<PAGE>
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                           AINSWORTH LUMBER CO. LTD.,
                                    as Issuer
                                       and
                           AINSWORTH ENGINEERED CORP.
                             as Subsidiary Guarantor

             and THE BANK OF NOVA SCOTIA TRUST COMPANY OF NEW YORK,
                                   as Trustee


                             ---------------------

                          THIRD SUPPLEMENTAL INDENTURE

                         Dated as of September 22, 2004

                                     to the

                                    INDENTURE
                         Dated as of December 20, 2001,
              as supplemented by the First Supplemental Indenture,
                          dated as of February 27, 2004
                      and the Second Supplemental Indenture
                            dated as of May 19, 2004

                                     between

                           AINSWORTH LUMBER CO. LTD.,
                                   as Issuer,

                           AINSWORTH ENGINEERED CORP.
                             as Subsidiary Guarantor

                                       and

               THE BANK OF NOVA SCOTIA TRUST COMPANY OF NEW YORK,
                                   as Trustee

                             ---------------------


               13.875% Senior Secured Securities due July 15, 2007

================================================================================



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     THIRD SUPPLEMENTAL INDENTURE, dated as of September 22, 2004 (the "Third
Supplemental Indenture"), by and among Ainsworth Lumber Co. Ltd., a British
Columbia corporation (the "Issuer"), Ainsworth Engineered (USA), LLC, a Delaware
Limited Liability Corporation (the "Guaranteeing Subsidiary"), Ainsworth
Engineered Corp., a Nova Scotia Unlimited Liability Company (the "Prior
Guarantor") and The Bank of Nova Scotia Trust Company of New York, as Trustee
(the "Trustee").

     WHEREAS, the Issuer issued 13.875% Senior Secured Securities due July 15,
2007 (the "Securities") pursuant to the Indenture, dated as of December 20,
2001, among Ainsworth Lumber Co. Ltd. (as Issuer), Ainsworth Lumber Inc. (as
Subsidiary Guarantor) and The Bank of Nova Scotia Trust Company of New York (as
Trustee), as supplemented by the First Supplemental Indenture, dated as of
February 27, 2004 and the Second Supplemental Indenture, dated as of May 19,
2004 (as supplemented, the "Indenture"), which Securities have previously been
guaranteed by the Prior Guarantor pursuant to the provisions of the Indenture;

     WHEREAS, Steen River Forest Products Ltd., a former Guarantor under the
Indenture, was formally dissolved, effective March 3, 2004, and is no longer in
existence;

     WHEREAS, the Indenture provides that under certain circumstances a
Subsidiary providing a Subsidiary Guarantee shall execute and deliver to the
Trustee a supplemental indenture pursuant to which such Subsidiary shall
unconditionally guarantee all of the Company's Obligations under the Securities
and the Indenture (the "Subsidiary Guarantee");

     WHEREAS, pursuant to Section 901 of the Indenture, the Trustee is
authorized to execute and deliver this Third Supplemental Indenture; and

     WHEREAS, all things necessary to make this Third Supplemental Indenture a
valid supplement to the Indenture have been done.

     NOW THEREFORE, in consideration of the foregoing and for other good and
valuable consideration, the receipt of which is hereby acknowledged, the parties
mutually covenant and agree for the equal and ratable benefit of the Holders of
the Securities as follows:

     SECTION 1. Certain Definitions. Unless otherwise stated, all capitalized
terms used but not defined herein shall have the meanings ascribed thereto in
the Indenture.

     SECTION 2. Agreement to Guarantee. The Guaranteeing Subsidiary hereby
agrees as follows:


     (a) Along with all Subsidiary Guarantors named in the Indenture, to jointly
and severally unconditionally guarantee to each Holder of a Security
authenticated and delivered by the Trustee and to the Trustee and its successors
and assigns,


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irrespective of the validity and enforceability of the Indenture,
the Securities or the obligations of the Company hereunder or thereunder, that:

          (i) the principal of and interest and premium, if any, on the
          Securities will be promptly paid in full when due, whether at
          maturity, by acceleration, redemption or otherwise, according to the
          terms thereof and of the Indenture; and

          (ii) in case of failure by the Company to punctually pay the principal
          of and interest and premium, if any, on the Securities when due,
          whether at stated maturity, by acceleration or otherwise, the
          Subsidiary Guarantors shall be jointly and severally obligated to pay
          the same immediately.

     (b) The Guaranteeing Subsidiary will be subject to all the provisions of
the Indenture applicable to a Subsidiary Guarantor.

