6.5	6.4
Total liabilities		4,797.4	4,131.3
Commitments and contingencies (Note 12)
Convertible Preferred Stock; $0.01 par value; aggregate liquidation preference $400.0; 0.4 shares authorized, issued and outstanding as of September 30, 2020 and 2019	[1]	399.2	399.2
Stockholders' equity:
Common stock (voting); $0.01 par value; 100.0 shares authorized; 69.0 and 68.6 shares issued and outstanding as of September 30, 2020 and 2019, respectively		0.7	0.7
Undesignated preferred stock; 5.0 shares authorized, none issued or outstanding		0	0
Additional paid-in capital		1,100.6	1,083
Retained earnings		694.3	799.2
Accumulated other comprehensive income (loss)		(34.7)	(20.6)
Total stockholders' equity		1,760.9	1,862.3
Total liabilities and stockholders' equity		$ 6,957.5	$ 6,392.8
[1]	See Note 3 for additional information.

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Statement Of Financial Position [Abstract]
Accounts receivable, allowances	$ 19.2	$ 13.1
Convertible preferred stock, par value	$ 0.01	$ 0.01
Convertible preferred stock. aggregate liquidation preference	$ 400	$ 400
Convertible preferred stock, shares authorized	400,000	400,000
Convertible preferred stock, shares issued	400,000	400,000
Convertible preferred stock, shares outstanding	400,000	400,000
Common stock (voting), par value	$ 0.01	$ 0.01
Common stock (voting), shares authorized	100,000,000	100,000,000
Common Stock (voting), issued	69,000,000	68,600,000
Common Stock (voting), outstanding	69,000,000	68,600,000
Undesignated Preferred Stock, shares authorized	5,000,000	5,000,000
Undesignated Preferred Stock, issued	0	0
Undesignated Preferred Stock, outstanding	0	0

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Income Statement [Abstract]
Net sales		$ 6,943.9	$ 7,105.2	$ 6,418.3
Cost of products sold		5,244.7	5,368.6	4,825
Gross profit		1,699.2	1,736.6	1,593.3
Operating expense:
Selling, general and administrative		1,273	1,311	1,187.2
Depreciation		70.1	70.7	60.3
Amortization		321	207.1	141.2
Total operating expense		1,664.1	1,588.8	1,388.7
Income (loss) from operations		35.1	147.8	204.6
Interest expense, financing costs, and other		128.1	158.6	136.5
Loss on debt extinguishment		14.7	0	1.2
Income (loss) before provision for income taxes		(107.7)	(10.8)	68.1
Provision for (benefit from) income taxes		(26.8)	(0.2)	(30.5)
Net income (loss)		(80.9)	(10.6)	98.6
Dividends on Preferred Stock		24	24	18
Net income (loss) attributable to common shareholders		$ (104.9)	$ (34.6)	$ 80.6
Weighted-average common stock outstanding:
Basic	[1]	68,800,000	68,400,000	68,000,000
Diluted	[1]	68,800,000	68,400,000	69,200,000
Net income (loss) per share:
Basic	[1]	$ (1.52)	$ (0.51)	$ 1.07
Diluted	[1]	$ (1.52)	$ (0.51)	$ 1.05
[1]	See Note 5 for detailed calculations and further discussion.

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Statement Of Income And Comprehensive Income [Abstract]
Net income (loss)	$ (80.9)	$ (10.6)	$ 98.6
Other comprehensive income (loss):
Foreign currency translation adjustment	(0.7)	(1.7)	(2.7)
Unrealized gain (loss) due to change in fair value of derivatives, net of tax	(13.4)	(1.6)	0
Total other comprehensive income (loss)	(14.1)	(3.3)	(2.7)
Comprehensive income (loss)	$ (95)	$ (13.9)	$ 95.9

Consolidated Statements of Stoc

Consolidated Statements of Stockholders' Equity - USD ($) $ in Millions	Total	Common Stock [Member]	APIC [Member]	[1]	Retained Earnings [Member]	AOCI [Member]	[2]
Balance at Sep. 30, 2017	$ 1,781.8	$ 0.7	$ 1,047.5		$ 748.2	$ (14.6)
Balance (in shares) at Sep. 30, 2017		67,700,000
Issuance of common stock, net of shares withheld for taxes	3.5		3.5
Issuance of common stock, net of shares withheld for taxes (Shares)		400,000
Issuance costs related to secondary offering of common stock	(0.5)		(0.5)
Stock-based compensation	16.5		16.5
Other comprehensive income (loss)	(2.7)					(2.7)
Net income (loss)	98.6				98.6
Dividends on Preferred Stock	(13)				(13)
Balance at Sep. 30, 2018	1,884.2	$ 0.7	1,067		833.8	(17.3)
Balance (in shares) at Sep. 30, 2018		68,100,000
Issuance of common stock, net of shares withheld for taxes	(0.4)		(0.4)
Issuance of common stock, net of shares withheld for taxes (Shares)		400,000
Stock-based compensation	16.4		16.4
Other comprehensive income (loss)	(3.3)					(3.3)
Net income (loss)	(10.6)				(10.6)
Dividends on Preferred Stock	(24)				(24)
Balance at Sep. 30, 2019	1,862.3	$ 0.7	1,083		799.2	(20.6)
Balance (in shares) at Sep. 30, 2019		68,600,000
Issuance of common stock, net of shares withheld for taxes	0.4		0.4
Issuance of common stock, net of shares withheld for taxes (Shares)		400,000
Stock-based compensation	17.2		17.2
Other comprehensive income (loss)	(14.1)					(14.1)
Net income (loss)	(80.9)				(80.9)
Dividends on Preferred Stock	(24)				(24)
Balance at Sep. 30, 2020	$ 1,760.9	$ 0.7	$ 1,100.6		$ 694.3	$ (34.7)
Balance (in shares) at Sep. 30, 2020		69,000,000
[1]	Additional Paid-in Capital (“APIC”)
[2]	Accumulated Other Comprehensive Income (Loss) ("AOCI")

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Operating Activities
Net income (loss)	$ (80.9)	$ (10.6)	$ 98.6
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	391.1	277.8	201.5
Stock-based compensation	17.2	16.4	16.5
Certain interest expense and other financing costs	11.5	12.1	17.3
Beneficial lease amortization	0	2.3	0
Loss on debt extinguishment	14.7	0	1.2
Gain on sale of fixed assets and other	(3.5)	(3.8)	(1.3)
Deferred income taxes	(25.6)	(2.6)	(30.1)
338(h)(10) election refund	(5.1)	0	0
Changes in operating assets and liabilities:
Accounts receivable	78.4	(18.5)	(45)
Inventories	73.4	(82.8)	(65.1)
Prepaid expenses and other current assets	(73.6)	(70.8)	57.6
Accounts payable and accrued expenses	72.2	92.1	287.4
Other assets and liabilities	9.5	1.1	0.8
Net cash provided by (used in) operating activities	479.3	212.7	539.4
Investing Activities
Purchases of property and equipment	(48.5)	(57)	(46)
Acquisition of businesses, net	5.1	(164)	(2,740.5)
Proceeds from the sale of assets	4.4	9.3	2.1
Net cash provided by (used in) investing activities	(39)	(211.7)	(2,784.4)
Financing Activities
Borrowings under revolving lines of credit	2,038	2,100.1	2,807.7
Payments under revolving lines of credit	(1,870)	(2,114)	(2,707.7)
Borrowings under term loan	0	0	970
Payments under term loan	(9.7)	(9.7)	(445.8)
Borrowings under senior notes	300	0	1,300
Payment under senior notes	(309.6)	0	0
Payment of debt issuance costs	(4.3)	(0.8)	(65.8)
Payments under equipment financing facilities and finance leases	(8.6)	(10)	(11.6)
Proceeds from issuance of convertible preferred stock	0	0	400
Payment of stock issuance costs	0	0	(1.3)
Payment of dividends on Preferred Stock	(24)	(24)	(13)
Proceeds from issuance of common stock related to equity awards	3.3	3.3	7.5
Payment of taxes related to net share settlement of equity awards	(2.9)	(3.7)	(4)
Net cash provided by (used in) financing activities	112.2	(58.8)	2,236
Effect of exchange rate changes on cash and cash equivalents	(0.2)	0.2	0.6
Net increase (decrease) in cash and cash equivalents	552.3	(57.6)	(8.4)
Cash and cash equivalents, beginning of period	72.3	129.9	138.3
Cash and cash equivalents, end of period	624.6	72.3	129.9
Cash paid during the period for:
Interest	130.3	146.4	111.3
Income taxes paid (received), net of refunds	$ (5.4)	$ (8.5)	$ 35.1

Company Overview

Company Overview	12 Months Ended
	Sep. 30, 2020
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Company Overview	1. Company Overview Beacon Roofing Supply, Inc. (the “Company”) was incorporated in the state of Delaware on August 22, 1997 and is the largest publicly traded distributor of residential and non-residential roofing materials and complementary building products in the United States and Canada. On January 15, 2020, the Company announced the rebranding of its exterior product branches with the trade name “Beacon Building Products” (the “Rebranding”). The new name, and a related logo, were adopted at over 450 Beacon one-step exterior products branches. The Company’s interior, insulation, weatherproofing and two-step branches continue to operate under legacy brand names. The Company operates its business under regional and local trade names and services customers in all 50 states throughout the U.S. and 6 provinces in Canada. The Company’s material subsidiaries are Beacon Sales Acquisition, Inc. and Beacon Roofing Supply Canada Company.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Sep. 30, 2020
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All inter-company transactions have been eliminated. Certain prior period amounts have been reclassified to conform to current period presentation. Use of Estimates The preparation of consolidated financial statements in conformity with United States generally accepted accounting principles (“GAAP”) requires management to make estimates and assumptions that affect the amounts reported in these consolidated financial statements and accompanying notes. Significant items subject to such estimates include inventories, purchase price allocations, recoverability of goodwill and intangibles, and income taxes. Assumptions made in the development of these estimates contemplate the impact of the novel coronavirus (“COVID‑19”) on the economy and the Company’s anticipated results; however, actual amounts could differ materially from these estimates. Fiscal Year The fiscal years presented are the years ended September 30, 2020 (“2020”), September 30, 2019 (“2019”), and September 30, 2018 (“2018”). Each of the Company’s first three quarters ends on the last day of the calendar month. Segment Information Operating segments are defined as components of a business that can earn revenue and incur expenses for which discrete financial information is evaluated on a regular basis by the chief operating decision maker (“CODM”) in order to decide how to allocate resources and assess performance. The Company’s CODM, the Chief Executive Officer, reviews consolidated results of operations to make decisions, therefore the Company views its operations and manages its business as one operating segment. Cash and Cash Equivalents The Company considers all highly liquid investments with maturities of three months or less when purchased to be cash equivalents. Cash and cash equivalents also include unsettled credit card transactions. Cash equivalents are comprised of money market funds which invest primarily in commercial paper or bonds with a rating of A-1 or better, and bank certificates of deposit. Accounts Receivable Accounts receivable are derived from unpaid invoiced amounts and are recorded at their net realizable value. The allowance for doubtful accounts is calculated based on actual historical write-offs and current economic factors and represents the Company’s best estimate of its credit exposure. Each month the Company reviews its receivables on a customer-by-customer basis and any balances that are deemed uncollectible are written off against the allowance after all means of collection have been exhausted and the potential for recovery is considered remote. The Company’s accounts receivable are primarily from customers in the building industry located in the United States and Canada, and no single customer represented at least 10% of the Company’s revenue during the year ended September 30, 2020 or accounts receivable as of September 30, 2020. Concentrations of Risk Financial instruments that potentially subject the Company to significant concentration of credit risk consist primarily of cash, cash equivalents and accounts receivable. The Company maintains the majority of its cash and cash equivalents with one financial institution, which management believes to be financially sound and with minimal credit risk. The Company’s deposits typically exceed amounts guaranteed by the Federal Deposit Insurance Corporation. Inventories Inventories, consisting substantially of finished goods, are valued at the lower of cost or market (net realizable value). Cost is determined using the moving weighted-average cost method. The Company’s arrangements with vendors typically provide for rebates after it makes a special purchase and/or monthly, quarterly and/or annual rebates of a specified amount of consideration payable when a number of measures have been achieved. Annual rebates are generally related to a specified cumulative level of purchases on a calendar-year basis. The Company accounts for such rebates as a reduction of the inventory value until the product is sold, at which time such rebates reduce cost of sales in the consolidated statements of operations. Throughout the year, the Company estimates the amount of the periodic rebates based upon the expected level of purchases. The Company continually revises these estimates to reflect actual rebates earned based on actual purchase levels. Amounts due from vendors under these arrangements are included in “prepaid expenses and other current assets” in the accompanying consolidated balance sheets. Property and Equipment Property and equipment acquired in connection with acquisitions are recorded at fair value as of the date of the acquisition and depreciated utilizing the straight-line method over the estimated remaining lives. All other additions are recorded at cost, and depreciation is computed using the straight-line method. The Company reviews the estimated useful lives of its fixed assets on an ongoing basis and the following table summarizes the estimates currently used: Asset Class Estimated Useful Life Buildings and improvements 40 years Equipment 3 to 7 years Furniture and fixtures 7 years Leasehold improvements Shorter of the estimated useful life or the term of the lease, considering renewal options expected to be exercised. Business Combinations The Company records acquisitions resulting in the consolidation of a business using the acquisition method of accounting. Under this method, the acquiring Company records the assets acquired, including intangible assets that can be identified and named, and liabilities assumed based on their estimated fair values at the date of acquisition. The Company uses an income approach to determine the fair value of acquired intangible assets, specifically the multi-period excess earnings method for customer relationships and the relief from royalty method for trade names. Various Level 3 fair value assumptions are used in the determination of these estimated fair values, including items such as sales growth rates, cost synergies, customer attrition rates, discount rates, and other prospective financial information. The purchase price in excess of the fair value of the assets acquired and liabilities assumed is recorded as goodwill. Estimates associated with the accounting for acquisitions may change as additional information becomes available regarding the assets acquired and liabilities assumed. Transaction costs associated with acquisitions are expensed as incurred. Goodwill and Intangibles On an annual basis and at interim periods when circumstances require, the Company tests the recoverability of its goodwill and indefinite-lived intangible assets. Examples of such indicators include a significant change in the business climate, unexpected competition, loss of key personnel or a decline in the Company’s market capitalization below the Company’s net book value. The Company performs impairment assessments at the reporting unit level, which is defined as an operating segment or one level below an operating segment, also known as a component. The Company currently has four components which it evaluates for aggregation by examining the distribution methods, sales mix, and operating results of each component to determine if these characteristics will be sustained over a long-term basis. For purposes of this evaluation, the Company expects its components to exhibit similar economic characteristics 3-5 years after events such as an acquisition within the Company’s core roofing business or management/business restructuring. Components that exhibit similar economic characteristics are subsequently aggregated into a single reporting unit. Based on the Company’s most recent impairment assessment performed as of August 31, 2020, it was determined that all of the Company’s components exhibited similar economic characteristics, and therefore should be aggregated into a single reporting unit (collectively, the “Reporting Unit”). To test for the recoverability of goodwill and indefinite-lived intangible assets, the Company first performs a qualitative assessment based on economic, industry and company-specific factors for all or selected reporting units to determine whether the existence of events and circumstances indicates that it is more likely than not that the goodwill or indefinite-lived intangible asset is impaired. Based on the results of the qualitative assessment, two additional steps in the impairment assessment may be required. The first step would require a comparison of each reporting unit’s fair value to the respective carrying value. If the carrying value exceeds the fair value, a second step is performed to measure the amount of impairment loss on a relative fair value basis, if any. Based on the Company’s most recent qualitative impairment assessment performed as of August 31, 2020, the Company concluded that there were no indicators of impairment, and that therefore it was more likely than not that the fair value of the goodwill and indefinite-lived intangible assets exceeded their net carrying amount, and therefore the quantitative two-step impairment test was not required. The Company amortizes certain identifiable intangible assets that have finite lives, currently consisting of non-compete agreements, customer relationships and trade names. Non-compete agreements are amortized on a straight-line basis over the terms of the associated contractual agreements; customer relationship assets are amortized on an accelerated basis based on the expected cash flows generated by the existing customers; and trade names are amortized on an accelerated basis over a five or ten year period. Amortizable intangible assets are tested for impairment, when deemed necessary, based on undiscounted cash flows and, if impaired, are written down to fair value based on either discounted cash flows or appraised values. In connection with certain financing arrangements, the Company has debt issuance costs that are amortized over the lives of the associated financings. Evaluation of Long-Lived Assets The Company evaluates the recoverability of its long-lived assets for impairment whenever events or circumstances indicate that the carrying amount of the assets may not be recoverable. Recoverability is measured by comparing the carrying amount of the asset to the future undiscounted cash flows the asset is expected to generate. If the asset is considered to be impaired, the amount of any impairment is measured as the difference between the carrying value and the fair value of the impaired asset. Fair Value Measurement The Company applies fair value accounting for all financial assets and liabilities that are reported at fair value in the financial statements on a recurring basis. Fair value is defined as the price that would be received from selling an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. The accounting guidance establishes a defined three-tier hierarchy to classify and disclose the fair value of assets and liabilities on both the date of their initial measurement as well as all subsequent periods. The hierarchy prioritizes the inputs used to measure fair value by the lowest level of input that is available and significant to the fair value measurement. The three levels are described as follows: • Level 1 : Observable inputs. Quoted prices in active markets for identical assets and liabilities; • Level 2 : Observable inputs other than the quoted price. Includes quoted prices for similar instruments, quoted prices for identical or similar instruments in inactive markets and amounts derived from valuation models where all significant inputs are observable in active markets; and • Level 3 : Unobservable inputs. Includes amounts derived from valuation models where one or more significant inputs are unobservable and require the Company to develop relevant assumptions. The Company evaluates its financial assets and liabilities subject to fair value measurements on a recurring basis to determine the appropriate level of classification as of each reporting period. Financial Derivatives The Company has entered into interest rate swaps to minimize the risks and costs associated with financing activities, as well as to maintain an appropriate mix of fixed-rate and floating-rate debt. The swap agreements are contracts to exchange variable-rate for fixed-interest rate payments over the life of the agreements. The Company's derivative instruments are designated as cash flow hedges, for which the Company records the effective portions of changes in their fair value, net of tax, in other comprehensive income. The Company recognizes any ineffective portion of the hedges in the consolidated statement of operations through interest expense, financing costs and other. Net Sales The Company records net sales when performance obligations with the customer are satisfied. A performance obligation is a promise to transfer a distinct good to the customer and is the unit of account. The transaction price is allocated to each distinct performance obligation and recognized as net sales when, or as, the performance obligation is satisfied. All contracts have a single performance obligation as the promise to transfer the individual good is not separately identifiable from other promises and is, therefore, not distinct. Performance obligations are satisfied at a point in time and net sales are recognized when the customer accepts the delivery of a product or takes possession of a product with rights and rewards of ownership. For goods shipped by third party carriers, the Company recognizes revenue upon shipment since the terms are generally FOB shipping point at which time control passes to the customer. The Company also arranges for certain products to be shipped directly from the manufacturer to the customer. The Company recognizes the gross revenue for these sales upon shipment as the terms are FOB shipping point at which time control passes to the customer. The Company enters into agreements with customers to offer rebates, generally based on achievement of specified sales levels and various marketing allowances that are common industry practice. Reductions to net sales for customer programs and incentive offerings, including promotions and other volume-based incentives, are estimated using the most likely amount method and recorded in the period in which the sale occurs. Provisions for early payment discounts are accrued in the same period in which the sale occurs. The Company does not have any material payment terms as payment is received shortly after the transfer of control of the products to the customer. Commissions to internal sales teams are paid to obtain contracts. As these contracts are less than one year, these costs are expensed as incurred. The Company includes shipping and handling costs billed to customers in net sales. Related costs are accounted for as fulfillment activities and are recognized as cost of products sold when control of the products transfers to the customer. Leases The Company mostly operates in leased facilities, which are accounted for as operating leases. The leases typically provide for a base rent plus real estate taxes and insurance. Certain of the leases provide for escalating rents over the lives of the leases, and rent expense is recognized over the terms of those leases on a straight-line basis. The real estate leases expire between 2020 and 2038. In addition, the Company leases equipment such as trucks and forklifts. Equipment leases are primarily accounted for as operating leases; however, the Company also accounts for some equipment leases as finance leases. The equipment leases expire between 2020 and 2027. The Company determines if an arrangement is a lease at inception. Operating lease assets and liabilities are included on the consolidated balance sheets. Finance lease assets are included in property and equipment, net. The current portion of the finance lease liabilities is included in accrued expenses, and the noncurrent portion is included in other long-term liabilities. Operating lease assets and liabilities are recognized at the present value of the future lease payments at the lease commencement date. The interest rate used to determine the present value of the future lease payments is the Company’s incremental borrowing rate, because the interest rates implicit in most of the leases are not readily determinable. The incremental borrowing rate is estimated to approximate the interest rate on a collateralized basis with similar terms and payments. Operating lease assets include any prepaid lease payments and lease incentives. The Company’s lease terms include periods under options to extend or terminate the lease when it is reasonably certain that those options will be exercised. The Company generally uses the base, non-cancelable lease term when determining the lease assets and liabilities. Operating lease expense is recognized on a straight-line basis over the lease term. The Company’s lease agreements generally contain lease and non-lease components. Non-lease components primarily include payments for maintenance and utilities. The Company has elected to combine fixed payments for non-lease components with lease payments and account for them together as a single lease component, which increases the lease assets and liabilities. Payments under the Company’s lease agreements are primarily fixed. However, certain lease agreements contain variable payments, which are expensed as incurred and are not included in the operating lease assets and liabilities. These amounts include payments affected by the Consumer Price Index and reimbursements to landlords for items such as property insurance and common area costs. The Company’s lease agreements do not contain any material residual value guarantees or material restrictive covenants. Stock-Based Compensation The Company applies the fair value method to recognize compensation expense for stock-based awards. Using this method, the estimated grant-date fair value of the award is recognized on a straight-line basis over the requisite service period based on the portion of the award that is expected to vest. The Company estimates forfeitures at the time of grant and revises the estimates, if necessary, in subsequent periods if actual forfeitures differ from those estimates. For awards with a performance-based vesting condition, the Company accrues stock-based compensation expense if it is probable that the performance condition will be achieved. Stock-based compensation expense for restricted stock units is measured based on the fair value of the Company’s common stock on the grant date. The Company utilizes the Black-Scholes option pricing model to estimate the grant-date fair value of option awards. The exercise price of option awards is set to equal the estimated fair value of the common stock at the date of the grant. The following weighted-average assumptions are also used to calculate the estimated fair value of option awards: • Expected volatility : The expected volatility of the Company’s shares is estimated using the historical stock price volatility over the most recent period commensurate with the estimated expected term of the awards. • Expected term : For employee stock option awards, the Company determines the weighted average expected term equal to the weighted period between the vesting period and the contract life of all outstanding options. • Dividend yield : The Company has not paid dividends and does not anticipate paying a cash dividend in the foreseeable future and, accordingly, uses an expected dividend yield of zero. • Risk-free interest rate : The Company bases the risk-free interest rate on the implied yield available on a U.S. Treasury note with a term equal to the estimated expected term of the awards. Foreign Currency Translation The Company’s operations located outside of the United States where the local currency is the functional currency are translated into U.S. dollars using the current rate method. Results of operations are translated at the average rate of exchange for the period. Assets and liabilities are translated at the closing rates on the period end date. Gains and losses on translation of these accounts are accumulated and reported as a separate component of equity and other comprehensive income (loss). Gains and losses on foreign currency transactions are recognized in the consolidated statements of operations as a component of interest expense, financing costs, and other. Income Taxes The Company accounts for income taxes using the liability method, which requires it to recognize a current tax liability or asset for current taxes payable or refundable and a deferred tax liability or asset for the estimated future tax effects of temporary differences between the financial statement and tax reporting bases of assets and liabilities to the extent that they are realizable. Deferred tax expense (benefit) results from the net change in deferred tax assets and liabilities during the year. FASB ASC Topic 740 prescribes a recognition threshold and measurement attribute for the financial statement recognition and measurement of a tax position taken or expected to be taken in a tax return. Based on this guidance, the Company analyzes its filing positions in all of the federal and state jurisdictions where it is required to file income tax returns, as well as all open tax years in these jurisdictions. Tax benefits from uncertain tax positions are recognized if it is more likely than not that the position is sustainable based solely on its technical merits. Net Income (Loss) per Share Basic net income (loss) per share is calculated by dividing net income (loss) attributable to common shareholders by the weighted-average number of common shares outstanding during the period, without consideration for common share equivalents or the conversion of Preferred Stock. Common share equivalents consist of the incremental common shares issuable upon the exercise of stock options and vesting of restricted stock unit awards. Diluted net income (loss) per common share is calculated by dividing net income (loss) attributable to common shareholders by the fully diluted weighted-average number of common shares outstanding during the period. Holders of Preferred Stock participate in dividends on an as-converted basis when declared on common shares. As a result, Preferred Stock is classified as a participating security and thereby requires the allocation of income that would have otherwise been available to common shareholders when calculating net income (loss) per share. Diluted net income (loss) per share is calculated by utilizing the most dilutive result of the if-converted and two-class methods. In both methods, net income (loss) attributable to common shareholders and the weighted-average common shares outstanding are adjusted to account for the impact of the assumed issuance of potential common shares that are dilutive, subject to dilution sequencing rules. Recent Accounting Pronouncements—Adopted In February 2016, the FASB issued ASU 2016-02, “ Leases In February 2018, the FASB issued ASU 2018-02, “Income Statement – Reporting Comprehensive Income.” Recent Accounting Pronouncements—Not Yet Adopted In June 2016, the FASB issued ASU 2016-13, “ Financial Instruments—Credit Losses: Measurement of Credit Losses on Financial Instruments In January 2017, the FASB issued ASU 2017-04, “ Simplifying the Accounting for Goodwill Impairment.” In December 2019, the FASB issued ASU 2019-12, “Income Taxes – Simplifying the Accounting for Income Taxes.” In March 2020, the FASB issued ASU 2020-04, “ Reference Rate Reform (Topic 848), Facilitation of the Effects of Reference Rate Reform on Financial Reporting In October 2020, the FASB issued ASU 2020-06, “ Debt – Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging – Contracts in Entity’s Own Equity (Subtopic 815-40), Accounting for Convertible Instruments and Contracts in an Entity’s Own Equity

