EXHIBIT (a) (6)
 
                          OFFER TO PURCHASE FOR CASH
                    ALL OUTSTANDING SHARES OF COMMON STOCK
 
                                      AND
 
    ALL OUTSTANDING SHARES OF $1.85 CUMULATIVE CONVERTIBLE PREFERENCE STOCK
 
                                      OF
 
                               BIRD CORPORATION
 
                                      AT
 
                      $5.50 NET PER SHARE OF COMMON STOCK
 
                                      AND
 
 $20 (WHICH AMOUNT SHALL NOT BE ADJUSTED FOR ANY DIVIDENDS ACCRUED AND UNPAID
  THROUGH THE EXPIRATION DATE) NET PER SHARE OF $1.85 CUMULATIVE CONVERTIBLE
                               PREFERENCE STOCK
 
                                      BY
 
                             BI EXPANSION II CORP.
                         A Wholly Owned Subsidiary of
                            CERTAINTEED CORPORATION
                    An Indirect Wholly Owned Subsidiary of
                           COMPAGNIE DE SAINT-GOBAIN
 
     THE OFFER AND WITHDRAWAL RIGHTS WILL EXPIRE AT 12:00 MIDNIGHT,
       NEW YORK CITY TIME, ON FRIDAY, FEBRUARY 13, 1998 UNLESS THE
                           OFFER IS EXTENDED.
 
To Our Clients:
 
  Enclosed for your consideration is an Offer to Purchase dated January 16,
1998 (the "Offer to Purchase"), and the related Letter of Transmittal (which,
together with any amendments or supplements thereto, collectively constitute
the "Offer") relating to the offer by BI Expansion II Corp., a Massachusetts
corporation (the "Purchaser") and a wholly owned subsidiary of CertainTeed
Corporation, a Delaware corporation ("CertainTeed") which is an indirect
wholly owned subsidiary of Compagnie de Saint-Gobain, a French corporation
("Saint-Gobain"), to purchase for cash all outstanding shares of Common Stock,
par value $1.00 per share (the "Common Shares"), of Bird Corporation, a
Massachusetts corporation (the "Company"), and all outstanding shares of $1.85
Cumulative Convertible Preference Stock, par value $1.00 per share (the
"Preference Shares"), of the Company, upon the terms and subject to the
conditions set forth in the Offer. The Common Shares and the Preference Shares
are collectively referred to as "Shares". Also enclosed is the Letter to
Stockholders of the Company from the President of the Company accompanied by
the Company's Solicitation/Recommendation Statement on Schedule 14D-9.
 
  WE ARE THE HOLDER OF RECORD OF SHARES HELD BY US FOR YOUR ACCOUNT. A TENDER
OF SUCH SHARES CAN BE MADE ONLY BY US AS THE HOLDER OF RECORD AND PURSUANT TO
YOUR INSTRUCTIONS. THE LETTER OF TRANSMITTAL IS FURNISHED TO YOU FOR YOUR
INFORMATION ONLY AND CANNOT BE USED TO TENDER SHARES HELD BY US FOR YOUR
ACCOUNT.
 
  We request instructions as to whether you wish to tender any of or all the
Shares held by us for your account pursuant to the terms and conditions set
forth in the Offer.

 
  Your attention is directed to the following:
 
    1. The offer price is $5.50 per Common Share and $20, which amount shall
  not be adjusted for any dividends accrued and unpaid through the Expiration
  Date (as defined below), per Preference Share, net to the seller in cash,
  without interest thereon, upon the terms and subject to the conditions of
  the Offer.
 
    2. The Offer is being made for all outstanding Common Shares and
  Preference Shares.
 
    3. The Board of Directors of the Company has unanimously approved the
  Offer and the Merger (as defined below) and determined that the terms of
  the Offer and the Merger are fair to, and in the best interests of, the
  stockholders of the Company and unanimously recommends that the
  stockholders of the Company accept the Offer and tender their Common Shares
  and Preference Shares.
 
    4. The Offer is being made pursuant to the Agreement and Plan of Merger
  dated as of January 12, 1998 (the "Merger Agreement"), among CertainTeed,
  the Purchaser and the Company pursuant to which, following the consummation
  of the Offer and the satisfaction or waiver of certain conditions, the
  Purchaser will be merged with and into the Company, with the Company
  surviving the merger as a wholly owned subsidiary of CertainTeed (the
  "Merger"). In the Merger, each outstanding Share (other than Shares held by
  stockholders who perfect their appraisal rights under Massachusetts law,
  Shares held in the Company's treasury and Shares held directly by the
  Purchaser or CertainTeed) will be converted into the right to receive in
  the case of Common Shares $5.50 per Common Share and in the case of
  Preference Shares $20, which amount shall not be adjusted for any dividends
  accrued and unpaid through the effective date of the Merger, per Preference
  Share, in each case net to the seller in cash, without interest thereon, as
  set forth in the Merger Agreement and described in the Offer to Purchase.
 
    5. THE OFFER AND WITHDRAWAL RIGHTS EXPIRE AT 12:00 MIDNIGHT, NEW YORK
  CITY TIME, ON FRIDAY, FEBRUARY 13, 1998 (THE "EXPIRATION DATE"), UNLESS THE
  OFFER IS EXTENDED BY THE PURCHASER, IN WHICH EVENT THE TERM "EXPIRATION
  DATE" SHALL MEAN THE LATEST TIME AT WHICH THE OFFER, AS SO EXTENDED BY THE
  PURCHASER, WILL EXPIRE.
 