     (c) The obligations hereunder shall be absolute and unconditional,
irrespective of and shall be unaffected by the validity, regularity or
enforceability of the Securities or the Indenture, the absence of any action to
enforce the same, any release, amendment, waiver or indulgence granted to the
Company or any other guarantor, or any consent to departure from any requirement
of any other guarantee of all or of any of the Securities, or any other
circumstances which might otherwise constitute a legal or equitable discharge or
defense of a surety or guarantor; provided, however, that, notwithstanding the
foregoing, no such release, amendment, waiver or indulgence shall, without the
consent of the Guaranteeing Subsidiary, increase the principal amount of the
Securities, or increase the interest rate thereon, or alter the Stated Maturity
thereof.

     (d) If, after the occurrence and during the continuance of an Event of
Default, the Trustee or any of the Holders are prevented by applicable law from
exercising their respective rights to accelerate the maturity of the Securities,
to collect interest on the Securities, or to enforce or exercise any other right
or remedy with respect to the Securities, the Guaranteeing Subsidiary shall pay
to the Trustee for the account of the Holders, upon demand therefor, the amount
that would otherwise have been due and payable had such rights and remedies been
permitted to be exercised by the Trustee or any of the Holders.

     (e) The following is hereby waived: diligence, presentment, demand for
payment, any requirement that the Trustee or any of the Holders protect, secure,
perfect or insure any security interest in or other Lien on any property subject
thereto or exhaust any right or take any action against the Company or any other
Person or any collateral, filing of claims with a court in the event of
insolvency or bankruptcy of the Company, any right to require a proceeding first
against the Company, protest or notice with respect to the Securities, the
indebtedness evidenced thereby, the Indenture and this Supplemental Indenture
and all demands whatsoever.



                                      S-2
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     (f) This Subsidiary Guarantee shall not be discharged except by complete
performance of the obligations contained in the Securities, the Indenture and
this Supplemental Indenture, and the Guaranteeing Subsidiary accepts all
obligations of a Subsidiary Guarantor under the Indenture.

     (g) The Guaranteeing Subsidiary shall be subrogated to all rights of the
Holders of the Securities against the Company in respect of any amounts paid by
the Guaranteeing Subsidiary on account of the Securities pursuant to the
provisions of the Subsidiary Guarantee or the Indenture, as supplemented by this
Supplemental Indenture; provided, however, that the Guaranteeing Subsidiary
shall not be entitled to enforce or to receive any payments arising out of, or
based upon, such right of subrogation until the principal of (and premium, if
any) and interest on the Securities shall have been paid in full.

     (h) The Guaranteeing Subsidiary shall, to the extent that it makes or is
required to make any payment in respect of this Subsidiary Guarantee, be
entitled to seek contribution from the other Subsidiary Guarantors to the extent
permitted by applicable law; provided, however, that the Guaranteeing Subsidiary
shall not be entitled to enforce or receive any payments arising out of, or
based upon, such right of contribution until the principal of (and premium, if
any) and interest on all Securities issued under the Indenture shall have been
paid in full.

     (i) This Subsidiary Guarantee shall remain in full force and effect and
continue to be effective should any petition be filed by or against the Company
for liquidation or reorganization, should the Company become insolvent or make
an assignment for the benefit of creditors or should a receiver or trustee be
appointed for all or any significant part of the Company's assets, and shall, to
the fullest extent permitted by law, continue to be effective or be reinstated,
as the case may be, if at any time payment and performance of the Securities
are, pursuant to applicable law, rescinded or reduced in amount, or must
otherwise be restored or returned by any Holder of the Securities, whether as a
"voidable preference", "fraudulent transfer" or otherwise, all as though such
payment or performance had not been made. In the event that any payment or any
part thereof, is rescinded, reduced, restored or returned, the Security shall,
to the fullest extent permitted by law, be reinstated and deemed reduced only by
such amount paid and not so rescinded, reduced, restored or returned.

     (j) In case any provision of this Subsidiary Guarantee shall be invalid,
illegal or unenforceable, the validity, legality, and enforceability of the
remaining provisions shall not in any way be affected or impaired thereby.

     (k) This Subsidiary Guarantee shall be a general unsecured obligation of
such Guaranteeing Subsidiary, ranking pari passu with any other future senior
Indebtedness of the Guaranteeing Subsidiary, if any.

     (l) Each payment to be made by the Guaranteeing Subsidiary in respect of
this Subsidiary Guarantee shall be made without set-off, counterclaim, reduction
or diminution of any kind or nature.



                                      S-3
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     SECTION 3. Execution and Delivery. The Guaranteeing Subsidiary agrees that,
subject to Section 1203 of the Indenture, this Subsidiary Guarantee shall remain
in full force and effect until the entire principal of, premium, if any, and
interest on the Securities shall have been paid in full or otherwise discharged
in accordance with the provisions of the Securities and the Indenture and all
amounts owing to the Trustee under the Indenture have been paid, notwithstanding
any failure to endorse on each Security a notation of this Subsidiary Guarantee.