Acquisitions

Acquisitions	12 Months Ended
	Sep. 30, 2020
Business Combinations [Abstract]
Acquisitions	3. Acquisitions Allied Building Products Corp. On January 2, 2018 (the “Closing Date”), the Company completed its acquisition of all the outstanding capital stock of Allied (the “Allied Acquisition”), pursuant to a certain stock purchase agreement dated August 24, 2017 (the “Stock Purchase Agreement”), among the Company, Oldcastle, Inc., as parent, and Oldcastle Distribution, Inc., as seller, for approximately $2.625 billion in cash, subject to a working capital and certain other adjustments as set forth in the Stock Purchase Agreement (the “Purchase Price”). As of September 30, 2020, the adjusted Purchase Price for Allied was $2.88 billion, including increases of (i) $164.0 million related to the impact of the Section 338(h)(10) election under the current U.S. tax code and (ii) $88.1 million from a recorded net working capital adjustment. In connection with the Allied Acquisition, on the Closing Date the Company entered into (i) a new term loan agreement with Citibank, N.A., providing for a term loan B facility with an initial commitment of $970.0 million and (ii) an amended and restated credit agreement with Wells Fargo Bank, N.A., providing for a senior secured asset-based revolving credit facility with an initial commitment of $1.30 billion. Base borrowing rates on these facilities are at LIBOR plus 1.25% and LIBOR plus 2.25%, respectively. In connection with the Allied Acquisition, on the Closing Date, the Company completed the sale of 400,000 shares of Series A Cumulative Convertible Participating Preferred Stock, par value $0.01 per share (the “Preferred Stock”), with an aggregate liquidation preference of $400.0 million, at a purchase price of $1,000 per share, to CD&R Boulder Holdings, L.P., pursuant to an investment agreement, dated as of August 24, 2017, with CD&R Boulder Holdings, L.P. and Clayton, Dubilier & Rice Fund IX, L.P. (solely for the purpose of limited provisions therein) (the “Convertible Preferred Stock Purchase”). The $400.0 million in proceeds from the Convertible Preferred Stock Purchase were used to finance, in part, the Purchase Price. The Preferred Stock is convertible perpetual participating preferred stock of the Company, and conversion of the Preferred Stock into $ 0.01 par value shares of the Company’s common stock will be at a conversion price of $ 41.26 per share. The Preferred Stock accumulates dividends at a rate of 6.0 % per annum (payable in cash or in-kind, subject to certain conditions). The Preferred Stock is not mandatorily redeemable; therefore, it is classified as mezzanine equity on the Company’s consolidated balance sheets and has a balance of $ 399.2 million (the $ 400.0 million proceeds received on the Closing Date, net of $ 0.8 million of unamortized issuance costs) as of September 30, 2020 . Allied’s results of operations have been included with Company’s consolidated results beginning January 2, 2018. Allied distributed products in 208 locations across 31 states as of the date of the close. The Allied Acquisition has been accounted for as a business combination in accordance with the requirements of ASC 805, “ Business Combinations January 2, 2018 January 2, 2018 (as reported at March 31, 2018) Adjustments (as adjusted at March 31, 2019) Cash $ 19.3 $ (19.2 ) $ 0.2 Accounts receivable 315.5 22.1 337.5 Inventory 322.7 (7.9 ) 314.8 Prepaid and other current assets 59.3 16.2 75.4 Property, plant, and equipment 139.5 (0.2 ) 139.4 Goodwill 1,130.6 102.1 1,232.8 Intangible assets 1,037.0 — 1,037.0 Current liabilities (271.3 ) 12.0 (259.3 ) Non-current liabilities (6.8 ) 6.1 (0.7 ) Total purchase price $ 2,745.8 $ 131.2 $ 2,877.1 The purchase accounting entries above include the impact of the Section 338(h)(10) election under the current U.S. tax code. The Company made this election on October 15, 2018 and has reflected the $164.0 million impact of this election in the purchase price and its fiscal year 2018 tax provision accordingly. In the year ended September 30, 2020, the Company received a net $5.1 million refund as the final true-up of the $164.0 million payment, which was recorded in interest expense, financing costs, and other in the consolidated statements of operations. The Company determined that $1.01 billion of goodwill related to the acquisition of Allied remains deductible for tax purposes as of September 30, 2020. All of the Company’s goodwill and indefinite-lived trade name are tested for impairment annually, and all acquired goodwill and intangible assets are subject to review for impairment should future indicators of impairment develop. There were no material contingencies assumed as part of the Allied Acquisition. Additional Acquisitions – Fiscal Year 2018 During fiscal year 2018, the Company acquired 7 branches from the following two acquisitions: • On May 1, 2018, the Company acquired Tri-State Builder’s Supply, a wholesale supplier of roofing, siding, windows, doors and related building products with 1 branch located in Duluth, Minnesota and annual sales of approximately $6 million. The Company has finalized the acquisition accounting entries for this transaction. • On July 16, 2018, the Company acquired Atlas Supply, Inc., the Pacific Northwest’s leading distributor of sealants, coatings, adhesives and related weatherproofing products, with 6 branches operating in Seattle, Tacoma, Spokane, and Mountlake Terrace in Washington, as well as locations in Portland, Oregon and Boise, Idaho, and annual sales of approximately $37 million. The Company has finalized the acquisition accounting entries for this transaction. The Company has recorded purchase accounting entries for these transactions that recognized the acquired assets and liabilities at their estimated fair values as of the respective acquisition dates. These transactions resulted in goodwill of $7.6 million ($6.5 million of which remains deductible for tax purposes as of September 30, 2020) and $11.4 million in intangible assets.

Net Sales

Net Sales	12 Months Ended
	Sep. 30, 2020
Revenue From Contract With Customer [Abstract]
Net Sales	4. Net Sales The following table presents the Company’s net sales by product line and geography for the years ended September 30, 2020 and 2019 (in millions): U.S. Canada Total Year Ended September 30, 2020 Residential roofing products $ 3,043.2 $ 56.4 $ 3,099.6 Non-residential roofing products 1,534.5 112.1 1,646.6 Complementary building products 2,188.7 9.0 2,197.7 Total net sales $ 6,766.4 $ 177.5 $ 6,943.9 Year Ended September 30, 2019 Residential roofing products $ 3,023.2 $ 56.4 $ 3,079.6 Non-residential roofing products 1,582.8 122.4 1,705.2 Complementary building products 2,312.5 7.9 2,320.4 Total net sales $ 6,918.5 $ 186.7 $ 7,105.2

Net Income (Loss) Per Share

Net Income (Loss) Per Share	12 Months Ended
	Sep. 30, 2020
Earnings Per Share [Abstract]
Net Income (Loss) Per Share	5. Net Income (Loss) Per Share The following table presents the components and calculations of basic and diluted net income (loss) per share for each period presented (in millions, except per share amounts): Year Ended September 30, 2020 2019 2018 Net income (loss) $ (80.9 ) $ (10.6 ) $ 98.6 Dividends on Preferred Stock 24.0 24.0 18.0 Net income (loss) attributable to common shareholders $ (104.9 ) $ (34.6 ) $ 80.6 Undistributed income allocated to participating securities — — (7.7 ) Net income (loss) attributable to common shareholders - basic and diluted $ (104.9 ) $ (34.6 ) $ 72.9 Weighted-average common shares outstanding - basic 68.8 68.4 68.0 Effect of common share equivalents — — 1.2 Weighted-average common shares outstanding - diluted 68.8 68.4 69.2 Net income (loss) per share - basic $ (1.52 ) $ (0.51 ) $ 1.07 Net income (loss) per share - diluted $ (1.52 ) $ (0.51 ) $ 1.05 The following table includes the number of shares that may be dilutive common shares in the future. These shares were not included in the computation of diluted net income (loss) per share because the effect was either anti-dilutive or the requisite performance conditions were not met (in millions): Year Ended September 30, 2020 2019 2018 Stock options 2.0 1.3 0.4 Restricted stock units 0.3 0.1 0.2 Preferred Stock 9.7 9.7 7.2

Stock-based Compensation

Stock-based Compensation	12 Months Ended
	Sep. 30, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-based Compensation	6. Stock-based Compensation On December 23, 2019, the Board of Directors of the Company approved the Beacon Roofing Supply, Inc. Second Amended and Restated 2014 Stock Plan (the “2014 Plan”). On February 11, 2020, the shareholders of the Company approved an additional 4,850,000 shares under the 2014 Plan. The 2014 Plan, which was originally approved by the shareholders on February 12, 2014, provides for discretionary awards of stock options, stock awards, restricted stock units, and stock appreciation rights to selected employees and non-employee directors. The 2014 Plan mandates that all forfeited, expired, and withheld shares, including those from the predecessor plan, be returned to the 2014 Plan and made available for issuance. As of September 30, 2020, there were 5.7 million shares of common stock available for issuance. The 2014 Plan is the only plan maintained by the Company pursuant to which equity awards are granted. For all equity awards granted prior to October 1, 2014, in the event of a change in control of the Company, all awards are immediately vested. Beginning in fiscal 2015, equity awards contained a “double trigger” change in control mechanism. Unless an award is continued or assumed by a public company in an equitable manner, an award shall become fully vested immediately prior to a change in control (at 100% of the grant target in the case of a performance-based restricted stock unit award). If an award is so continued or assumed, vesting will continue in accordance with the terms of the award, unless there is a qualifying termination within one-year following the change in control, in which event the award shall immediately become fully vested (at 100% of the grant target in the case of a performance-based restricted stock unit award). Stock Options Non-qualified stock options generally expire 10 years after the grant date and, except under certain conditions, the options are subject to continued employment and vest in three annual installments over the three-year The fair values of the options granted for the year ended September 30, 2020 were estimated on the dates of grants using the Black-Scholes option-pricing model with the following weighted-average assumptions: Year Ended September 30, 2020 2019 2018 Risk-free interest rate 1.61 % 2.86 % 2.10 % Expected volatility 34.26 % 29.68 % 26.43 % Expected life (in years) 5.26 5.22 5.46 Dividend yield — — — The following table summarizes all stock option activity for the periods presented (in millions, except per share and time period amounts): Options Outstanding Weighted- Average Exercise Price Weighted- Average Remaining Contractual Term (Years) Aggregate Intrinsic Value 1 Balance as of September 30, 2019 2.3 $ 32.61 6.1 $ 12.0 Granted 0.5 31.95 Exercised (0.2 ) 19.51 Canceled/Forfeited (0.1 ) 37.30 Expired — 31.68 Balance as of September 30, 2020 2.5 $ 33.09 5.9 $ 6.9 Vested and expected to vest after September 30, 2020 2.4 $ 33.13 5.9 $ 6.8 Exercisable as of September 30, 2020 1.6 $ 33.69 4.5 $ 5.1 ____________________________________________________________________ 1 During the years ended September 30, 2020, 2019, and 2018, the Company recorded stock-based compensation expense related to stock options of $4.4 million, $4.1 million, and $3.9 million, respectively. As of September 30, 2020, there was $4.9 million of total unrecognized compensation The following table summarizes additional information on stock options for the periods presented (in millions, except per share amounts): Year Ended September 30, 2020 2019 2018 Weighted-average fair value of stock options granted $ 10.35 $ 8.91 $ 15.86 Total grant date fair value of stock options vested $ 4.3 $ 3.9 $ 4.2 Total intrinsic value of stock options exercised $ 2.1 $ 2.8 $ 9.6 Restricted Stock Units Restricted stock unit (“RSU”) awards granted to employees are subject to continued employment and generally vest on the third anniversary of the grant date. The Company also grants certain RSU awards to management that contain one or more additional vesting conditions tied directly to a defined performance metric for the Company. The actual number of RSUs that will vest can range from 0% to 200 % of the original grant amount, depending upon actual Company performance below or above the established performance metric targets. The Company estimates performance in relation to the defined targets when determining the projected number of RSUs that are expected to vest and calculating the related stock-based compensation expense. RSUs granted to non-employee directors are subject to continued service and vest on the first anniversary of the grant date (except under certain conditions). Generally, the common shares underlying the RSUs are not eligible for distribution until the non-employee director’s service on the Board has terminated, and for non-employee director RSU grants made prior to fiscal year 2014, the share distribution date is six months The following table summarizes all restricted stock unit activity for the periods presented (in millions, except per share amounts): RSUs Outstanding Weighted-Average Grant Date Fair Value Balance as of September 30, 2019 1.1 $ 37.48 Granted 0.5 31.81 Released (0.3 ) 44.87 Canceled/Forfeited (0.1 ) 33.69 Balance as of September 30, 2020 1.2 $ 33.55 Vested and expected to vest after September 30, 2020 1.0 $ 34.68 During the years ended September 30, 2020, 2019, and 2018, the Company recorded stock-based compensation expense related to RSUs of $12.8 million, $12.3 million, and $12.6 million, respectively. As of September 30, 2020, there was $12.9 million of total unrecognized compensation cost related to unvested The following table summarizes additional information on RSUs for the period presented (in millions, except per share amounts): Year Ended September 30, 2020 2019 2018 Weighted-average fair value of RSUs granted $ 31.81 $ 28.02 $ 57.40 Total grant date fair value of RSUs vested $ 14.4 $ 16.1 $ 6.7 Total intrinsic value of RSUs released $ 9.8 $ 11.5 $ 11.0