    6. The Offer is conditioned upon, among other things, (1) there being
  validly tendered and not withdrawn prior to the Expiration Date such number
  of Common Shares that would constitute at least 66 2/3% of all outstanding
  Common Shares (determined on a fully diluted basis), (2) there being
  validly tendered and not withdrawn prior to the Expiration Date such number
  of Preference Shares that would constitute at least 66 2/3% of all
  outstanding Preference Shares, (3) any waiting period under the Hart-Scott-
  Rodino Antitrust Improvements Act of 1976, as amended, and the regulations
  thereunder applicable to the purchase of Shares pursuant to the Offer
  having expired or been terminated and (4) all consents, approvals, orders
  or authorizations of, or registrations, declarations or filings with any
  governmental authority required or necessary in connection with the Offer,
  the Merger and the Merger Agreement and the transactions contemplated by
  the Merger Agreement having been obtained and being in full force and
  effect.
 
    7. Any stock transfer taxes applicable to a sale of Shares to the
  Purchaser will be borne by the Purchaser, except as otherwise provided in
  Instruction 6 of the Letter of Transmittal.
 
  Your instructions to us should be forwarded promptly to permit us to submit
a tender on your behalf prior to the Expiration Date.
 
  If you wish to have us tender any of or all the Shares held by us for your
account, please so instruct us by completing, executing, detaching and
returning to us the instruction form on the detachable part hereof. An
envelope to return your instructions to us is enclosed. If you authorize the
tender of your Shares, all such Shares will be tendered unless otherwise
specified on the detachable part hereof. Your instructions should be forwarded
to us in ample time to permit us to submit a tender on your behalf prior to
the Expiration Date.
 
  Payment for Shares accepted for payment pursuant to the Offer will in all
cases be made only after timely receipt by ChaseMellon Shareholder Services,
L.L.C. (the "Depositary") of (a) certificates for (or a timely Book-Entry
Confirmation (as defined in the Offer to Purchase) with respect to) such
Shares, (b) a Letter of Transmittal (or fax thereof), properly completed and
duly executed, with any required signature guarantees, or,
 
                                       2

 
in the case of a book-entry transfer effected pursuant to the procedure set
forth in Section 2 of the Offer to Purchase, an Agent's Message, and (c) any
other documents required by the Letter of Transmittal. Accordingly, tendering
stockholders may be paid at different times depending upon when certificates
for Shares or Book-Entry Confirmations with respect to Shares are actually
received by the Depositary. UNDER NO CIRCUMSTANCES WILL INTEREST BE PAID ON
THE PURCHASE PRICE OF THE SHARES TO BE PAID BY THE PURCHASER, REGARDLESS OF
ANY EXTENSION OF THE OFFER OR ANY DELAY IN MAKING SUCH PAYMENT.
 
  The Offer is not being made to, nor will tenders be accepted from, or on
behalf of, holders of Shares in any jurisdiction in which the making or
acceptance of the Offer would not be in compliance with the laws of such
jurisdiction. None of the Purchaser, CertainTeed or Saint-Gobain is aware of
any jurisdiction in which the making of the Offer or the acceptance thereof
would not be in compliance with the laws of such jurisdiction. To the extent
the Purchaser, CertainTeed or Saint-Gobain becomes aware of any state law that
would limit the class of offerees in the Offer, the Purchaser will amend the
Offer and, depending on the timing of such amendment, if any, will extend the
Offer to provide adequate dissemination of such information to holders of
Shares prior to the expiration of the Offer. In any jurisdiction where the
securities, blue sky or other laws require the Offer to be made by a licensed
broker or dealer, the Offer is being made on behalf of the Purchaser by
McFarland Dewey Securities Co., L.P., the Dealer Manager for the Offer, or one
or more registered brokers or dealers that are licensed under the laws of such
jurisdiction.
 
                                       3

 
                         INSTRUCTIONS WITH RESPECT TO
                        THE OFFER TO PURCHASE FOR CASH
                    ALL OUTSTANDING SHARES OF COMMON STOCK
 
                                      AND
 
    ALL OUTSTANDING SHARES OF $1.85 CUMULATIVE CONVERTIBLE PREFERENCE STOCK
 
                                      OF
 
                               BIRD CORPORATION
 
  The undersigned acknowledge(s) receipt of your letter, the Offer to Purchase
of BI Expansion II Corp. dated January 16, 1998 (the "Offer to Purchase"), and
the related Letter of Transmittal relating to shares of Common Stock, par
value $1 per share (the "Common Shares"), and $1.85 Cumulative Convertible
Preference Stock, par value $1 per share (the "Preference Shares"), of Bird
Corporation, a Massachusetts corporation. The Common Shares and the Preference
Shares are collectively referred to as the "Shares".
 
  This will instruct you to tender the number of Shares indicated below held
by you for the account of the undersigned, on the terms and subject to the
conditions set forth in the Offer to Purchase and related Letter of
Transmittal.
 
  Number of Shares to be Tendered:*                     SIGN HERE
 _____________________ Common Shares      _____________________________________
 _________________ Preference Shares      _____________________________________
                                                      SIGNATURE(S)
 
                                          _____________________________________
Daytime Area Code and Tel. No. ______     _____________________________________
 
                                                (PLEASE PRINT NAME(S) AND
Taxpayer Identification No. or                        ADDRESS(ES))
Social Security No. _________________
- --------
Dated: _______________________ , 1998
* Unless otherwise indicated, it will be assumed that all your Shares are to
 be tendered.
 
                                       4