     SECTION 4. Releases. Notwithstanding Section 3 above, if:


          (i) the Subsidiary Guarantor ceases to be a Restricted Subsidiary in
          compliance with the applicable provisions of the Indenture;

          (ii) the Securities are defeased and discharged pursuant to Section
          1502 of the Indenture; or

          (iii) all or substantially all of the assets of the Guaranteeing
          Subsidiary or all of the Capital Stock of the Guaranteeing Subsidiary
          are sold (including by issuance, amalgamation, merger, consolidation
          or otherwise) by the Company or any Restricted Subsidiary in a
          transaction constituting an Asset Disposition and in which the Net
          Available Proceeds from such Assets Disposition are applied in
          accordance with requirements of Section 1013 of the Indenture,

     then, in each case of (i), (ii) or (iii), upon delivery by the Company of
an Officers' Certificate and an Opinion of Counsel stating that all conditions
precedent contained in the Indenture provided for relating to the release of the
Guaranteeing Subsidiary from its obligations under the Subsidiary Guarantee and
Article Twelve of the Indenture have been complied with, the Guaranteeing
Subsidiary or the Person acquiring such assets (in the event of a sale or other
disposition of all or substantially all of the assets or Capital Stock of such
Guaranteeing Subsidiary) shall be released and discharged of its obligations
under the Subsidiary Guarantee and under Article Twelve of the Indenture without
any action on the part of the Trustee or any Holder, and the Trustee shall
execute any documents reasonably required in order to acknowledge the release of
the Guaranteeing Subsidiary from its obligations under this Subsidiary Guarantee
endorsed on the Securities and under this Subsidiary Guarantee and Article
Twelve of the Indenture.

     SECTION 5. No Recourse Against Others. No past, present or future director,
officer, employee, incorporator, shareholder or agent of the Guaranteeing
Subsidiary, as such, shall have any liability for any obligations of the Company
or any Subsidiary under the Securities, any Subsidiary Guarantees, the Indenture
or this Supplemental Indenture or for any claim based on, in respect of, or by
reason of, such obligations or their creation. Each Holder of the Securities by
accepting a Security waives and releases all such liability. The waiver and
release are part of the consideration for the issuance of the Securities. Such
waiver may not be effective to


                                      S-4
<PAGE>

waive liabilities under the federal securities laws and it is the view of the
Commission that such a waiver is against public policy.

     SECTION 6. Governing Law. This Third Supplemental Indenture shall be
governed by and construed in accordance with the laws of the State of New York,
as applied to contracts made and performed within the State of New York.

     SECTION 7. Counterparts. This Third Supplemental Indenture may be signed in
any number of counterparts, each of which shall be an original, with the same
effect as if the signatures thereto and hereto were upon the same instrument.

     SECTION 8. Severability. In case any provision in this Third Supplemental
Indenture shall be invalid, illegal or unenforceable, the validity, legality and
enforceability of the remaining provisions shall not in any way be affected or
impaired thereby.

     SECTION 9. Ratification. Except as expressly amended hereby, each provision
of the Indenture shall remain in full force and effect and, as amended hereby,
the Indenture is in all respects agreed to, ratified and confirmed by each of
the Issuer, the Prior Guarantor, the Guaranteeing Subsidiary and the Trustee.

     SECTION 10. Trustee Makes No Representation. The Trustee makes no
representation as to the validity or sufficiency of this Third Supplemental
Indenture. The statements and recitals herein are deemed to be those of the
Issuer and the Guaranteeing Subsidiary and not of the Trustee.

     SECTION 11. Successors and Assigns. All covenants and agreements in this
Third Supplemental Indenture by the Issuer, the Prior Guarantor, the
Guaranteeing Subsidiary, the Trustee and the Holders shall bind their respective
successors and assigns, whether so expressed or not.

     SECTION 12. Effect of Headings. The Section headings herein are for
convenience only and shall not effect the construction hereof.


                                      S-5
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     IN WITNESS WHEREOF, the parties hereto have caused this Third Supplemental
Indenture to be duly executed as of the date first above written.


                                    AINSWORTH LUMBER CO. LTD.,
                                    as Issuer


                                    By:    /s/ Catherine Ainsworth
                                           -------------------------------------
                                           Catherine Ainsworth
                                           Chief Operating Officer


                                    AINSWORTH ENGINEERED CORP.,
                                    as Subsidiary Guarantor


                                    By:    /s/ Catherine Ainsworth
                                           -------------------------------------
                                           Catherine Ainsworth
                                           Secretary


                                    AINSWORTH ENGINEERED (USA), LLC
                                    as Subsidiary Guarantor


                                    By:    /s/ Catherine Ainsworth
                                           -------------------------------------
                                           Catherine Ainsworth
                                           Secretary


                                    THE BANK OF NOVA SCOTIA TRUST
                                    COMPANY OF NEW YORK,
                                    as Trustee


                                    By:
                                           -------------------------------------
                                           Name:  Warren Goshine
                                           Title:  Vice President




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