Prepaid Expenses and Other Curr

Prepaid Expenses and Other Current Assets	12 Months Ended
	Sep. 30, 2020
Prepaid Expenses And Other Current Assets [Abstract]
Prepaid Expenses and Other Current Assets	7. Prepaid Expenses and Other Current Assets The following table summarizes the significant components of prepaid expenses and other current assets (in millions): September 30, 2020 2019 Vendor rebates $ 326.4 $ 262.8 Other 51.9 52.8 Total prepaid expenses and other current assets $ 378.3 $ 315.6

Property and Equipment

Property and Equipment	12 Months Ended
	Sep. 30, 2020
Property Plant And Equipment [Abstract]
Property and Equipment	8. Property and Equipment The following table provides a detailed breakout of property and equipment, by type (in millions): September 30, 2020 2019 Land and buildings $ 87.3 $ 83.4 Equipment 453.1 448.1 Furniture and fixtures 42.5 40.0 Finance lease assets 11.6 — Total property and equipment 594.5 571.5 Accumulated depreciation (350.8 ) (311.1 ) Total property and equipment, net $ 243.7 $ 260.4 Depreciation expense for the years ended September 30, 2020, 2019, and 2018 was $70.1 million, $70.7 million, and $60.3 million, respectively

Goodwill and Intangible Assets

Goodwill and Intangible Assets	12 Months Ended
	Sep. 30, 2020
Goodwill And Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets	9. Goodwill and Intangible Assets The Company considered the adverse impact of the COVID-19 pandemic on its operations as part of its annual impairment analyses of intangible assets and goodwill and concluded there was no impairment to record in fiscal year 2020. Goodwill The following table sets forth the change in the carrying amount of goodwill during the years ended September 30, 2020 and 2019, respectively (in millions): Balance as of September 30, 2018 $ 2,491.8 Acquisitions 1 (0.5 ) Translation and other adjustments (0.7 ) Balance as of September 30, 2019 $ 2,490.6 Balance as of September 30, 2019 $ 2,490.6 Translation and other adjustments (0.2 ) Balance as of September 30, 2020 $ 2,490.4 ___________________________________________ 1 Reflects purchase accounting adjustments related to fiscal year 2018 acquisition of Atlas Supply, Inc. (see Note 3 for further discussion). The changes in the carrying amount of goodwill for the years ended September 30, 2020 and 2019 were driven primarily by purchase accounting and foreign currency translation adjustments. Intangible Assets In connection with transactions finalized for the year ended September 30, 2018, the Company recorded intangible assets of $1.05 billion ($920.8 million of customer relationships, $7.0 million of beneficial lease arrangements, and $120.0 million of indefinite-lived trademarks). The following table summarizes intangible assets by category (in millions, except time period amounts): September 30, Weighted- Average Remaining Life 1 2020 2019 (Years) Amortizable intangible assets: Non-compete agreements $ 0.2 $ 2.8 1.7 Customer relationships 1,481.1 1,530.9 16.4 Trademarks 7.2 10.5 6.1 Beneficial lease arrangements — 8.1 — Total amortizable intangible assets 1,488.5 1,552.3 Accumulated amortization (738.6 ) (619.9 ) Total amortizable intangible assets, net $ 749.9 $ 932.4 Indefinite-lived trademarks 51.3 193.1 Total intangibles, net $ 801.2 $ 1,125.5 ___________________________________________ 1 As of September 30, 2020. In the second quarter of fiscal year 2020, in connection with the Rebranding, the Company incurred non-cash accelerated intangible asset amortization of $142.6 million related to the write-off of certain trade names, primarily Allied (exterior products only), Roofing Supply Group and JGA. The Company used an income approach, specifically the relief from royalty method, to determine the fair value of remaining indefinite-lived trademarks. Various Level 3 fair value assumptions were used in the determination of the estimated fair value, including items such as sales growth rates, royalty rates, discount rates, and other prospective financial information. For the years ended September 30, 2020, 2019, and 2018, the Company recorded $321.0 million, $207.1 million, and $141.2 million, respectively, of amortization expense relating to the above-listed intangible assets. The intangible asset lives range from 5 to 20 years and the weighted- average The following table summarizes the estimated future amortization expense for intangible assets (in millions): Year Ending September 30, 2021 $ 147.9 2022 120.4 2023 97.3 2024 78.7 2025 63.6 Thereafter 242.0 Total future amortization expense $ 749.9

Financing Arrangements

Financing Arrangements	12 Months Ended
	Sep. 30, 2020
Debt Disclosure [Abstract]
Financing Arrangements	10. Financing Arrangements The following table summarizes all financing arrangements from the respective periods presented (in millions): September 30, 2020 2019 Revolving Lines of Credit 2023 ABL: U.S. Revolver 1 $ 251.1 $ 81.0 Current portion — — Borrowings under revolving lines of credit, net $ 251.1 $ 81.0 Long-term Debt, net Term Loans: 2025 Term Loan 2 $ 922.3 $ 926.5 Current portion (9.7 ) (9.7 ) Long-term borrowings under term loan 912.6 916.8 Senior Notes: 2023 Senior Notes 3 — 294.9 2025 Senior Notes 4 1,285.7 1,282.9 2026 Senior Notes 5 295.9 — Current portion — — Long-term borrowings under senior notes 1,581.6 1,577.8 Long-term debt, net $ 2,494.2 $ 2,494.6 Equipment Financing Facilities, net Equipment financing facilities 6 $ 2.6 $ 6.9 Capital lease obligations 7 — 6.7 Current portion (2.6 ) (9.0 ) Long-term obligations under equipment financing, net $ — $ 4.6 ___________________________________________________ 1 2 3 4 5 6 7 Debt Refinancing 2026 Senior Notes On October 9, 2019, the Company, and certain subsidiaries of the Company as guarantors, executed a private offering of $300.0 million aggregate principal amount of 4.50% Senior Notes due 2026 (the “2026 Senior Notes”) at an issue price of 100%. The 2026 Senior Notes mature on November 15, 2026 and bear interest at a rate of 4.50% per annum, payable on May 15 and November 15 of each year, commencing on May 15, 2020. The 2026 Senior Notes and related subsidiary guarantees were offered and sold in a private transaction exempt from the registration requirements of the Securities Act of 1933, as amended (the “Securities Act”), to qualified institutional buyers in accordance with Rule 144A under the Securities Act and to non-U.S. persons outside of the United States pursuant to Regulation S under the Securities Act. The 2026 Senior Notes and related subsidiary guarantees have not been, and will not be, registered under the Securities Act or the securities laws of any state or other jurisdiction, and may not be offered or sold in the United States absent registration or an applicable exemption from On October 28, 2019, the Company used the net proceeds from the offering, together with cash on hand and available borrowings under the 2023 ABL (as defined The intent of the transaction was to take advantage of lower market interest rates by refinancing the existing 2023 Senior Notes with the 2026 Senior Notes. The Company accounted for the refinance as a debt extinguishment of the 2023 Senior Notes and an issuance of the 2026 Senior Notes. As a result, the Company recorded a loss on debt extinguishment of $14.7 million in the three months ended December 31, 2019. The Company has capitalized debt issuance costs of $4.8 million related to the 2026 Senior Notes, which are being amortized over the term of the financing arrangements. As of September 30, 2020, the outstanding balance on the 2026 Senior Notes, net of $4.1 million of unamortized debt issuance costs, was $295.9 million. Financing - Allied Acquisition In connection with the Allied Acquisition, the Company entered into various financing arrangements totaling $3.57 billion, including an asset-based revolving line of credit of $1.30 billion (“2023 ABL”), $525.0 million of which was drawn at closing, and a $970.0 million term loan (“2025 Term Loan”). The Company also raised an additional $1.30 billion through the issuance of senior notes (the “2025 Senior Notes”). The proceeds from these financing arrangements were used to finance the Allied Acquisition, to refinance or otherwise extinguish all third-party indebtedness, to pay fees and expenses associated with the acquisition, and to provide working capital and funds for other general corporate purposes. The Company capitalized new debt issuance costs totaling approximately $65.3 million related to the 2023 ABL, the 2025 Term Loan and the 2025 Senior Notes, which are being amortized over the term of the financing arrangements. 2023 ABL On January 2, 2018, the Company entered into a $1.30 billion asset-based revolving line of credit with Wells Fargo Bank, N.A. and a syndicate of other lenders. The 2023 ABL, as amended to date, provides for revolving loans in both the United States (“2023 U.S. Revolver”) in an amount up to $1.25 billion and Canada (“2023 Canada Revolver”) in an amount up to $50.0 million, in each case subject to a borrowing base. The 2023 ABL has a maturity date of January 2, 2023. The 2023 ABL has various borrowing tranches with an interest rate based, at the Company’s option, on a base rate, plus an applicable margin, or a reserve adjusted LIBOR rate, plus an applicable margin. The applicable margin ranges from 0.25% to 0.75% per annum with respect to base rate borrowings and from 1.25% to 1.75% per annum with respect to LIBOR borrowings. The current unused commitment fees on the 2023 ABL are 0.25% per annum. On July 28, 2020, the Company amended the 2023 ABL to provide for, among other things, a mechanism for replacing LIBOR with the secured overnight financing rate published by the Federal Reserve Bank of New York or other alternate benchmark rate selected by the administrative agent and the Company. There is one financial covenant under the 2023 ABL, which is the Fixed Charge Coverage Ratio (the “FCCR”). The FCCR is calculated by dividing Consolidated EBITDA, less Capital Expenditures, by Consolidated Fixed Charges (all terms as defined in the agreement). Per the covenant, the The 2023 ABL is secured by a first priority lien over substantially all of the Company’s and each guarantor’s accounts, chattel paper, deposit accounts, books, records and inventory (as well as intangibles related thereto), subject to certain customary exceptions (the “ABL Priority Collateral”), and a second priority lien over substantially all of the Company’s and each guarantor’s other assets, including all of the equity interests of any subsidiary held by the Company or any guarantor, subject to certain customary exceptions (the “Term Priority Collateral”). The 2023 ABL is guaranteed jointly, severally, fully and unconditionally by the Company’s active United States subsidiaries. In March 2020, the Company elected to draw down approximately $725 million from its revolving lines of credit. This was a proactive measure to increase the Company's cash position and preserve financial flexibility in light of current uncertainty in global markets resulting from the COVID-19 pandemic. During the second half of fiscal 2020, the Company used a portion of its operating cash flows to fully repay these additional borrowings. As of September 30, 2020, the total balance outstanding on the 2023 ABL, net of $5.9 million of unamortized debt issuance costs, was $251.1 million. The Company also has outstanding standby letters of credit related to the 2023 U.S. Revolver in the amount of $13.0 million as of September 30, 2020. 2025 Term Loan On January 2, 2018, the Company entered into a $970.0 million Term Loan with Citibank N.A., and a syndicate of other lenders. The 2025 Term Loan requires quarterly principal payments in the amount of $2.4 million, with the remaining outstanding principal to be paid on its January 2, 2025 maturity date. The interest rate is based, at the Company’s option, on a base rate, plus an applicable margin, or a reserve adjusted LIBOR rate, plus an applicable margin. The applicable margin is 1.25% per annum with respect to base rate borrowings and 2.25% per annum with respect to LIBOR borrowings. The 2025 Term Loan is secured by a first priority lien on the Term Priority Collateral and a second priority lien on the ABL Priority Collateral. Certain excluded As of September 30, 2020 2025 Senior Notes On October 25, 2017, Beacon Escrow Corporation, a wholly owned subsidiary of the Company (the “Escrow Issuer”), completed a private offering of $1.30 billion aggregate principal amount of 4.875% Senior Notes due 2025 at an issue price of 100%. The 2025 Senior Notes bear interest at a rate of 4.875% per annum, payable semi-annually in arrears, beginning May 1, 2018. The Company anticipates repaying the 2025 Senior Notes at the maturity date of November 1, 2025. Per the terms of the Escrow Agreement, the net proceeds from the 2025 Senior Notes remained in escrow until they were used to fund a portion of the purchase price of the Allied Acquisition payable at closing on January 2, 2018. Upon closing of the Allied Acquisition on January 2, 2018, (i) the Escrow Issuer merged with and into the Company, and the Company assumed all obligations under the 2025 Senior Notes; and (ii) all existing domestic subsidiaries of the Company (including the entities acquired in the Allied Acquisition As of September 30, 2020, the outstanding Financing - RSG Acquisition 2023 Senior Notes On October 1, 2015, in connection with the acquisition of Roofing Supply Group, the Company raised $300.0 million by issuing 6.38% Senior Notes due 2023 (the “2023 Senior redemption Other Information The following table presents annual principal payments for all outstanding financing arrangements for each of the next five years and thereafter (in millions): Year Ending September 30, 2023 ABL 2025 Term Loan Senior Notes 1 Equipment Financing Facilities Total 2021 $ — $ 9.7 $ — $ 2.6 $ 12.3 2022 — 9.7 — — 9.7 2023 257.0 9.7 — — 266.7 2024 — 9.7 — — 9.7 2025 — 906.9 — — 906.9 Thereafter — — 1,600.0 — 1,600.0 Total debt 257.0 945.7 1,600.0 2.6 2,805.3 Unamortized debt issuance costs (5.9 ) (23.4 ) (18.4 ) — (47.7 ) Total long-term debt $ 251.1 $ 922.3 $ 1,581.6 $ 2.6 $ 2,757.6 ___________________________________________________ 1 Under the terms of the 2023 ABL, the 2025 Term Loan, the 2025 Senior Notes and the 2026 Senior Notes, the Company is limited in making certain restricted payments, including dividends on its common stock. Based on the provisions in the respective debt agreements and given the Company’s intention to not pay common stock dividends in the foreseeable future, the Company does not believe that the restrictions are significant.

Leases

Leases	12 Months Ended
	Sep. 30, 2020
Leases [Abstract]
Leases	11. Leases The following table summarizes components of operating lease costs recognized within selling, general and administrative expenses (in millions): Year Ended September 30, 2020 Operating lease costs $ 124.5 Variable lease costs 10.4 Total operating lease costs $ 134.9 The following table presents supplemental cash flow information related to operating leases (in millions): Year Ended September 30, 2020 Operating cash flows for operating lease liabilities $ 118.7 As of September 30, 2020, the Company’s operating leases had a weighted-average remaining lease term of 5.6 years and a weighted-average discount rate of 3.87%. The following table summarizes future lease payments under operating leases as of September 30, 2020 (in millions): Year Ending September 30, 2021 $ 115.1 2022 101.4 2023 84.3 2024 67.4 2025 40.8 Thereafter 81.8 Total future lease payments 490.8 Imputed interest (49.9 ) Total operating lease liabilities $ 440.9

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Sep. 30, 2020
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	12. Commitments and Contingencies The Company is subject to loss contingencies pursuant to various federal, state and local environmental laws and regulations; however, the Company is not aware of any reasonably possible losses that would have a material impact on its results of operations, financial position, or liquidity. Potential loss contingencies include possible obligations to remove or mitigate the effects on the environment of the placement, storage, disposal or release of certain chemical or other substances by the Company or by other parties. In connection with its acquisitions, the Company’s practice is to request indemnification for any and all known material liabilities of significance as of the respective dates of acquisition. Historically, environmental liabilities have not had a material impact on the Company’s results of operations, financial position or liquidity. The Company is subject to litigation from time to time in the ordinary course of business; however, the Company does not expect the results, if any, to have a material adverse impact on its results of operations, financial position or liquidity.

Accumulated Other Comprehensive

Accumulated Other Comprehensive Income (Loss)	12 Months Ended
	Sep. 30, 2020
Equity [Abstract]
Accumulated Other Comprehensive Income (Loss)	13. Accumulated Other Comprehensive Income (Loss) Other comprehensive income (loss) is comprised of certain gains and losses that are excluded from net income under GAAP and instead recorded as a separate element of stockholders’ equity. The following table summarizes the components of and changes in accumulated other comprehensive loss (in millions): Foreign Derivative Currency Translation Financial Instruments AOCI Balance as of September 30, 2017 $ (14.6 ) $ — $ (14.6 ) Other comprehensive loss before reclassifications (2.7 ) — (2.7 ) Reclassifications out of other comprehensive loss — — — Balance as of September 30, 2018 $ (17.3 ) $ — $ (17.3 ) Other comprehensive income before reclassifications (1.7 ) (1.6 ) (3.3 ) Reclassifications out of other comprehensive loss — — — Balance as of September 30, 2019 $ (19.0 ) $ (1.6 ) $ (20.6 ) Other comprehensive income before reclassifications (0.7 ) (13.4 ) (14.1 ) Reclassifications out of other comprehensive loss — — — Balance as of September 30, 2020 $ (19.7 ) $ (15.0 ) $ (34.7 ) Gains (losses) on derivative instruments are recognized in the consolidated statements of operations in interest expense, financing costs, and other.

