<PAGE>
================================================================================

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

                            and THE BANK OF NEW YORK,
                                   as Trustee


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

                          FIFTH SUPPLEMENTAL INDENTURE

                         Dated as of September 22, 2004

                                     to the

                                    INDENTURE
                           Dated as of July 10, 1997,
              as supplemented by the First Supplemental Indenture,
                         dated as of February 14, 2001,
                       the Second Supplemental Indenture,
                         dated as of December 20, 2001,
                      and the Third Supplemental Indenture
                        dated as of February 27, 2004 and
                        the Fourth 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 NEW YORK,
                                   as Trustee

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


               12 1/2% Senior Secured Securities due July 15, 2007

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



<PAGE>



     FIFTH SUPPLEMENTAL INDENTURE, dated as of September 22, 2004 (the "Fifth
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 New York, as successor to Bank of Montreal Trust
Company as Trustee (the "Trustee").

     WHEREAS, the Issuer issued 12 1/2% Senior Secured Securities due July 15,
2007 (the "Securities") pursuant to the Indenture, dated as of July 10, 1997,
among Ainsworth Lumber Co. Ltd. (as Issuer), Ainsworth Lumber Inc. (as
Subsidiary Guarantor) and Bank of Montreal Trust Company (as predecessor
Trustee), as supplemented by the First Supplemental Indenture, dated as of
February 14, 2001, the Second Supplemental Indenture, dated as of December 20,
2001, the Third Supplemental Indenture dated as of February 27, 2004 and the
Fourth 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 Fifth Supplemental Indenture; and

     WHEREAS, all things necessary to make this Fifth 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:




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     (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, 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 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.

     (c) The obligations of the Guaranteeing Subsidiary under the Subsidiary
Guarantee are independent of the obligations guaranteed by the Guaranteeing
Subsidiary hereunder, and a separate action or actions may be brought and
prosecuted by the Trustee on behalf of, or by, the Holders, subject to the terms
and conditions set forth in the Indenture, against the Guaranteeing Subsidiary
to enforce the Subsidiary Guarantee, irrespective of whether any action is
brought against the Company or whether the Company is joined in any such action
or actions.

     (d) In the event of a default in payment of principal (or premium, if any)
or interest on a Security, whether at its stated maturity, by acceleration,
purchase or otherwise, legal proceedings may be instituted by the Trustee on
behalf of, or by, the Holder of such Security, subject to the terms and
conditions set forth in the Indenture, directly against the Guaranteeing
Subsidiary to enforce the Subsidiary Guarantee without first proceeding against
the Company or any other Guarantor. 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 Holder, upon demand therefor, the amount


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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 of
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
Indenture and this Supplemental Indenture and all demands whatsoever.

     (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) If any Holder or the Trustee is required by any court or otherwise to
return to the Company, the Subsidiary Guarantors (including the Guaranteeing
Subsidiary), or any custodian, trustee, liquidator or other similar official
acting in relation to either the Company or the Subsidiary Guarantors, any
amount paid either to the Trustee or such Holder, this Subsidiary Guarantee, to
the extent theretofore discharged, shall be reinstated in full force and effect.

     (h) 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.

     (i) As between the Guaranteeing Subsidiary, on the one hand, and the
Holders and the Trustee, on the other hand, (x) the maturity of the obligations
guaranteed hereby may be accelerated as provided in Article 5 of the Indenture
for the purposes of this Subsidiary Guarantee, notwithstanding any stay,
injunction or other prohibition preventing such acceleration in respect of the
obligations guaranteed hereby, and (y) in the event of any declaration of
acceleration of such obligations as provided in Article 5 of the Indenture, such
obligations (whether or not due and payable) shall forthwith become due and
payable by the Guaranteeing Subsidiary for the purpose of this Subsidiary
Guarantee.

     (j) The Guaranteeing Subsidiary shall have the right to seek contribution
from any non-paying Guarantor so long as the exercise of such right does not
impair the rights of the Holders under this Subsidiary Guarantee.


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     (k) Pursuant to Section 1201 of the Indenture, after giving effect to all
other contingent and fixed liabilities that are relevant under any applicable
Bankruptcy or fraudulent conveyance laws, and after giving effect to any
collections from, rights to receive contribution from or payments made by or on
behalf of any other Subsidiary Guarantor in respect of the obligations of such
other Subsidiary Guarantor under Article 12 of the Indenture, this new
Subsidiary Guarantee shall be limited to the maximum amount permissible such
that the obligations of such Guaranteeing Subsidiary under this Subsidiary
Guarantee will not constitute a fraudulent transfer or conveyance.

     (l) The 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, 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 obligee on the Securities and
Subsidiary Guarantee, 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.

     (m) 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.

     (n) 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.

     (o) 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.

     SECTION 3. Execution and Delivery. The Guaranteeing Subsidiary agrees that
the Subsidiary Guarantee shall remain in full force and effect 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


                                      S-4
<PAGE>

          (iii) all or substantially all of the assets of the Subsidiary
          Guarantor or all of the Capital Stock of the Subsidiary Guarantor 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,

     and, 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
Subsidiary Guarantor from its obligations under the Subsidiary Guarantee and
Article Twelve of the Indenture have been complied with, the Subsidiary
Guarantor 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
Subsidiary Guarantor) 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
Subsidiary Guarantor from its obligations under the Subsidiary Guarantee
endorsed on the Securities and under 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 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 Fifth 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 Fifth 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 Fifth 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,


                                      S-5
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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 Fifth 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
Fifth 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-6

<PAGE>



     IN WITNESS WHEREOF, the parties hereto have caused this Fifth 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
                                           -------------------------------------
                                           Name:  Catherine Ainsworth
                                           Title:  Secretary

                                    AINSWORTH ENGINEERED (USA), LLC,
                                    as Subsidiary Guarantor


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


                                    THE BANK OF NEW YORK,
                                    as Trustee


                                    By:    Patricia Phillips-Coward
                                           -------------------------------------
                                           Name:  Assistant Vice-President
                                           Title:  Patricia Phillips-Coward





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