| Cover | Mar. 29, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Mar. 29, 2021 |
| Entity File Number | 001-36873 |
| Entity Registrant Name | Summit Materials, Inc. |
| Entity Central Index Key | 0001621563 |
| Entity Tax Identification Number | 47-1984212 |
| Entity Incorporation, State or Country Code | DE |
| Entity Address, Address Line One | 1550 Wynkoop Street |
| Entity Address, Address Line Two | 3rd Floor |
| Entity Address, City or Town | Denver |
| Entity Address, State or Province | CO |
| Entity Address, Postal Zip Code | 80202 |
| City Area Code | 303 |
| Local Phone Number | 893-0012 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Class A Common Stock (par value, $0.01 per share) |
| Trading Symbol | SUM |
| Security Exchange Name | NYSE |
| Entity Emerging Growth Company | false |