Income Taxes

Income Taxes	12 Months Ended
	Sep. 30, 2020
Income Tax Disclosure [Abstract]
Income Taxes	14. Income Taxes The Company recorded a provision for (benefit from) income taxes of $(26.8) million, $(0.2) million and $(30.5) million for the years ended September 30, 2020, 2019 and 2018, respectively. On March 27, 2020, the U.S. federal government officially signed into law the Coronavirus Aid, Relief, and Economic Security Act (the “CARES Act”). ASC 740, “ Accounting for Income Taxes five-year Other provisions in the CARES Act impacting the Company include the ability to carry back losses due to the technical correction for fiscal year filers with an NOL in the 2017-2018 straddle year, the technical correction regarding qualified improvement property, the increase in Section 163(j) interest limitation percentage, and the allowance of remaining AMT credits to be fully refundable in 2019. The Company carried back losses from 2018 to 2016 during the year to generate an immaterial tax benefit. The increase in Section 163(j) interest limitation percentage allowed the Company to deduct all interest expense from 2020 and utilize the Section 163(j) deferred tax asset carryover generated in 2019. The remaining provisions did not have a material impact on the Company’s tax provision (benefit). On December 22, 2017, the U.S. federal government officially signed into law the Tax Cut and Jobs Act of 2017 (“TCJA”). ASC 740, Accounting for Income Taxes, required companies to recognize the effect of tax law changes in the period of enactment even though the effective date for most provisions was for tax years beginning after December 31, 2017, or in the case of certain other provisions, January 1, 2018. The Company has fully implemented the federal TCJA provisions into its ASC 740 analysis. State conformity to the TCJA law changes have been communicated by the state and local jurisdictions; therefore, the Company has made adjustments related to the potential impact in its financial statements. The following table summarizes the components of the income tax provision (benefit) (in millions): Year Ended September 30, 2020 2019 2018 Current: Federal 1 $ (1.1 ) $ — $ (4.4 ) Foreign 1.4 0.6 0.5 State 0.6 1.6 3.5 Total current taxes 0.9 2.2 (0.4 ) Deferred: Federal (21.3 ) (1.5 ) (35.2 ) Foreign 0.2 — (0.2 ) State (6.6 ) (0.9 ) 5.3 Total deferred taxes (27.7 ) (2.4 ) (30.1 ) Provision for (benefit from) income taxes $ (26.8 ) $ (0.2 ) $ (30.5 ) _____________________________ 1 The following table is a reconciliation of the statutory federal income tax rate to the Company’s effective income tax rate for the periods presented: Year Ended September 30, 2020 2019 2018 U.S. federal income taxes at statutory rate 21.0 % 21.0 % 24.5 % State income taxes, net of federal benefit 4.1 % (3.3 %) 5.2 % Share-based payments (0.6 %) (4.7 %) (3.9 %) Deferred tax asset/liability remeasurement 1 0.6 % 0.0 % (73.5 %) Repatriation transition tax 1 0.0 % 4.3 % 1.8 % Non-deductible meals and entertainment (0.9 %) (13.9 %) 2.2 % Other 0.7 % (1.8 %) (1.2 %) Effective tax rate 24.9 % 1.6 % (44.9 %) _____________________________ 1 2020 includes the impact of carryback of NOLs to 2016 tax year and realization of 35% statutory rate. 2018 includes Impact of Tax Cut and Jobs Act of 2017. Deferred income taxes reflect the tax consequences of temporary differences between the amounts of assets and liabilities for financial reporting purposes and such amounts as measured by tax law. These temporary differences are determined according to ASC 740 Income Taxes. The following table presents t emporary differences that give rise to deferred tax assets and liabilities for the periods presented (in millions ): September 30, 2020 2019 Deferred tax assets: Deferred compensation $ 10.8 $ 11.1 Allowance for doubtful accounts 5.7 4.2 Accrued vacation and other 10.4 5.2 Inventory valuation 13.7 14.3 Tax loss carryforwards 1 11.5 18.3 Unrealized loss on financial derivatives 4.8 0.5 Lease liability 111.6 1.6 Total deferred tax assets 168.5 55.2 Deferred tax liabilities: Excess tax over book depreciation and amortization (128.7 ) (159.1 ) Lease right-of-use asset (113.8 ) — Total deferred tax liabilities (242.5 ) (159.1 ) Net deferred income tax liabilities $ (74.0 ) $ (103.9 ) _____________________________ 1 Comprised of net operating loss, foreign tax, and alternative minimum tax carryforwards The Company acquired $135.3 million of federal and state net operating loss (“NOL”) carryforwards as part of its acquisition of RSG in fiscal year 2016. For the year ended September 30, 2020, the Company utilized $4.3 million of federal NOLs. As of September 30, 2020, the Company had a total federal NOL carryforward balance of $27.8 million, portions of which are set to expire at various dates through 2035. The Company’s non-domestic subsidiary, BRSCC, is treated as a controlled foreign corporation. BRSCC’s taxable income, which reflects all of the Company’s Canadian operations, is being taxed only in Canada and would generally be taxed in the United States only upon an actual or deemed distribution. The Company expects that BRSCC’s earnings will be indefinitely reinvested for the foreseeable future; therefore, no United States deferred tax asset or liability for the differences between the book basis and the tax basis of BRSCC has been recorded as of September 30, 2020. As of September 30, 2020, the Company’s goodwill balance on its consolidated balance sheet was $2.49 billion, of which there remains an amortizable tax basis of $1.25 billion for income tax purposes. As of September 30, 2020, there were no uncertain tax positions which, if recognized, would affect the Company’s effective tax rate. The Company’s accounting policy is to recognize any interest and penalties related to income tax matters in income tax expense in the consolidated statements of operations. The Company has operations in 50 U.S. states and 6 provinces in Canada. The Company is currently under audit in certain state and local jurisdictions for various years. These audits may involve complex issues, which may require an extended period of time to resolve. Additional taxes are reasonably possible; however, the amounts cannot be estimated at this time or would not be significant. The Company is no longer subject to U.S. federal income tax examinations for any fiscal years ended on or before September 30, 2016. For the majority of states, the Company is also no longer subject to tax examinations for any fiscal years ended on or before September 30, 2016. In Canada, the Company is no longer subject to tax examinations for any fiscal years ended on or before September 30, 2016. For the Canadian provinces, the Company is no longer subject to tax examinations for any fiscal years ended on or before September 30, 2016.

Geographic Data

Geographic Data	12 Months Ended
	Sep. 30, 2020
Segment Reporting [Abstract]
Geographic Data	15. Geographic Data The following tables summarize certain geographic information for the periods presented (in millions): September 30, 2020 2019 Long-lived assets: U.S. $ 985.8 $ 1,182.5 Canada 9.9 12.4 Total long-lived assets $ 995.7 $ 1,194.9

Allowance for Doubtful Accounts

Allowance for Doubtful Accounts	12 Months Ended
	Sep. 30, 2020
Allowance For Doubtful Accounts [Abstract]
Allowance for Doubtful Accounts	16. Allowance for Doubtful Accounts The following table summarizes changes in the valuation of the allowance for doubtful accounts (in millions): Year Ended September 30, Beginning Balance Charged to Operations Write-offs Ending Balance 2020 $ 13.1 $ 19.2 $ (13.1 ) $ 19.2 2019 17.6 9.5 (14.0 ) 13.1 2018 11.8 11.0 (5.2 ) 17.6 The increase in valuation as of September 30, 2020 is primarily due to the recording of additional allowance in response to certain customer bankruptcies.

Fair Value Measurement

Fair Value Measurement	12 Months Ended
	Sep. 30, 2020
Fair Value Disclosures [Abstract]
Fair Value Measurement	17. Fair Value Measurement As of September 30, 2020, the carrying amount of cash and cash equivalents, accounts receivable, prepaid and other current assets, accounts payable and accrued expenses approximated fair value because of the short-term nature of these instruments. The Company measures its cash equivalents at amortized cost, which approximates fair value based upon quoted market prices (Level 1). As of September 30, 2020, based upon recent trading prices (Level 2 — market approach), the fair value of the Company’s $300.0 million Senior Notes due in 2026 was $309.0 million and the fair value of the $1.30 billion Senior Notes due in 2025 was $1.28 billion. As of September 30, 2020, the fair value of the Company’s term loan and revolving asset-based line of credit approximated the amount outstanding. The Company estimates the fair value of these financing arrangements by discounting the future cash flows of each instrument using estimated market rates of debt instruments with similar maturities and credit profiles (Level 3).

Employee Benefit Plans

Employee Benefit Plans	12 Months Ended
	Sep. 30, 2020
Compensation And Retirement Disclosure [Abstract]
Employee Benefit Plans	18. Employee Benefit Plans The Company maintains defined contribution plans covering all full-time employees of the Company who have 90 days of service and are at least 21 years old. An eligible employee may elect to make a before-tax contribution of between 1% and 100% of his or her compensation through payroll deductions, not to exceed the annual limit set by law. The Company currently matches the first 50% of participant contributions limited to 6% of a participant’s gross compensation (maximum Company match is 3%). The combined total expense for this plan and a similar plan for Canadian employees was $12.1 million, $11.7 million, and $11.8 million for the years ended September 30, 2020, 2019, and 2018, respectively. The Company sponsors an external pension fund for certain of its foreign employees who belong to a local union. Pension contributions are made to government-sponsored social security pension plans in accordance with local legal requirements. Annual contributions were $1.7 million, $1.0 million, and $0.2 million for the years ended September 30, 2020, 2019, and 2018, respectively. The Company also participates in multi-employer defined benefit plans for which it is not the sponsor. The aggregated expense for these plans was $2.5 million, $2.6 million, and $1.8 million for the years ended September 30, 2020, 2019, and 2018, respectively. Withdrawal from participation in one of these plans requires the Company to make a lump-sum contribution to the plan, and the Company’s withdrawal liability depends on the extent of the plan’s funding of vested benefits, among other factors. During the year ended September 30, 2020, the Company withdrew from the Central States Pension Fund and Local 408 Pension Fund, both of which were reported to have underfunded liabilities. The Company reached a settlement agreement with each fund, and the lump-sum contributions made to exit the funds did not have a material impact on its results of operations.

Financial Derivatives

Financial Derivatives	12 Months Ended
	Sep. 30, 2020
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Financial Derivatives	19. Financial Derivatives The Company uses interest rate derivative instruments to manage the risk related to fluctuating cash flows from interest rate changes by converting a portion of its variable-rate borrowings into fixed-rate borrowings. On September 11, 2019, the Company entered into two interest rate swap agreements to manage the interest rate risk associated with the variable-rate on the 2025 Term Loan. Each swap agreement has a notional amount of $250 million. One agreement (the “5-year swap”) will expire on August 30, 2024 and swaps the thirty-day LIBOR with a fixed-rate of 1.49%. The second agreement (the “3-year swap”) will expire on August 30, 2022 and swaps the thirty-day LIBOR with a fixed-rate of 1.50%. At the inception of the swap agreements, the Company determined that both swaps qualified for cash flow hedge accounting under ASC 815. Therefore, changes in the fair value of the effective portions of the swaps, net of taxes, will be recognized in other comprehensive income each period, then reclassified into the consolidated statements of operations as a component of interest expense, financing costs, and other in the period in which the hedged transaction affects earnings. Any ineffective portions of the hedges are immediately recognized in earnings as a component of interest expense, financing costs and other. The effectiveness of the swaps will be assessed qualitatively by the Company during the lives of the hedges by a) comparing the current terms of the hedges with the related hedged debt to assure they continue to coincide and b) through an evaluation of the ability of the counterparty to the hedges to honor their obligations under the hedges. The Company performed a qualitative analysis as of September 30, 2020 and concluded that the swap agreements continue to meet the requirements under ASC 815 to qualify for cash flow hedge accounting. As of September 30, 2020, the fair value of the 3-year and 5-year swaps, net of tax, were $5.0 million and $10.0 million, respectively, both in favor of the counterparty. These amounts are included in accrued expenses in the accompanying consolidated balance sheets. The Company records any differences paid or received on its interest rate hedges to interest expense, financing costs and other. The following table summarizes the combined fair values, net of tax, of the interest rate derivative instruments (in millions): Assets/(Liabilities) as of September 30, Instrument Fair Value Hierarchy 2020 2019 Designated interest rate swaps 1 Level 2 $ (15.0 ) $ (1.6 ) _______________________ 1 Assets are included on the consolidated balance sheets in prepaid expenses and other current assets, while liabilities are included in accrued expenses. The fair value of the interest rate swaps is determined through the use of a pricing model, which utilizes verifiable inputs such as market interest rates that are observable at commonly quoted intervals (generally referred to as the “LIBOR Curve”) for the full terms of the hedge agreements. These values reflect a Level 2 measurement under the applicable fair value hierarchy. The following table summarizes the amounts of gain (loss) on the interest rate derivative instruments recognized in other comprehensive income (in millions): Year Ended September 30, Instrument 2020 2019 2018 Designated interest rate swaps $ (13.4 ) $ (1.6 ) $ —

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Sep. 30, 2020
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The consolidated financial statements include the accounts of the Company and its wholly-owned subsidiaries. All inter-company transactions have been eliminated. Certain prior period amounts have been reclassified to conform to current period presentation.
Use of Estimates	Use of Estimates The preparation of consolidated financial statements in conformity with United States generally accepted accounting principles (“GAAP”) requires management to make estimates and assumptions that affect the amounts reported in these consolidated financial statements and accompanying notes. Significant items subject to such estimates include inventories, purchase price allocations, recoverability of goodwill and intangibles, and income taxes. Assumptions made in the development of these estimates contemplate the impact of the novel coronavirus (“COVID‑19”) on the economy and the Company’s anticipated results; however, actual amounts could differ materially from these estimates.
Fiscal Year	Fiscal Year The fiscal years presented are the years ended September 30, 2020 (“2020”), September 30, 2019 (“2019”), and September 30, 2018 (“2018”). Each of the Company’s first three quarters ends on the last day of the calendar month.
Segment Information	Segment Information Operating segments are defined as components of a business that can earn revenue and incur expenses for which discrete financial information is evaluated on a regular basis by the chief operating decision maker (“CODM”) in order to decide how to allocate resources and assess performance. The Company’s CODM, the Chief Executive Officer, reviews consolidated results of operations to make decisions, therefore the Company views its operations and manages its business as one operating segment.
Cash and Cash Equivalents	Cash and Cash Equivalents The Company considers all highly liquid investments with maturities of three months or less when purchased to be cash equivalents. Cash and cash equivalents also include unsettled credit card transactions. Cash equivalents are comprised of money market funds which invest primarily in commercial paper or bonds with a rating of A-1 or better, and bank certificates of deposit.
Accounts Receivable	Accounts Receivable Accounts receivable are derived from unpaid invoiced amounts and are recorded at their net realizable value. The allowance for doubtful accounts is calculated based on actual historical write-offs and current economic factors and represents the Company’s best estimate of its credit exposure. Each month the Company reviews its receivables on a customer-by-customer basis and any balances that are deemed uncollectible are written off against the allowance after all means of collection have been exhausted and the potential for recovery is considered remote. The Company’s accounts receivable are primarily from customers in the building industry located in the United States and Canada, and no single customer represented at least 10% of the Company’s revenue during the year ended September 30, 2020 or accounts receivable as of September 30, 2020.
Concentrations of Risk	Concentrations of Risk Financial instruments that potentially subject the Company to significant concentration of credit risk consist primarily of cash, cash equivalents and accounts receivable. The Company maintains the majority of its cash and cash equivalents with one financial institution, which management believes to be financially sound and with minimal credit risk. The Company’s deposits typically exceed amounts guaranteed by the Federal Deposit Insurance Corporation.
Inventories	Inventories Inventories, consisting substantially of finished goods, are valued at the lower of cost or market (net realizable value). Cost is determined using the moving weighted-average cost method. The Company’s arrangements with vendors typically provide for rebates after it makes a special purchase and/or monthly, quarterly and/or annual rebates of a specified amount of consideration payable when a number of measures have been achieved. Annual rebates are generally related to a specified cumulative level of purchases on a calendar-year basis. The Company accounts for such rebates as a reduction of the inventory value until the product is sold, at which time such rebates reduce cost of sales in the consolidated statements of operations. Throughout the year, the Company estimates the amount of the periodic rebates based upon the expected level of purchases. The Company continually revises these estimates to reflect actual rebates earned based on actual purchase levels. Amounts due from vendors under these arrangements are included in “prepaid expenses and other current assets” in the accompanying consolidated balance sheets.
Property and Equipment	Property and Equipment Property and equipment acquired in connection with acquisitions are recorded at fair value as of the date of the acquisition and depreciated utilizing the straight-line method over the estimated remaining lives. All other additions are recorded at cost, and depreciation is computed using the straight-line method. The Company reviews the estimated useful lives of its fixed assets on an ongoing basis and the following table summarizes the estimates currently used: Asset Class Estimated Useful Life Buildings and improvements 40 years Equipment 3 to 7 years Furniture and fixtures 7 years Leasehold improvements Shorter of the estimated useful life or the term of the lease, considering renewal options expected to be exercised.
Business Combinations	Business Combinations The Company records acquisitions resulting in the consolidation of a business using the acquisition method of accounting. Under this method, the acquiring Company records the assets acquired, including intangible assets that can be identified and named, and liabilities assumed based on their estimated fair values at the date of acquisition. The Company uses an income approach to determine the fair value of acquired intangible assets, specifically the multi-period excess earnings method for customer relationships and the relief from royalty method for trade names. Various Level 3 fair value assumptions are used in the determination of these estimated fair values, including items such as sales growth rates, cost synergies, customer attrition rates, discount rates, and other prospective financial information. The purchase price in excess of the fair value of the assets acquired and liabilities assumed is recorded as goodwill. Estimates associated with the accounting for acquisitions may change as additional information becomes available regarding the assets acquired and liabilities assumed. Transaction costs associated with acquisitions are expensed as incurred.
Goodwill and Intangibles	Goodwill and Intangibles On an annual basis and at interim periods when circumstances require, the Company tests the recoverability of its goodwill and indefinite-lived intangible assets. Examples of such indicators include a significant change in the business climate, unexpected competition, loss of key personnel or a decline in the Company’s market capitalization below the Company’s net book value. The Company performs impairment assessments at the reporting unit level, which is defined as an operating segment or one level below an operating segment, also known as a component. The Company currently has four components which it evaluates for aggregation by examining the distribution methods, sales mix, and operating results of each component to determine if these characteristics will be sustained over a long-term basis. For purposes of this evaluation, the Company expects its components to exhibit similar economic characteristics 3-5 years after events such as an acquisition within the Company’s core roofing business or management/business restructuring. Components that exhibit similar economic characteristics are subsequently aggregated into a single reporting unit. Based on the Company’s most recent impairment assessment performed as of August 31, 2020, it was determined that all of the Company’s components exhibited similar economic characteristics, and therefore should be aggregated into a single reporting unit (collectively, the “Reporting Unit”). To test for the recoverability of goodwill and indefinite-lived intangible assets, the Company first performs a qualitative assessment based on economic, industry and company-specific factors for all or selected reporting units to determine whether the existence of events and circumstances indicates that it is more likely than not that the goodwill or indefinite-lived intangible asset is impaired. Based on the results of the qualitative assessment, two additional steps in the impairment assessment may be required. The first step would require a comparison of each reporting unit’s fair value to the respective carrying value. If the carrying value exceeds the fair value, a second step is performed to measure the amount of impairment loss on a relative fair value basis, if any. Based on the Company’s most recent qualitative impairment assessment performed as of August 31, 2020, the Company concluded that there were no indicators of impairment, and that therefore it was more likely than not that the fair value of the goodwill and indefinite-lived intangible assets exceeded their net carrying amount, and therefore the quantitative two-step impairment test was not required. The Company amortizes certain identifiable intangible assets that have finite lives, currently consisting of non-compete agreements, customer relationships and trade names. Non-compete agreements are amortized on a straight-line basis over the terms of the associated contractual agreements; customer relationship assets are amortized on an accelerated basis based on the expected cash flows generated by the existing customers; and trade names are amortized on an accelerated basis over a five or ten year period. Amortizable intangible assets are tested for impairment, when deemed necessary, based on undiscounted cash flows and, if impaired, are written down to fair value based on either discounted cash flows or appraised values. In connection with certain financing arrangements, the Company has debt issuance costs that are amortized over the lives of the associated financings.
Evaluation of Long-Lived Assets	Evaluation of Long-Lived Assets The Company evaluates the recoverability of its long-lived assets for impairment whenever events or circumstances indicate that the carrying amount of the assets may not be recoverable. Recoverability is measured by comparing the carrying amount of the asset to the future undiscounted cash flows the asset is expected to generate. If the asset is considered to be impaired, the amount of any impairment is measured as the difference between the carrying value and the fair value of the impaired asset.
Fair Value Measurement	Fair Value Measurement The Company applies fair value accounting for all financial assets and liabilities that are reported at fair value in the financial statements on a recurring basis. Fair value is defined as the price that would be received from selling an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. The accounting guidance establishes a defined three-tier hierarchy to classify and disclose the fair value of assets and liabilities on both the date of their initial measurement as well as all subsequent periods. The hierarchy prioritizes the inputs used to measure fair value by the lowest level of input that is available and significant to the fair value measurement. The three levels are described as follows: • Level 1 : Observable inputs. Quoted prices in active markets for identical assets and liabilities; • Level 2 : Observable inputs other than the quoted price. Includes quoted prices for similar instruments, quoted prices for identical or similar instruments in inactive markets and amounts derived from valuation models where all significant inputs are observable in active markets; and • Level 3 : Unobservable inputs. Includes amounts derived from valuation models where one or more significant inputs are unobservable and require the Company to develop relevant assumptions. The Company evaluates its financial assets and liabilities subject to fair value measurements on a recurring basis to determine the appropriate level of classification as of each reporting period.
Financial Derivatives	Financial Derivatives The Company has entered into interest rate swaps to minimize the risks and costs associated with financing activities, as well as to maintain an appropriate mix of fixed-rate and floating-rate debt. The swap agreements are contracts to exchange variable-rate for fixed-interest rate payments over the life of the agreements. The Company's derivative instruments are designated as cash flow hedges, for which the Company records the effective portions of changes in their fair value, net of tax, in other comprehensive income. The Company recognizes any ineffective portion of the hedges in the consolidated statement of operations through interest expense, financing costs and other.
Net Sales	Net Sales The Company records net sales when performance obligations with the customer are satisfied. A performance obligation is a promise to transfer a distinct good to the customer and is the unit of account. The transaction price is allocated to each distinct performance obligation and recognized as net sales when, or as, the performance obligation is satisfied. All contracts have a single performance obligation as the promise to transfer the individual good is not separately identifiable from other promises and is, therefore, not distinct. Performance obligations are satisfied at a point in time and net sales are recognized when the customer accepts the delivery of a product or takes possession of a product with rights and rewards of ownership. For goods shipped by third party carriers, the Company recognizes revenue upon shipment since the terms are generally FOB shipping point at which time control passes to the customer. The Company also arranges for certain products to be shipped directly from the manufacturer to the customer. The Company recognizes the gross revenue for these sales upon shipment as the terms are FOB shipping point at which time control passes to the customer. The Company enters into agreements with customers to offer rebates, generally based on achievement of specified sales levels and various marketing allowances that are common industry practice. Reductions to net sales for customer programs and incentive offerings, including promotions and other volume-based incentives, are estimated using the most likely amount method and recorded in the period in which the sale occurs. Provisions for early payment discounts are accrued in the same period in which the sale occurs. The Company does not have any material payment terms as payment is received shortly after the transfer of control of the products to the customer. Commissions to internal sales teams are paid to obtain contracts. As these contracts are less than one year, these costs are expensed as incurred. The Company includes shipping and handling costs billed to customers in net sales. Related costs are accounted for as fulfillment activities and are recognized as cost of products sold when control of the products transfers to the customer.
Leases	Leases The Company mostly operates in leased facilities, which are accounted for as operating leases. The leases typically provide for a base rent plus real estate taxes and insurance. Certain of the leases provide for escalating rents over the lives of the leases, and rent expense is recognized over the terms of those leases on a straight-line basis. The real estate leases expire between 2020 and 2038. In addition, the Company leases equipment such as trucks and forklifts. Equipment leases are primarily accounted for as operating leases; however, the Company also accounts for some equipment leases as finance leases. The equipment leases expire between 2020 and 2027. The Company determines if an arrangement is a lease at inception. Operating lease assets and liabilities are included on the consolidated balance sheets. Finance lease assets are included in property and equipment, net. The current portion of the finance lease liabilities is included in accrued expenses, and the noncurrent portion is included in other long-term liabilities. Operating lease assets and liabilities are recognized at the present value of the future lease payments at the lease commencement date. The interest rate used to determine the present value of the future lease payments is the Company’s incremental borrowing rate, because the interest rates implicit in most of the leases are not readily determinable. The incremental borrowing rate is estimated to approximate the interest rate on a collateralized basis with similar terms and payments. Operating lease assets include any prepaid lease payments and lease incentives. The Company’s lease terms include periods under options to extend or terminate the lease when it is reasonably certain that those options will be exercised. The Company generally uses the base, non-cancelable lease term when determining the lease assets and liabilities. Operating lease expense is recognized on a straight-line basis over the lease term. The Company’s lease agreements generally contain lease and non-lease components. Non-lease components primarily include payments for maintenance and utilities. The Company has elected to combine fixed payments for non-lease components with lease payments and account for them together as a single lease component, which increases the lease assets and liabilities. Payments under the Company’s lease agreements are primarily fixed. However, certain lease agreements contain variable payments, which are expensed as incurred and are not included in the operating lease assets and liabilities. These amounts include payments affected by the Consumer Price Index and reimbursements to landlords for items such as property insurance and common area costs. The Company’s lease agreements do not contain any material residual value guarantees or material restrictive covenants.
Stock-Based Compensation	Stock-Based Compensation The Company applies the fair value method to recognize compensation expense for stock-based awards. Using this method, the estimated grant-date fair value of the award is recognized on a straight-line basis over the requisite service period based on the portion of the award that is expected to vest. The Company estimates forfeitures at the time of grant and revises the estimates, if necessary, in subsequent periods if actual forfeitures differ from those estimates. For awards with a performance-based vesting condition, the Company accrues stock-based compensation expense if it is probable that the performance condition will be achieved. Stock-based compensation expense for restricted stock units is measured based on the fair value of the Company’s common stock on the grant date. The Company utilizes the Black-Scholes option pricing model to estimate the grant-date fair value of option awards. The exercise price of option awards is set to equal the estimated fair value of the common stock at the date of the grant. The following weighted-average assumptions are also used to calculate the estimated fair value of option awards: • Expected volatility : The expected volatility of the Company’s shares is estimated using the historical stock price volatility over the most recent period commensurate with the estimated expected term of the awards. • Expected term : For employee stock option awards, the Company determines the weighted average expected term equal to the weighted period between the vesting period and the contract life of all outstanding options. • Dividend yield : The Company has not paid dividends and does not anticipate paying a cash dividend in the foreseeable future and, accordingly, uses an expected dividend yield of zero. • Risk-free interest rate : The Company bases the risk-free interest rate on the implied yield available on a U.S. Treasury note with a term equal to the estimated expected term of the awards.
Foreign Currency Translation	Foreign Currency Translation The Company’s operations located outside of the United States where the local currency is the functional currency are translated into U.S. dollars using the current rate method. Results of operations are translated at the average rate of exchange for the period. Assets and liabilities are translated at the closing rates on the period end date. Gains and losses on translation of these accounts are accumulated and reported as a separate component of equity and other comprehensive income (loss). Gains and losses on foreign currency transactions are recognized in the consolidated statements of operations as a component of interest expense, financing costs, and other.
Income Taxes	Income Taxes The Company accounts for income taxes using the liability method, which requires it to recognize a current tax liability or asset for current taxes payable or refundable and a deferred tax liability or asset for the estimated future tax effects of temporary differences between the financial statement and tax reporting bases of assets and liabilities to the extent that they are realizable. Deferred tax expense (benefit) results from the net change in deferred tax assets and liabilities during the year. FASB ASC Topic 740 prescribes a recognition threshold and measurement attribute for the financial statement recognition and measurement of a tax position taken or expected to be taken in a tax return. Based on this guidance, the Company analyzes its filing positions in all of the federal and state jurisdictions where it is required to file income tax returns, as well as all open tax years in these jurisdictions. Tax benefits from uncertain tax positions are recognized if it is more likely than not that the position is sustainable based solely on its technical merits.
Net Income (Loss) per Share	Net Income (Loss) per Share Basic net income (loss) per share is calculated by dividing net income (loss) attributable to common shareholders by the weighted-average number of common shares outstanding during the period, without consideration for common share equivalents or the conversion of Preferred Stock. Common share equivalents consist of the incremental common shares issuable upon the exercise of stock options and vesting of restricted stock unit awards. Diluted net income (loss) per common share is calculated by dividing net income (loss) attributable to common shareholders by the fully diluted weighted-average number of common shares outstanding during the period. Holders of Preferred Stock participate in dividends on an as-converted basis when declared on common shares. As a result, Preferred Stock is classified as a participating security and thereby requires the allocation of income that would have otherwise been available to common shareholders when calculating net income (loss) per share. Diluted net income (loss) per share is calculated by utilizing the most dilutive result of the if-converted and two-class methods. In both methods, net income (loss) attributable to common shareholders and the weighted-average common shares outstanding are adjusted to account for the impact of the assumed issuance of potential common shares that are dilutive, subject to dilution sequencing rules.
Recent Accounting Pronouncements - Adopted and Not Yet Adopted	Recent Accounting Pronouncements—Adopted In February 2016, the FASB issued ASU 2016-02, “ Leases In February 2018, the FASB issued ASU 2018-02, “Income Statement – Reporting Comprehensive Income.” Recent Accounting Pronouncements—Not Yet Adopted In June 2016, the FASB issued ASU 2016-13, “ Financial Instruments—Credit Losses: Measurement of Credit Losses on Financial Instruments In January 2017, the FASB issued ASU 2017-04, “ Simplifying the Accounting for Goodwill Impairment.” In December 2019, the FASB issued ASU 2019-12, “Income Taxes – Simplifying the Accounting for Income Taxes.” In March 2020, the FASB issued ASU 2020-04, “ Reference Rate Reform (Topic 848), Facilitation of the Effects of Reference Rate Reform on Financial Reporting In October 2020, the FASB issued ASU 2020-06, “ Debt – Debt with Conversion and Other Options (Subtopic 470-20) and Derivatives and Hedging – Contracts in Entity’s Own Equity (Subtopic 815-40), Accounting for Convertible Instruments and Contracts in an Entity’s Own Equity

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Sep. 30, 2020
Accounting Policies [Abstract]
Summary of Property and Equipment Estimated Useful Life	All other additions are recorded at cost, and depreciation is computed using the straight-line method. The Company reviews the estimated useful lives of its fixed assets on an ongoing basis and the following table summarizes the estimates currently used: Asset Class Estimated Useful Life Buildings and improvements 40 years Equipment 3 to 7 years Furniture and fixtures 7 years Leasehold improvements Shorter of the estimated useful life or the term of the lease, considering renewal options expected to be exercised.

Acquisitions (Tables)

Acquisitions (Tables)	12 Months Ended
	Sep. 30, 2020
Business Combinations [Abstract]
Schedule of Recognized Identified Assets Acquired and Liabilities Assumed	As of March 31, 2019, the Company had finalized the purchase accounting entries for the Allied Acquisition, detailed as follows (in millions): January 2, 2018 January 2, 2018 (as reported at March 31, 2018) Adjustments (as adjusted at March 31, 2019) Cash $ 19.3 $ (19.2 ) $ 0.2 Accounts receivable 315.5 22.1 337.5 Inventory 322.7 (7.9 ) 314.8 Prepaid and other current assets 59.3 16.2 75.4 Property, plant, and equipment 139.5 (0.2 ) 139.4 Goodwill 1,130.6 102.1 1,232.8 Intangible assets 1,037.0 — 1,037.0 Current liabilities (271.3 ) 12.0 (259.3 ) Non-current liabilities (6.8 ) 6.1 (0.7 ) Total purchase price $ 2,745.8 $ 131.2 $ 2,877.1

Net Sales (Tables)

Net Sales (Tables)	12 Months Ended
	Sep. 30, 2020
Revenue From Contract With Customer [Abstract]
Summary of Net Sales by Product Line and Geography	The following table presents the Company’s net sales by product line and geography for the years ended September 30, 2020 and 2019 (in millions): U.S. Canada Total Year Ended September 30, 2020 Residential roofing products $ 3,043.2 $ 56.4 $ 3,099.6 Non-residential roofing products 1,534.5 112.1 1,646.6 Complementary building products 2,188.7 9.0 2,197.7 Total net sales $ 6,766.4 $ 177.5 $ 6,943.9 Year Ended September 30, 2019 Residential roofing products $ 3,023.2 $ 56.4 $ 3,079.6 Non-residential roofing products 1,582.8 122.4 1,705.2 Complementary building products 2,312.5 7.9 2,320.4 Total net sales $ 6,918.5 $ 186.7 $ 7,105.2

Net Income (Loss) Per Share (Ta

Net Income (Loss) Per Share (Tables)	12 Months Ended
	Sep. 30, 2020
Earnings Per Share [Abstract]
Components and Calculation of Basic and Diluted Net Income (Loss) Per Share	The following table presents the components and calculations of basic and diluted net income (loss) per share for each period presented (in millions, except per share amounts): Year Ended September 30, 2020 2019 2018 Net income (loss) $ (80.9 ) $ (10.6 ) $ 98.6 Dividends on Preferred Stock 24.0 24.0 18.0 Net income (loss) attributable to common shareholders $ (104.9 ) $ (34.6 ) $ 80.6 Undistributed income allocated to participating securities — — (7.7 ) Net income (loss) attributable to common shareholders - basic and diluted $ (104.9 ) $ (34.6 ) $ 72.9 Weighted-average common shares outstanding - basic 68.8 68.4 68.0 Effect of common share equivalents — — 1.2 Weighted-average common shares outstanding - diluted 68.8 68.4 69.2 Net income (loss) per share - basic $ (1.52 ) $ (0.51 ) $ 1.07 Net income (loss) per share - diluted $ (1.52 ) $ (0.51 ) $ 1.05
Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share	The following table includes the number of shares that may be dilutive common shares in the future. These shares were not included in the computation of diluted net income (loss) per share because the effect was either anti-dilutive or the requisite performance conditions were not met (in millions): Year Ended September 30, 2020 2019 2018 Stock options 2.0 1.3 0.4 Restricted stock units 0.3 0.1 0.2 Preferred Stock 9.7 9.7 7.2

Stock-based Compensation (Table

Stock-based Compensation (Tables)	12 Months Ended
	Sep. 30, 2020
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Fair Values of Options, Black-Scholes Option-Pricing Model, Weighted-Average Assumptions	The fair values of the options granted for the year ended September 30, 2020 were estimated on the dates of grants using the Black-Scholes option-pricing model with the following weighted-average assumptions: Year Ended September 30, 2020 2019 2018 Risk-free interest rate 1.61 % 2.86 % 2.10 % Expected volatility 34.26 % 29.68 % 26.43 % Expected life (in years) 5.26 5.22 5.46 Dividend yield — — —
Stock Options Outstanding and Activity During the Period	The following table summarizes all stock option activity for the periods presented (in millions, except per share and time period amounts): Options Outstanding Weighted- Average Exercise Price Weighted- Average Remaining Contractual Term (Years) Aggregate Intrinsic Value 1 Balance as of September 30, 2019 2.3 $ 32.61 6.1 $ 12.0 Granted 0.5 31.95 Exercised (0.2 ) 19.51 Canceled/Forfeited (0.1 ) 37.30 Expired — 31.68 Balance as of September 30, 2020 2.5 $ 33.09 5.9 $ 6.9 Vested and expected to vest after September 30, 2020 2.4 $ 33.13 5.9 $ 6.8 Exercisable as of September 30, 2020 1.6 $ 33.69 4.5 $ 5.1 ____________________________________________________________________ 1
Stock Option Grants, Vesting, and Exercises	The following table summarizes additional information on stock options for the periods presented (in millions, except per share amounts): Year Ended September 30, 2020 2019 2018 Weighted-average fair value of stock options granted $ 10.35 $ 8.91 $ 15.86 Total grant date fair value of stock options vested $ 4.3 $ 3.9 $ 4.2 Total intrinsic value of stock options exercised $ 2.1 $ 2.8 $ 9.6
Restricted Shares and Units Outstanding and Activity During the Period	The following table summarizes all restricted stock unit activity for the periods presented (in millions, except per share amounts): RSUs Outstanding Weighted-Average Grant Date Fair Value Balance as of September 30, 2019 1.1 $ 37.48 Granted 0.5 31.81 Released (0.3 ) 44.87 Canceled/Forfeited (0.1 ) 33.69 Balance as of September 30, 2020 1.2 $ 33.55 Vested and expected to vest after September 30, 2020 1.0 $ 34.68
Schedule Of Restricted Stock Units Additional Information	The following table summarizes additional information on RSUs for the period presented (in millions, except per share amounts): Year Ended September 30, 2020 2019 2018 Weighted-average fair value of RSUs granted $ 31.81 $ 28.02 $ 57.40 Total grant date fair value of RSUs vested $ 14.4 $ 16.1 $ 6.7 Total intrinsic value of RSUs released $ 9.8 $ 11.5 $ 11.0

Prepaid Expenses and Other Cu_2

Prepaid Expenses and Other Current Assets (Tables)	12 Months Ended
	Sep. 30, 2020
Prepaid Expenses And Other Current Assets [Abstract]
Schedule of Prepaid Expenses and Other Current Assets	The following table summarizes the significant components of prepaid expenses and other current assets (in millions): September 30, 2020 2019 Vendor rebates $ 326.4 $ 262.8 Other 51.9 52.8 Total prepaid expenses and other current assets $ 378.3 $ 315.6

Property and Equipment (Tables)

Property and Equipment (Tables)	12 Months Ended
	Sep. 30, 2020
Property Plant And Equipment [Abstract]
Property and Equipment, net	The following table provides a detailed breakout of property and equipment, by type (in millions): September 30, 2020 2019 Land and buildings $ 87.3 $ 83.4 Equipment 453.1 448.1 Furniture and fixtures 42.5 40.0 Finance lease assets 11.6 — Total property and equipment 594.5 571.5 Accumulated depreciation (350.8 ) (311.1 ) Total property and equipment, net $ 243.7 $ 260.4

Goodwill and Intangible Assets

Goodwill and Intangible Assets (Tables)	12 Months Ended
	Sep. 30, 2020
Goodwill And Intangible Assets Disclosure [Abstract]
Summary of Changes in goodwill	Goodwill The following table sets forth the change in the carrying amount of goodwill during the years ended September 30, 2020 and 2019, respectively (in millions): Balance as of September 30, 2018 $ 2,491.8 Acquisitions 1 (0.5 ) Translation and other adjustments (0.7 ) Balance as of September 30, 2019 $ 2,490.6 Balance as of September 30, 2019 $ 2,490.6 Translation and other adjustments (0.2 ) Balance as of September 30, 2020 $ 2,490.4 ___________________________________________ 1 Reflects purchase accounting adjustments related to fiscal year 2018 acquisition of Atlas Supply, Inc. (see Note 3 for further discussion).
Summary of Intangible Assets	The following table summarizes intangible assets by category (in millions, except time period amounts): September 30, Weighted- Average Remaining Life 1 2020 2019 (Years) Amortizable intangible assets: Non-compete agreements $ 0.2 $ 2.8 1.7 Customer relationships 1,481.1 1,530.9 16.4 Trademarks 7.2 10.5 6.1 Beneficial lease arrangements — 8.1 — Total amortizable intangible assets 1,488.5 1,552.3 Accumulated amortization (738.6 ) (619.9 ) Total amortizable intangible assets, net $ 749.9 $ 932.4 Indefinite-lived trademarks 51.3 193.1 Total intangibles, net $ 801.2 $ 1,125.5 ___________________________________________ 1 As of September 30, 2020.
Summary of Estimated Future Amortization	The following table summarizes the estimated future amortization expense for intangible assets (in millions): Year Ending September 30, 2021 $ 147.9 2022 120.4 2023 97.3 2024 78.7 2025 63.6 Thereafter 242.0 Total future amortization expense $ 749.9

Financing Arrangements (Tables)

Financing Arrangements (Tables)	12 Months Ended
	Sep. 30, 2020
Debt Disclosure [Abstract]
Schedule of Long-term Debt Instruments	The following table summarizes all financing arrangements from the respective periods presented (in millions): September 30, 2020 2019 Revolving Lines of Credit 2023 ABL: U.S. Revolver 1 $ 251.1 $ 81.0 Current portion — — Borrowings under revolving lines of credit, net $ 251.1 $ 81.0 Long-term Debt, net Term Loans: 2025 Term Loan 2 $ 922.3 $ 926.5 Current portion (9.7 ) (9.7 ) Long-term borrowings under term loan 912.6 916.8 Senior Notes: 2023 Senior Notes 3 — 294.9 2025 Senior Notes 4 1,285.7 1,282.9 2026 Senior Notes 5 295.9 — Current portion — — Long-term borrowings under senior notes 1,581.6 1,577.8 Long-term debt, net $ 2,494.2 $ 2,494.6 Equipment Financing Facilities, net Equipment financing facilities 6 $ 2.6 $ 6.9 Capital lease obligations 7 — 6.7 Current portion (2.6 ) (9.0 ) Long-term obligations under equipment financing, net $ — $ 4.6 ___________________________________________________ 1 2 3 4 5 6 7
Schedule of Maturities of Long-term Debt	The following table presents annual principal payments for all outstanding financing arrangements for each of the next five years and thereafter (in millions): Year Ending September 30, 2023 ABL 2025 Term Loan Senior Notes 1 Equipment Financing Facilities Total 2021 $ — $ 9.7 $ — $ 2.6 $ 12.3 2022 — 9.7 — — 9.7 2023 257.0 9.7 — — 266.7 2024 — 9.7 — — 9.7 2025 — 906.9 — — 906.9 Thereafter — — 1,600.0 — 1,600.0 Total debt 257.0 945.7 1,600.0 2.6 2,805.3 Unamortized debt issuance costs (5.9 ) (23.4 ) (18.4 ) — (47.7 ) Total long-term debt $ 251.1 $ 922.3 $ 1,581.6 $ 2.6 $ 2,757.6 ___________________________________________________ 1

Leases (Tables)

Leases (Tables)	12 Months Ended
	Sep. 30, 2020
Leases [Abstract]
Components of Operating Lease Costs Recognized Within Selling, General and Administrative Expenses	The following table summarizes components of operating lease costs recognized within selling, general and administrative expenses (in millions): Year Ended September 30, 2020 Operating lease costs $ 124.5 Variable lease costs 10.4 Total operating lease costs $ 134.9
Summary of Supplemental Cash Flow Information Related to Operating Leases	The following table presents supplemental cash flow information related to operating leases (in millions): Year Ended September 30, 2020 Operating cash flows for operating lease liabilities $ 118.7
Summary of Future Lease Payments Under Operating Leases	The following table summarizes future lease payments under operating leases as of September 30, 2020 (in millions): Year Ending September 30, 2021 $ 115.1 2022 101.4 2023 84.3 2024 67.4 2025 40.8 Thereafter 81.8 Total future lease payments 490.8 Imputed interest (49.9 ) Total operating lease liabilities $ 440.9

Accumulated Other Comprehensi_2

Accumulated Other Comprehensive Income (Loss) (Tables)	12 Months Ended
	Sep. 30, 2020
Equity [Abstract]
Reclassification out of Accumulated Other Comprehensive Income	The following table summarizes the components of and changes in accumulated other comprehensive loss (in millions): Foreign Derivative Currency Translation Financial Instruments AOCI Balance as of September 30, 2017 $ (14.6 ) $ — $ (14.6 ) Other comprehensive loss before reclassifications (2.7 ) — (2.7 ) Reclassifications out of other comprehensive loss — — — Balance as of September 30, 2018 $ (17.3 ) $ — $ (17.3 ) Other comprehensive income before reclassifications (1.7 ) (1.6 ) (3.3 ) Reclassifications out of other comprehensive loss — — — Balance as of September 30, 2019 $ (19.0 ) $ (1.6 ) $ (20.6 ) Other comprehensive income before reclassifications (0.7 ) (13.4 ) (14.1 ) Reclassifications out of other comprehensive loss — — — Balance as of September 30, 2020 $ (19.7 ) $ (15.0 ) $ (34.7 )

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Sep. 30, 2020
Income Tax Disclosure [Abstract]
Schedule Of Income Tax Provision (Benefit)	The following table summarizes the components of the income tax provision (benefit) (in millions): Year Ended September 30, 2020 2019 2018 Current: Federal 1 $ (1.1 ) $ — $ (4.4 ) Foreign 1.4 0.6 0.5 State 0.6 1.6 3.5 Total current taxes 0.9 2.2 (0.4 ) Deferred: Federal (21.3 ) (1.5 ) (35.2 ) Foreign 0.2 — (0.2 ) State (6.6 ) (0.9 ) 5.3 Total deferred taxes (27.7 ) (2.4 ) (30.1 ) Provision for (benefit from) income taxes $ (26.8 ) $ (0.2 ) $ (30.5 ) _____________________________ 1
Principal Reason for the Difference Between Effective Income Tax Rate and the Statutory Federal Income	The following table is a reconciliation of the statutory federal income tax rate to the Company’s effective income tax rate for the periods presented: Year Ended September 30, 2020 2019 2018 U.S. federal income taxes at statutory rate 21.0 % 21.0 % 24.5 % State income taxes, net of federal benefit 4.1 % (3.3 %) 5.2 % Share-based payments (0.6 %) (4.7 %) (3.9 %) Deferred tax asset/liability remeasurement 1 0.6 % 0.0 % (73.5 %) Repatriation transition tax 1 0.0 % 4.3 % 1.8 % Non-deductible meals and entertainment (0.9 %) (13.9 %) 2.2 % Other 0.7 % (1.8 %) (1.2 %) Effective tax rate 24.9 % 1.6 % (44.9 %) _____________________________ 1 2020 includes the impact of carryback of NOLs to 2016 tax year and realization of 35% statutory rate. 2018 includes Impact of Tax Cut and Jobs Act of 2017.
Components of the Company's Deferred Taxes	temporary differences are determined according to ASC 740 Income Taxes. The following table presents t emporary differences that give rise to deferred tax assets and liabilities for the periods presented (in millions ): September 30, 2020 2019 Deferred tax assets: Deferred compensation $ 10.8 $ 11.1 Allowance for doubtful accounts 5.7 4.2 Accrued vacation and other 10.4 5.2 Inventory valuation 13.7 14.3 Tax loss carryforwards 1 11.5 18.3 Unrealized loss on financial derivatives 4.8 0.5 Lease liability 111.6 1.6 Total deferred tax assets 168.5 55.2 Deferred tax liabilities: Excess tax over book depreciation and amortization (128.7 ) (159.1 ) Lease right-of-use asset (113.8 ) — Total deferred tax liabilities (242.5 ) (159.1 ) Net deferred income tax liabilities $ (74.0 ) $ (103.9 ) _____________________________ 1 Comprised of net operating loss, foreign tax, and alternative minimum tax carryforwards

Geographic Data (Tables)

Geographic Data (Tables)	12 Months Ended
	Sep. 30, 2020
Segment Reporting [Abstract]
Schedule of Geographic Information	The following tables summarize certain geographic information for the periods presented (in millions): September 30, 2020 2019 Long-lived assets: U.S. $ 985.8 $ 1,182.5 Canada 9.9 12.4 Total long-lived assets $ 995.7 $ 1,194.9

Allowance for Doubtful Accoun_2

Allowance for Doubtful Accounts (Tables)	12 Months Ended
	Sep. 30, 2020
Allowance For Doubtful Accounts [Abstract]
Allowance for Doubtful Accounts	The following table summarizes changes in the valuation of the allowance for doubtful accounts (in millions): Year Ended September 30, Beginning Balance Charged to Operations Write-offs Ending Balance 2020 $ 13.1 $ 19.2 $ (13.1 ) $ 19.2 2019 17.6 9.5 (14.0 ) 13.1 2018 11.8 11.0 (5.2 ) 17.6

Financial Derivatives (Tables)

Financial Derivatives (Tables)	12 Months Ended
	Sep. 30, 2020
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Summary of Combined Fair Values, Net of Tax of Interest Rate Derivative Instruments	The Company records any differences paid or received on its interest rate hedges to interest expense, financing costs and other. The following table summarizes the combined fair values, net of tax, of the interest rate derivative instruments (in millions): Assets/(Liabilities) as of September 30, Instrument Fair Value Hierarchy 2020 2019 Designated interest rate swaps 1 Level 2 $ (15.0 ) $ (1.6 ) _______________________ 1 Assets are included on the consolidated balance sheets in prepaid expenses and other current assets, while liabilities are included in accrued expenses.
Summary of Amounts of Gain (Loss) on Interest Rate Derivative Instruments Recognized in Other Comprehensive Income	The following table summarizes the amounts of gain (loss) on the interest rate derivative instruments recognized in other comprehensive income (in millions): Year Ended September 30, Instrument 2020 2019 2018 Designated interest rate swaps $ (13.4 ) $ (1.6 ) $ —

Company Overview - Additional I

Company Overview - Additional Information (Detail)	12 Months Ended
	Sep. 30, 2020StateProvince
Company Overview [Line Items]
Date of incorporation	Aug. 22, 1997
U.S. [Member]
Company Overview [Line Items]
Number of states in which entity operates | State	50
Canada [Member]
Company Overview [Line Items]
Number of provinces in which entity operates | Province	6

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions	Oct. 01, 2020USD ($)	Sep. 30, 2020USD ($)SegmentCustomer	Sep. 30, 2019USD ($)	Sep. 30, 2018USD ($)	Oct. 01, 2019USD ($)	Sep. 30, 2017USD ($)
Significant Accounting Policies [Line Items]
Number of operating segments | Segment		1
Number of customers, with at least 10% of revenue | Customer		0
Number of customers, with at least 10% of accounts receivable | Customer		0
Expected dividend yield		0.00%	0.00%	0.00%
Operating lease assets		$ 443.3
Operating lease liabilities		440.9
Accounts receivable reserve		$ 19.2	$ 13.1	$ 17.6		$ 11.8
Accounting Standards Update 2016-02 [Member]
Significant Accounting Policies [Line Items]
Operating lease assets					$ 483.5
Operating lease liabilities					$ 476
Accounting Standards Update 2016-13 [Member] | Restatement Adjustment [Member] | Subsequent Event [Member]
Significant Accounting Policies [Line Items]
Accounts receivable reserve	$ 4.3
Retained earnings adjustment	$ 4.3
Trade Names [Member]
Significant Accounting Policies [Line Items]
Finite-lived intangible assets, amortization method		accelerated basis over a five or ten year period
Minimum [Member]
Significant Accounting Policies [Line Items]
Amortization period		5 years
Minimum [Member] | Real Estate Property [Member]
Significant Accounting Policies [Line Items]
Operating lease expiration year		2020
Minimum [Member] | Equipment [Member]
Significant Accounting Policies [Line Items]
Operating lease expiration year		2020
Minimum [Member] | Trade Names [Member]
Significant Accounting Policies [Line Items]
Amortization period		5 years
Maximum [Member]
Significant Accounting Policies [Line Items]
Amortization period		20 years
Maximum [Member] | Real Estate Property [Member]
Significant Accounting Policies [Line Items]
Operating lease expiration year		2038
Maximum [Member] | Equipment [Member]
Significant Accounting Policies [Line Items]
Operating lease expiration year		2027
Maximum [Member] | Trade Names [Member]
Significant Accounting Policies [Line Items]
Amortization period		10 years
Major Supplier Concentration Risk [Member] | Minimum [Member] | Cost of Goods, Total [Member]
Significant Accounting Policies [Line Items]
Goodwill impairment test period		3 years
Major Supplier Concentration Risk [Member] | Maximum [Member] | Cost of Goods, Total [Member]
Significant Accounting Policies [Line Items]
Goodwill impairment test period		5 years

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Summary of Property and Equipment Estimated Useful Life (Detail)	12 Months Ended
	Sep. 30, 2020
Building and Building Improvements [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life	40 years
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life	7 years
Leasehold Improvements [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life	Shorter of the estimated useful life or the term of the lease, considering renewal options expected to be exercised.
Minimum [Member] | Equipment [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life	3 years
Maximum [Member] | Equipment [Member]
Property, Plant and Equipment [Line Items]
Estimated Useful Life	7 years

Acquisitions - Additional Infor

Acquisitions - Additional Information (Detail)		Jul. 16, 2018USD ($)Location	May 01, 2018USD ($)Location	Jan. 02, 2018USD ($)StateLocation$ / sharesshares	Sep. 30, 2020USD ($)$ / sharesshares	Sep. 30, 2019USD ($)$ / sharesshares	Mar. 31, 2019USD ($)	Sep. 30, 2018USD ($)Branch	Mar. 31, 2018USD ($)	Oct. 25, 2017USD ($)
Business Acquisition [Line Items]
Preferred stock, issuance | shares					0	0
Mezzanine equity	[1]				$ 399,200,000	$ 399,200,000
Common stock, par value | $ / shares					$ 0.01	$ 0.01
Number of business locations acquired | Branch								7
Business acquisitions purchase price allocation goodwill amount					$ 2,490,400,000	$ 2,490,600,000		$ 2,491,800,000
Senior Notes, Matures November 2025 [Member]
Business Acquisition [Line Items]
Debt instrument, aggregate principal amount										$ 1,300,000,000
Allied Acquisition [Member]
Business Acquisition [Line Items]
Adjusted purchase price					2,880,000,000		$ 2,877,100,000		$ 2,745,800,000
Net working capital adjustment					88,100,000
Business acquisition purchase price and tax position					164,000,000
Number of business locations acquired | Location				208
Number of states business location acquired | State				31
Business combination purchase price tax adjustment to be received, net					5,100,000
Business acquisition, goodwill, tax deductible amount					1,010,000,000
Business acquisitions purchase price allocation goodwill amount							1,232,800,000		1,130,600,000
Business acquisitions purchase price allocation intangible assets other than goodwill							$ 1,037,000,000		$ 1,037,000,000
Allied Acquisition [Member] | Investment Agreement [Member] | Series A Cumulative Convertible Participating Preferred Stock [Member]
Business Acquisition [Line Items]
Preferred stock, issuance | shares				400,000,000,000
Preferred stock, par value | $ / shares				$ 0.01
Preferred stock, liquidation preference value				$ 400,000,000
Preferred stock, liquidation purchase price per share | $ / shares				$ 1,000
Proceeds from convertible preferred stock				$ 400,000,000
Preferred stock conversion price per share | $ / shares				$ 41.26
Preferred stock dividend rate				6.00%
Mezzanine equity					399,200,000
Unamortized issuance costs					800,000
Allied Acquisition [Member] | Secured Term Loan B Facility [Member]
Business Acquisition [Line Items]
Long-term line of credit				$ 970,000,000
Allied Acquisition [Member] | Secured Term Loan B Facility [Member] | LIBOR [Member]
Business Acquisition [Line Items]
Base borrowing rates				1.25%
Allied Acquisition [Member] | Senior Notes, Matures November 2025 [Member]
Business Acquisition [Line Items]
Debt instrument, aggregate principal amount				$ 1,300,000,000
Allied Acquisition [Member] | Senior-secured Asset-based Revolving Line Of Credit [Member] | LIBOR [Member]
Business Acquisition [Line Items]
Base borrowing rates				2.25%
Allied Acquisition [Member] | Stock Purchase Agreement [Member]
Business Acquisition [Line Items]
Acquisition purchase price in cash				$ 2,625,000,000
Tri-State Builder Supply [Member]
Business Acquisition [Line Items]
Number of business locations acquired | Location			1
Business acquisition, sales reported by acquired entity for last annual period			$ 6,000,000
Atlas Supply, Inc. [Member]
Business Acquisition [Line Items]
Number of business locations acquired | Location		6
Business acquisition, sales reported by acquired entity for last annual period		$ 37,000,000
Series of Individually Immaterial Business Acquisitions [Member]
Business Acquisition [Line Items]
Business acquisition, goodwill, tax deductible amount					$ 6,500,000
Business acquisitions purchase price allocation goodwill amount								7,600,000
Business acquisitions purchase price allocation intangible assets other than goodwill								$ 11,400,000
[1]	See Note 3 for additional information.

Acquisitions - Schedule of Reco

Acquisitions - Schedule of Recognized Identified Assets Acquired and Liabilities Assumed (Detail) - USD ($) $ in Millions		6 Months Ended	12 Months Ended
		Mar. 31, 2019	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2018	Mar. 31, 2018
Business Combination Separately Recognized Transactions [Line Items]
Goodwill			$ 2,490.6	$ 2,490.4	$ 2,491.8
Goodwill	[1]		$ (0.5)
Allied Acquisition [Member]
Business Combination Separately Recognized Transactions [Line Items]
Cash		$ 0.2				$ 19.3
Accounts receivable		337.5				315.5
Inventory		314.8				322.7
Prepaid and other current assets		75.4				59.3
Property, plant, and equipment		139.4				139.5
Goodwill		1,232.8				1,130.6
Intangible assets		1,037				1,037
Current liabilities		(259.3)				(271.3)
Non-current liabilities		(0.7)				(6.8)
Total purchase price		2,877.1		$ 2,880		$ 2,745.8
Cash		(19.2)
Accounts receivable		22.1
Inventory		(7.9)
Prepaid and other current assets		16.2
Property, plant, and equipment		(0.2)
Goodwill		102.1
Current liabilities		12
Non-current liabilities		6.1
Total purchase price		$ 131.2
[1]	Reflects purchase accounting adjustments related to fiscal year 2018 acquisition of Atlas Supply, Inc. (see Note 3 for further discussion).

Net Sales - Summary of Net Sale

Net Sales - Summary of Net Sales by Product Line and Geography (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Disaggregation Of Revenue [Line Items]
Net sales	$ 6,943.9	$ 7,105.2	$ 6,418.3
Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	3,099.6	3,079.6
Non-Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1,646.6	1,705.2
Complementary Building Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	2,197.7	2,320.4
U.S. [Member]
Disaggregation Of Revenue [Line Items]
Net sales	6,766.4	6,918.5
U.S. [Member] | Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	3,043.2	3,023.2
U.S. [Member] | Non-Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1,534.5	1,582.8
U.S. [Member] | Complementary Building Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	2,188.7	2,312.5
Canada [Member]
Disaggregation Of Revenue [Line Items]
Net sales	177.5	186.7
Canada [Member] | Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	56.4	56.4
Canada [Member] | Non-Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	112.1	122.4
Canada [Member] | Complementary Building Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	$ 9	$ 7.9

Net Income (Loss) Per Share - C

Net Income (Loss) Per Share - Components and Calculation of Basic and Diluted Net Income (Loss) Per Share (Detail) - USD ($) $ / shares in Units, $ in Millions		12 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share [Abstract]
Net income (loss)		$ (80.9)	$ (10.6)	$ 98.6
Dividends on Preferred Stock		24	24	18
Net income (loss) attributable to common shareholders		(104.9)	(34.6)	80.6
Undistributed income allocated to participating securities				(7.7)
Net income (loss) attributable to common shareholders - basic and diluted		$ (104.9)	$ (34.6)	$ 72.9
Weighted-average common shares outstanding - basic	[1]	68,800,000	68,400,000	68,000,000
Effect of common share equivalents				1,200,000
Weighted-average common shares outstanding - diluted	[1]	68,800,000	68,400,000	69,200,000
Net income (loss) per share - basic	[1]	$ (1.52)	$ (0.51)	$ 1.07
Net income (loss) per share - diluted	[1]	$ (1.52)	$ (0.51)	$ 1.05
[1]	See Note 5 for detailed calculations and further discussion.

Net Income (Loss) Per Share - S

Net Income (Loss) Per Share - Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share (Detail) - shares	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net income (loss) per share	9,700,000	9,700,000	7,200,000
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net income (loss) per share	2,000,000	1,300,000	400,000
Restricted Stock Units [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from computation of diluted net income (loss) per share	300,000	100,000	200,000

Stock-based Compensation - Addi

Stock-based Compensation - Additional Information (Detail) - USD ($) $ in Millions	Feb. 11, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation number of additional shares authorized	4,850,000
Stock-based compensation number of shares authorized		5,700,000
Restricted Stock Awards [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares that will vest		100.00%
Stock Options [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Non-qualified options granted expiration period		10 years
Vesting period		3 years
Allocated stock-based compensation expense		$ 4.4	$ 4.1	$ 3.9
Unrecognized compensation cost related to unvested stock		$ 4.9
Unrecognized compensation cost related to unvested stock, expected weighted-average period of recognition		1 year 9 months 18 days
Restricted Stock Units (RSUs) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Allocated stock-based compensation expense		$ 12.8	$ 12.3	$ 12.6
Unrecognized compensation cost related to unvested stock		$ 12.9
Unrecognized compensation cost related to unvested stock, expected weighted-average period of recognition		1 year 8 months 12 days
Restricted Stock Units (RSUs) [Member] | Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares that will vest		0.00%
Restricted Stock Units (RSUs) [Member] | Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Percentage of shares that will vest		200.00%

Stock-based Compensation - Fair

Stock-based Compensation - Fair Values of Options, Black-Scholes Option-Pricing Model, Weighted-Average Assumptions (Detail)	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Risk-free interest rate	1.61%	2.86%	2.10%
Expected volatility	34.26%	29.68%	26.43%
Expected life (in years)	5 years 3 months 3 days	5 years 2 months 19 days	5 years 5 months 15 days
Dividend yield	0.00%	0.00%	0.00%

Stock-based compensation - Stoc

Stock-based compensation - Stock Options Outstanding and Activity During the Period (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Options Outstanding
Balance as of September 30, 2019	2,300,000
Granted	500,000
Exercised	(200,000)
Canceled/Forfeited	(100,000)
Balance as of September 30, 2020	2,500,000	2,300,000
Vested and expected to vest after September 30, 2020	2,400,000
Exercisable as of September 30, 2020	1,600,000
Weighted-Average Exercise Price
Beginning Balance	$ 32.61
Granted	31.95
Exercised	19.51
Canceled/Forfeited	37.30
Expired	31.68
Ending Balance	33.09	$ 32.61
Vested and expected to vest after September 30, 2020	33.13
Exercisable as of September 30, 2020	$ 33.69
Weighted-Average Remaining Contractual Life
Balance	5 years 10 months 24 days	6 years 1 month 6 days
Vested and expected to vest after September 30, 2020	5 years 10 months 24 days
Exercisable as of September 30, 2020	4 years 6 months
Aggregate Intrinsic Value
Balance	$ 6.9	$ 12
Vested and expected to vest after September 30, 2020	6.8
Exercisable as of September 30, 2020	$ 5.1

Stock-based compensation - St_2

Stock-based compensation - Stock Option Grants, Vesting, and Exercises (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Weighted-average fair value of stock options granted	$ 10.35	$ 8.91	$ 15.86
Total grant date fair value of stock options vested	$ 4.3	$ 3.9	$ 4.2
Total intrinsic value of stock options exercised	$ 2.1	$ 2.8	$ 9.6

Stock-based Compensation - Rest

Stock-based Compensation - Restricted Shares and Units Outstanding and Activity During the Period (Detail) - $ / shares	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Weighted - Average Grant Date Fair Value
Granted	$ 31.81	$ 28.02	$ 57.40
Restricted Stock Units (RSUs) [Member]
Outstanding
Balance as of September 30, 2019	1,100,000
Granted	500,000
Released	(300,000)
Canceled/Forfeited	(100,000)
Balance as of September 30, 2020	1,200,000	1,100,000
Vested and expected to vest after September 30, 2020	1,000,000
Weighted - Average Grant Date Fair Value
Balance as of September 30, 2019	$ 37.48
Granted	31.81
Released	44.87
Canceled/Forfeited	33.69
Balance as of September 30, 2020	33.55	$ 37.48
Vested and expected to vest after September 30, 2020	$ 34.68

Stock-based Compensation - Sche

Stock-based Compensation - Schedule Of Restricted Stock Units Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Weighted-average fair value of RSUs granted	$ 31.81	$ 28.02	$ 57.40
Total grant date fair value of RSUs vested	$ 14.4	$ 16.1	$ 6.7
Total intrinsic value of RSUs released	$ 9.8	$ 11.5	$ 11

Prepaid Expenses and Other Cu_3

Prepaid Expenses and Other Current Assets - Schedule of Prepaid Expenses and Other Current Assets (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Prepaid Expenses And Other Current Assets [Abstract]
Vendor rebates	$ 326.4	$ 262.8
Other	51.9	52.8
Total prepaid expenses and other current assets	$ 378.3	$ 315.6

Property and Equipment - Proper

Property and Equipment - Property and Equipment, net (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 594.5	$ 571.5
Accumulated depreciation	(350.8)	(311.1)
Total property and equipment, net	243.7	260.4
Land and Buildings [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	87.3	83.4
Equipment [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	453.1	448.1
Furniture and Fixtures [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	42.5	40
Finance Lease Assets [Member]
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 11.6	$ 0

Property and Equipment - Additi

Property and Equipment - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Property Plant And Equipment [Abstract]
Depreciation	$ 70.1	$ 70.7	$ 60.3

Goodwill and Intangible Asset_2

Goodwill and Intangible Assets - Summary of Changes in goodwill (Detail) - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2020	Sep. 30, 2019
Goodwill And Intangible Assets Disclosure [Abstract]
Beginning balance		$ 2,490.6	$ 2,491.8
Acquisitions	[1]		(0.5)
Translation and other adjustments		(0.2)	(0.7)
Ending balance		$ 2,490.4	$ 2,490.6
[1]	Reflects purchase accounting adjustments related to fiscal year 2018 acquisition of Atlas Supply, Inc. (see Note 3 for further discussion).

Goodwill and Intangible Asset_3

Goodwill and Intangible Assets - Additional Information (Detail) - USD ($) $ in Millions		3 Months Ended	12 Months Ended
		Mar. 31, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Goodwill And Intangible Assets [Line Items]
Finite-lived Intangible Assets Acquired					$ 1,050
Amortization			$ 321	$ 207.1	141.2
Minimum [Member]
Goodwill And Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life			5 years
Maximum [Member]
Goodwill And Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life			20 years
Weighted Average [Member]
Goodwill And Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life			16 years 4 months 24 days
Trademarks [Member]
Goodwill And Intangible Assets [Line Items]
Indefinite-lived Intangible Assets Acquired					120
Tradenames [Member] | Allied Acquisition [Member]
Goodwill And Intangible Assets [Line Items]
Amortization		$ 142.6
Customer Relationships [Member]
Goodwill And Intangible Assets [Line Items]
Finite-lived Intangible Assets Acquired					920.8
Customer Relationships [Member] | Weighted Average [Member]
Goodwill And Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	[1]		16 years 4 months 24 days
Beneficial Lease Arrangements [Member]
Goodwill And Intangible Assets [Line Items]
Finite-lived Intangible Assets Acquired					$ 7
[1]	As of September 30, 2020.

Goodwill and Intangible Asset_4

Goodwill and Intangible Assets - Summary of Intangible Assets (Detail) - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2020	Sep. 30, 2019
Finite Lived Intangible Assets [Line Items]
Amortizable intangible assets		$ 1,488.5	$ 1,552.3
Accumulated amortization		(738.6)	(619.9)
Total amortizable intangible assets, net		749.9	932.4
Indefinite-lived trademarks		51.3	193.1
Total intangibles, net		$ 801.2	1,125.5
Weighted Average [Member]
Finite Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life		16 years 4 months 24 days
Minimum [Member]
Finite Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life		5 years
Maximum [Member]
Finite Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life		20 years
Noncompete Agreements [Member]
Finite Lived Intangible Assets [Line Items]
Amortizable intangible assets		$ 0.2	2.8
Noncompete Agreements [Member] | Weighted Average [Member]
Finite Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	[1]	1 year 8 months 12 days
Customer Relationships [Member]
Finite Lived Intangible Assets [Line Items]
Amortizable intangible assets		$ 1,481.1	1,530.9
Customer Relationships [Member] | Weighted Average [Member]
Finite Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	[1]	16 years 4 months 24 days
Trademarks [Member]
Finite Lived Intangible Assets [Line Items]
Amortizable intangible assets		$ 7.2	10.5
Trademarks [Member] | Weighted Average [Member]
Finite Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	[1]	6 years 1 month 6 days
Beneficial Lease Arrangements [Member]
Finite Lived Intangible Assets [Line Items]
Amortizable intangible assets			$ 8.1
[1]	As of September 30, 2020.

Goodwill and Intangible Asset_5

Goodwill and Intangible Assets - Summary of Estimated Future Amortization (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Goodwill And Intangible Assets Disclosure [Abstract]
2021	$ 147.9
2022	120.4
2023	97.3
2024	78.7
2025	63.6
Thereafter	242
Total amortizable intangible assets, net	$ 749.9	$ 932.4

Financing Arrangements - Long-t

Financing Arrangements - Long-term Debt Instruments (Detail) - USD ($)		Sep. 30, 2020		Sep. 30, 2019		Jan. 02, 2018
Debt Instrument [Line Items]
Borrowings under revolving lines of credit/term loans, net		$ 251,100,000		$ 81,000,000
Current portion		0		0
Long-term borrowings under senior notes		1,581,600,000		1,577,800,000
Long-term debt, net		2,494,200,000		2,494,600,000
Equipment financing facilities	[1]	2,600,000		6,900,000
Capital lease obligations	[2]			6,700,000
Long-term obligations under equipment financing, net				4,600,000
2023 Senior Notes [Member]
Debt Instrument [Line Items]
Senior Notes	[3]			294,900,000
2025 Senior Notes [Member]
Debt Instrument [Line Items]
Senior Notes	[4]	1,285,700,000		1,282,900,000
2026 Senior Notes [Member]
Debt Instrument [Line Items]
Senior Notes	[5]	295,900,000
Equipment Financing Facilities [Member]
Debt Instrument [Line Items]
Current portion		(2,600,000)		(9,000,000)
Long-term obligations under equipment financing, net				4,600,000
Revolving Lines of Credit [Member]
Debt Instrument [Line Items]
Borrowings under revolving lines of credit/term loans, net		251,100,000		81,000,000
Revolving Lines of Credit [Member] | 2023 ABL [Member]
Debt Instrument [Line Items]
Borrowings under revolving lines of credit/term loans, net		251,100,000
Revolving Lines of Credit [Member] | 2023 ABL [Member] | U.S. [Member]
Debt Instrument [Line Items]
Total Borrowings under revolving lines of credit/term loans	[6]	251,100,000		81,000,000
Term Loan [Member]
Debt Instrument [Line Items]
Current portion		(9,700,000)		(9,700,000)
Borrowings under revolving lines of credit/term loans, net		912,600,000		916,800,000
Term Loan [Member] | 2025 Term Loan [Member]
Debt Instrument [Line Items]
Total Borrowings under revolving lines of credit/term loans		$ 922,300,000	[7]	$ 926,500,000	[7]	$ 970,000,000
[1]	Fixed interest rates ranging from 2.33% to 2.89% for all periods presented.
[2]	As of October 1, 2019, in connection with the adoption of ASU 2016-02, capital lease obligations that were formerly included in equipment financing facilities are included either in accrued expenses or other long-term liabilities on the consolidated balance sheets (see Notes 2 and 11 for further discussion)
[3]	Interest rate of 6.38% as of September 30, 2019.
[4]	Interest rate of 4.88% for all periods presented.
[5]	Interest rate of 4.50% as of September 30, 2020.
[6]	Effective rate on borrowings of 1.89% and 5.41% as of September 30, 2020 and 2019, respectively.
[7]	Interest rate of 2.41% and 4.36% as of September 30, 2020 and 2019, respectively.

Financing Arrangements - Long_2

Financing Arrangements - Long-term Debt Instruments (Parenthetical) (Detail)	Jan. 02, 2018	Sep. 30, 2020	Sep. 30, 2019	Oct. 25, 2017	Oct. 01, 2015
Debt Instrument [Line Items]
Debt interest rate at period end		4.50%
Debt instrument maturity date		Nov. 15, 2026
Equipment Financing Facilities [Member]
Debt Instrument [Line Items]
Debt instrument maturity date		Sep. 30, 2021	Sep. 30, 2021
Equipment Financing Facilities [Member] | Minimum [Member]
Debt Instrument [Line Items]
Equipment financing facilities, fixed interest rate percentage		2.33%	2.33%
Equipment Financing Facilities [Member] | Maximum [Member]
Debt Instrument [Line Items]
Equipment financing facilities, fixed interest rate percentage		2.89%	2.89%
2023 Senior Notes [Member]
Debt Instrument [Line Items]
Debt interest rate at period end			6.38%		6.38%
Debt instrument maturity date			Oct. 1, 2023
2025 Senior Notes [Member]
Debt Instrument [Line Items]
Debt interest rate at period end		4.88%	4.88%	4.875%
Debt instrument maturity date		Nov. 1, 2025	Nov. 1, 2025
Revolving Lines of Credit [Member] | 2023 ABL [Member]
Debt Instrument [Line Items]
Debt instrument maturity date	Jan. 2, 2023
Revolving Lines of Credit [Member] | 2023 ABL [Member] | U.S. [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Interest Rate at Period End		1.89%	5.41%
Line of Credit Facility, Expiration Date		Jan. 2, 2023	Jan. 2, 2023
Term Loan [Member] | Term Loan, Matures January 2025 [Member]
Debt Instrument [Line Items]
Debt interest rate at period end		2.41%	4.36%
Debt instrument maturity date	Jan. 2, 2025	Jan. 2, 2025	Jan. 2, 2025

Financing Arrangements - 2026 S

Financing Arrangements - 2026 Senior Notes - Additional Information (Detail) - USD ($)		Oct. 28, 2019	Oct. 09, 2019	Jan. 02, 2018	Dec. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Oct. 01, 2015
Debt Instrument [Line Items]
Debt instrument interest rate						4.50%
Debt instrument maturity date						Nov. 15, 2026
Loss on debt extinguishment						$ 14,700,000	$ 0	$ 1,200,000
Payments of debt issuance costs				$ 65,300,000		4,300,000	$ 800,000	$ 65,800,000
Unamortized debt issuance costs						47,700,000
Senior Notes, Matures November 2026 [Member]
Debt Instrument [Line Items]
Debt instrument, aggregate principal amount			$ 300,000,000
Debt instrument interest rate			4.50%
Debt instrument, issue price percentage			100.00%
Debt instrument maturity date			Nov. 15, 2026
Debt instrument, interest payable commencement date			May 15, 2020
Payments of debt issuance costs			$ 4,800,000
Unamortized debt issuance costs						4,100,000
Senior Notes Payable	[1]					$ 295,900,000
Senior Notes, Matures October 2023 [Member]
Debt Instrument [Line Items]
Debt instrument interest rate							6.38%		6.38%
Debt instrument maturity date							Oct. 1, 2023
Redemption of senior debt		$ 300,000,000
Debt redemption price percentage of principal amount		103.188%
Debt instrument, redemption description						On October 28, 2019, the Company used the net proceeds from the offering, together with cash on hand and available borrowings under the 2023 ABL (as defined below), to redeem all $300.0 million aggregate principal amount outstanding of the 2023 Senior Notes (as defined below) at a redemption price of 103.188%
Unamortized debt issuance costs		$ 5,100,000
Senior Notes Payable	[2]						$ 294,900,000
Refinancing the Existing 2023 Senior Notes with Issuance of 2026 Senior Notes [Member]
Debt Instrument [Line Items]
Loss on debt extinguishment					$ 14,700,000
[1]	Interest rate of 4.50% as of September 30, 2020.
[2]	Interest rate of 6.38% as of September 30, 2019.

Financing Arrangements - Financ

Financing Arrangements - Financing - Allied Acquisition - Additional Information (Detail) - USD ($)	Jan. 02, 2018	Mar. 31, 2020	Sep. 30, 2020		Sep. 30, 2019		Sep. 30, 2018
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 3,570,000,000
Borrowings under senior notes			$ 300,000,000		$ 0		$ 1,300,000,000
Payments of debt issuance costs	65,300,000		4,300,000		800,000		$ 65,800,000
Senior Notes, Matures November 2025 [Member]
Debt Instrument [Line Items]
Borrowings under senior notes	1,300,000,000
Revolving Lines of Credit [Member] | 2023 ABL [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	1,300,000,000
Proceeds from Lines of Credit	525,000,000	$ 725,000,000
Term Loan [Member] | Term Loan, Matures January 2025 [Member]
Debt Instrument [Line Items]
Long-term line of credit	$ 970,000,000		$ 922,300,000	[1]	$ 926,500,000	[1]
[1]	Interest rate of 2.41% and 4.36% as of September 30, 2020 and 2019, respectively.

Financing Arrangements - 2023 A

Financing Arrangements - 2023 ABL - Additional Information (Detail)	Jan. 02, 2018USD ($)	Mar. 31, 2020USD ($)	Sep. 30, 2020USD ($)Covenant	Sep. 30, 2019USD ($)
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 3,570,000,000
Debt instrument maturity date			Nov. 15, 2026
Borrowings under revolving lines of credit/term loans, net			$ 251,100,000	$ 81,000,000
Unamortized debt issuance costs			47,700,000
2023 ABL [Member]
Debt Instrument [Line Items]
Unamortized debt issuance costs			5,900,000
Revolving Lines of Credit [Member]
Debt Instrument [Line Items]
Borrowings under revolving lines of credit/term loans, net			$ 251,100,000	$ 81,000,000
Revolving Lines of Credit [Member] | 2023 ABL [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 1,300,000,000
Debt instrument maturity date	Jan. 2, 2023
Line of credit facility, unused fees			0.25%
Number of financial covenants | Covenant			1
Debt instrument covenant compliance			Per the covenant, the Company’s FCCR must be a minimum of 1.00 at the end of each fiscal quarter, calculated on a trailing four quarter basis (or under certain circumstances, at the end of each fiscal month, calculated on a trailing twelve-month basis). Compliance is only required at such times as borrowing availability (subject to certain adjustments) is less than the greater of (i) 10% of the lesser of the borrowing base or the aggregate commitments or (ii) $90.0 million, and for a period of thirty days thereafter. The Company was in compliance with this covenant as of September 30, 2020.
Debt Instrument Covenant Required Borrowings Base Percentage			10.00%
Debt Instrument Covenant Required Borrowings Availability			$ 90,000,000
Fixed charge coverage ratio			100.00%
Proceeds from Lines of Credit	$ 525,000,000	$ 725,000,000
Borrowings under revolving lines of credit/term loans, net			$ 251,100,000
Unamortized debt issuance costs			5,900,000
Revolving Lines of Credit [Member] | 2023 ABL [Member] | Base Rate [Member] | Minimum [Member]
Debt Instrument [Line Items]
Base rate borrowings	0.25%
Revolving Lines of Credit [Member] | 2023 ABL [Member] | Base Rate [Member] | Maximum [Member]
Debt Instrument [Line Items]
Base rate borrowings	0.75%
Revolving Lines of Credit [Member] | 2023 ABL [Member] | LIBOR [Member] | Minimum [Member]
Debt Instrument [Line Items]
Base rate borrowings	1.25%
Revolving Lines of Credit [Member] | 2023 ABL [Member] | LIBOR [Member] | Maximum [Member]
Debt Instrument [Line Items]
Base rate borrowings	1.75%
Revolving Lines of Credit [Member] | U.S. [Member] | 2023 ABL [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 1,250,000,000
Standby letters of credit outstanding			$ 13,000,000
Revolving Lines of Credit [Member] | Canada [Member] | 2023 ABL [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 50,000,000

Financing Arrangements - 2025 T

Financing Arrangements - 2025 Term Loan - Additional Information (Detail) - USD ($)	Jan. 02, 2018	Sep. 30, 2020		Sep. 30, 2019
Debt Instrument [Line Items]
Debt instrument maturity date		Nov. 15, 2026
Unamortized debt issuance costs		$ 47,700,000
Term Loan, Matures January 2025 [Member]
Debt Instrument [Line Items]
Unamortized debt issuance costs		23,400,000
Term Loan [Member] | Term Loan, Matures January 2025 [Member]
Debt Instrument [Line Items]
Long-term line of credit	$ 970,000,000	$ 922,300,000	[1]	$ 926,500,000	[1]
Debt Instrument, Frequency of Periodic Payment		quarterly
Debt Instrument, Periodic Payment	$ 2,400,000
Debt instrument maturity date	Jan. 2, 2025	Jan. 2, 2025		Jan. 2, 2025
Unamortized debt issuance costs		$ 23,400,000
Term Loan [Member] | Term Loan, Matures January 2025 [Member] | Base Rate [Member]
Debt Instrument [Line Items]
Base rate borrowings	1.25%
Term Loan [Member] | Term Loan, Matures January 2025 [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Base rate borrowings	2.25%
[1]	Interest rate of 2.41% and 4.36% as of September 30, 2020 and 2019, respectively.

Financing Arrangements - 2025 S

Financing Arrangements - 2025 Senior Notes - Additional Information (Detail) - USD ($)		12 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Oct. 25, 2017
Debt Instrument [Line Items]
Debt instrument interest rate		4.50%
Debt instrument maturity date		Nov. 15, 2026
Unamortized debt issuance costs		$ 47,700,000
Senior Notes, Matures November 2025 [Member]
Debt Instrument [Line Items]
Debt instrument, aggregate principal amount				$ 1,300,000,000
Debt instrument interest rate		4.88%	4.88%	4.875%
Debt instrument, issue price percentage				100.00%
Debt instrument maturity date		Nov. 1, 2025	Nov. 1, 2025
Unamortized debt issuance costs		$ 14,300,000
Senior Notes Payable	[1]	$ 1,285,700,000	$ 1,282,900,000
[1]	Interest rate of 4.88% for all periods presented.

Financing Arrangements - 2023 S

Financing Arrangements - 2023 Senior Notes - Additional Information (Detail) - USD ($) $ in Millions	Oct. 28, 2019	Oct. 01, 2015	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Debt Instrument [Line Items]
Borrowings under senior notes			$ 300	$ 0	$ 1,300
Debt instrument interest rate			4.50%
Unamortized debt issuance costs			$ 47.7
Senior Notes, Matures October 2023 [Member]
Debt Instrument [Line Items]
Borrowings under senior notes		$ 300
Debt instrument interest rate		6.38%		6.38%
Debt instrument, coupon rate		6.38%
Redemption of senior debt	$ 300
Debt redemption price percentage of principal amount	103.188%
Unamortized debt issuance costs	$ 5.1

Financing Arrangements - Schedu

Financing Arrangements - Schedule of Maturities of Long-term Debt (Detail) $ in Millions	Sep. 30, 2020USD ($)
Debt Instrument [Line Items]
2021	$ 12.3
2022	9.7
2023	266.7
2024	9.7
2025	906.9
Thereafter	1,600
Total debt	2,805.3
Unamortized debt issuance costs	(47.7)
Total long-term debt	2,757.6
2023 ABL [Member]
Debt Instrument [Line Items]
2021	0
2022	0
2023	257
2024	0
2025	0
Thereafter	0
Total debt	257
Unamortized debt issuance costs	(5.9)
Total long-term debt	251.1
2025 Term Loan [Member]
Debt Instrument [Line Items]
2021	9.7
2022	9.7
2023	9.7
2024	9.7
2025	906.9
Thereafter	0
Total debt	945.7
Unamortized debt issuance costs	(23.4)
Total long-term debt	922.3
Equipment Financing Facilities [Member]
Debt Instrument [Line Items]
2021	2.6
2022	0
2023	0
2024	0
2025	0
Thereafter	0
Total debt	2.6
Unamortized debt issuance costs	0
Total long-term debt	2.6
Senior Notes [Member]
Debt Instrument [Line Items]
2021	0	[1]
2022	0	[1]
2023	0	[1]
2024	0	[1]
2025	0	[1]
Thereafter	1,600	[1]
Total debt	1,600	[1]
Unamortized debt issuance costs	(18.4)	[1]
Total long-term debt	$ 1,581.6	[1]
[1]	Represent principal amounts for 2025 Senior Notes and 2026 Senior Notes.

Leases - Components of Operatin

Leases - Components of Operating Lease Costs Recognized Within Selling, General and Administrative Expenses (Detail) $ in Millions	12 Months Ended
	Sep. 30, 2020USD ($)
Leases [Abstract]
Operating lease costs	$ 124.5
Variable lease costs	10.4
Total operating lease costs	$ 134.9

Leases - Summary of Supplementa

Leases - Summary of Supplemental Cash Flow Information Related to Operating Leases (Detail) $ in Millions	12 Months Ended
	Sep. 30, 2020USD ($)
Leases [Abstract]
Operating cash flows for operating lease liabilities	$ 118.7

Leases - Additional Information

Leases - Additional Information (Detail)	Sep. 30, 2020
Leases [Abstract]
Operating lease, weighted-average remaining lease term	5 years 7 months 6 days
Operating lease, weighted-average discount rate	3.87%

Leases - Summary of Future Leas

Leases - Summary of Future Lease Payments Under Operating Leases (Detail) $ in Millions	Sep. 30, 2020USD ($)
Leases [Abstract]
2021	$ 115.1
2022	101.4
2023	84.3
2024	67.4
2025	40.8
Thereafter	81.8
Total future lease payments	490.8
Imputed interest	(49.9)
Total operating lease liabilities	$ 440.9

Accumulated Other Comprehensi_3

Accumulated Other Comprehensive Income (Loss) - Reclassification out of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance		$ 1,862.3	$ 1,884.2	$ 1,781.8
Other comprehensive income (loss) before reclassifications		(14.1)	(3.3)	(2.7)
Balance		1,760.9	1,862.3	1,884.2
Foreign Currency Translation
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance		(19)	(17.3)	(14.6)
Other comprehensive income (loss) before reclassifications		(0.7)	(1.7)	(2.7)
Balance		(19.7)	(19)	(17.3)
Derivative Financial Instruments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance		(1.6)
Other comprehensive income (loss) before reclassifications		(13.4)	(1.6)
Balance		(15)	(1.6)
AOCI [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance	[1]	(20.6)	(17.3)	(14.6)
Other comprehensive income (loss) before reclassifications		(14.1)	(3.3)	(2.7)
Balance	[1]	$ (34.7)	$ (20.6)	$ (17.3)
[1]	Accumulated Other Comprehensive Income (Loss) ("AOCI")

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail)	Dec. 31, 2017	Sep. 30, 2020USD ($)StateProvince	Sep. 30, 2019USD ($)	Sep. 30, 2018USD ($)	Sep. 30, 2016USD ($)
Provision for (benefit from) income taxes		$ (26,800,000)	$ (200,000)	$ (30,500,000)
Carryback period of net operating losses		5 years
Net operating loss carryback beginning date		Dec. 31, 2017
Net operating loss carryback ending date		Jan. 1, 2021
U.S. federal income taxes at statutory rate	35.00%	21.00%	21.00%	24.50%
Operating loss carryforwards		$ 27,800,000			$ 135,300,000
Net operating loss carryforward utilized		4,300,000
Goodwill		2,490,400,000	$ 2,490,600,000	$ 2,491,800,000
Goodwill amortizable for income tax		1,250,000,000
Uncertain tax positions		$ 0
U.S. [Member]
Number of States in which Entity Operates | State		50
Canada [Member]
Number of provinces in which entity operates | Province		6

Income Taxes - Summary of Compo

Income Taxes - Summary of Components of Income Tax Provision (Benefit) (Detail) - USD ($) $ in Millions		12 Months Ended
		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Current:
Federal	[1]	$ (1.1)	$ 0	$ (4.4)
Foreign		1.4	0.6	0.5
State		0.6	1.6	3.5
Total current taxes		0.9	2.2	(0.4)
Deferred:
Federal		(21.3)	(1.5)	(35.2)
Foreign		0.2	0	(0.2)
State		(6.6)	(0.9)	5.3
Total deferred taxes		(27.7)	(2.4)	(30.1)
Provision for (benefit from) income taxes		$ (26.8)	$ (0.2)	$ (30.5)
[1]	2018 tax benefit due to changes in the treatment of acquired fixed assets stemming from the Tax Cut and Jobs Act of 2017

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Statutory Federal Income Tax Rate (Detail)	Dec. 31, 2017	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Income Tax Disclosure [Abstract]
U.S. federal income taxes at statutory rate	35.00%	21.00%	21.00%	24.50%
State income taxes, net of federal benefit		4.10%	(3.30%)	5.20%
Share-based payments		(0.60%)	(4.70%)	(3.90%)
Deferred tax asset/liability remeasurement		0.60%	0.00%	(73.50%)
Repatriation transition tax		0.00%	4.30%	1.80%
Non-deductible meals and entertainment		(0.90%)	(13.90%)	2.20%
Other		0.70%	(1.80%)	(1.20%)
Effective tax rate		24.90%	1.60%	(44.90%)

Income Taxes - Reconciliation_2

Income Taxes - Reconciliation of Statutory Federal Income Tax Rate (Parenthetical) (Detail)	Dec. 31, 2017	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Income Tax Disclosure [Abstract]
U.S. federal income taxes at statutory rate	35.00%	21.00%	21.00%	24.50%

Income Taxes - Schedule of Temp

Income Taxes - Schedule of Temporary Differences That Give Rise to Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Deferred tax assets:
Deferred compensation	$ 10.8	$ 11.1
Allowance for doubtful accounts	5.7	4.2
Accrued vacation and other	10.4	5.2
Inventory valuation	13.7	14.3
Tax loss carryforwards	11.5	18.3
Unrealized loss on financial derivatives	4.8	0.5
Lease liability	111.6	1.6
Total deferred tax assets	168.5	55.2
Deferred tax liabilities:
Excess tax over book depreciation and amortization	(128.7)	(159.1)
Lease right-of-use asset	(113.8)
Total deferred tax liabilities	(242.5)	(159.1)
Net deferred income tax liabilities	$ (74)	$ (103.9)

Geographic Data - Schedule Of G

Geographic Data - Schedule Of Geographic Information (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019
Segment Reporting Information [Line Items]
Long-lived assets	$ 995.7	$ 1,194.9
U.S. [Member]
Segment Reporting Information [Line Items]
Long-lived assets	985.8	1,182.5
Canada [Member]
Segment Reporting Information [Line Items]
Long-lived assets	$ 9.9	$ 12.4

Allowance for Doubtful Accoun_3

Allowance for Doubtful Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Allowance For Doubtful Accounts [Abstract]
Balance at beginning of Year	$ 13.1	$ 17.6	$ 11.8
Charged to Operations	19.2	9.5	11
Write-offs	(13.1)	(14)	(5.2)
Balance at end of year	$ 19.2	$ 13.1	$ 17.6

Fair Value Measurement - Additi

Fair Value Measurement - Additional Information (Detail) - Fair Value, Inputs, Level 2 [Member] $ in Millions	12 Months Ended
	Sep. 30, 2020USD ($)
2026 Senior Notes [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Debt instrument maturity year	2026
2025 Senior Notes [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Debt instrument maturity year	2025
Carrying Value [Member] | 2026 Senior Notes [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Senior notes	$ 300
Carrying Value [Member] | 2025 Senior Notes [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Senior notes	1,300
Fair Value [Member] | 2026 Senior Notes [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Senior notes	309
Fair Value [Member] | 2025 Senior Notes [Member]
Fair Value, Concentration of Risk, Financial Statement Captions [Line Items]
Senior notes	$ 1,280

Employee Benefit Plans - Additi

Employee Benefit Plans - Additional Information (Details) $ in Millions	12 Months Ended
	Sep. 30, 2020USD ($)	Sep. 30, 2019USD ($)	Sep. 30, 2018USD ($)
Defined Contribution Plan Service Period	90 days
Description of Defined Contribution Pension and Other Postretirement Plans	The Company currently matches the first 50% of participant contributions limited to 6% of a participant’s gross compensation (maximum Company match is 3%).
Defined Contribution Plan, Maximum Annual Contributions Per Employee, Percent	100.00%
Defined Contribution Plan Minimum Annual Contributions Per Employee Percent	1.00%
Defined First 50% of Participant Contribution Plan	50.00%
Defined Contribution Plan, Cost Recognized	$ 12.1	$ 11.7	$ 11.8
Pension Contributions	1.7	1	0.2
Multi-employer defined benefit plans aggregated expense	$ 2.5	$ 2.6	$ 1.8
Minimum [Member]
Defined Contribution Plan Qualifying Age	21
Maximum [Member]
Defined Contribution Plan, Employer Matching Contribution, Percent of Employees' Gross Pay	6.00%
Defined Contribution Plan, Employer Matching Contribution, Percent of Match	3.00%

Financial Derivatives - Additio

Financial Derivatives - Additional Information (Detail)	Sep. 11, 2019USD ($)Agreement	Sep. 30, 2020USD ($)
Derivative [Line Items]
Number of interest rate swap agreements | Agreement	2
5-Year Swap [Member]
Derivative [Line Items]
Derivative, notional amount	$ 250,000,000
Term of derivative agreement	5 years
Derivative, maturity date	Aug. 30, 2024
Derivative, fixed interest rate	1.49%
Derivative fair value, net of tax		$ 10,000,000
3-Year Swap [Member]
Derivative [Line Items]
Derivative, notional amount	$ 250,000,000
Term of derivative agreement	3 years
Derivative, maturity date	Aug. 30, 2022
Derivative, fixed interest rate	1.50%
Derivative fair value, net of tax		$ 5,000,000

Financial Derivatives - Summary

Financial Derivatives - Summary of Combined Fair Values, Net of Tax of Interest Rate Derivative Instruments (Details) - USD ($) $ in Millions		Sep. 30, 2020	Sep. 30, 2019
Designated as Hedging Instrument [Member] | Interest Rate Swaps [Member] | Fair Value, Inputs, Level 2 [Member]
Derivative [Line Items]
Derivative fair value, net of tax	[1]	$ (15)	$ (1.6)
[1]	Assets are included on the consolidated balance sheets in prepaid expenses and other current assets, while liabilities are included in accrued expenses.

Financial Derivatives - Summa_2

Financial Derivatives - Summary of Amounts of Gain (Loss) on Interest Rate Derivative Instruments Recognized in Other Comprehensive Income (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Derivative [Line Items]
Gain (loss) on interest rate derivative instruments recognized in other comprehensive income	$ (13.4)	$ (1.6)	$ 0
Designated as Hedging Instrument [Member] | Interest Rate Swaps [Member]
Derivative [Line Items]
Gain (loss) on interest rate derivative instruments recognized in other comprehensive income	$ (13.4)	$ (1.6)