<PAGE>




         SECOND SUPPLEMENTAL INDENTURE (this "Supplemental Indenture"), dated as
of September 22, 2004, among Ainsworth Engineered (USA), LLC (the "Guaranteeing
Subsidiary"), a subsidiary of Ainsworth Lumber Co. Ltd., a corporation
incorporated under the laws of the Province of British Columbia (the "Company"),
the Company, Ainsworth Engineered Corp., a subsidiary of the Company (the "Prior
Guarantor"), and The Bank of New York, as Trustee under the Indenture referred
to below (the "Trustee").

                               W I T N E S S E T H

         WHEREAS, the Company has heretofore executed and delivered to the
Trustee an indenture, dated as of March 3, 2004 providing for the issuance of an
unlimited aggregate principal amount of Senior Notes due March 15, 2014 (the
"Notes"), as supplemented by the First Supplemental Indenture, dated as of May
19, 2004 among the Prior Guarantor, the Company and the Trustee (as
supplemented, the "Indenture");

         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 Notes and
the Indenture on the terms and conditions set forth herein (the "Subsidiary
Guarantee"); and

         WHEREAS, pursuant to Section 9.01 of the Indenture, the Trustee is
authorized to execute and deliver this Supplemental Indenture.

         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 Notes as follows:

         1. CAPITALIZED TERMS. Capitalized terms used herein without definition
shall have the meanings assigned to them in the Indenture.

         2. AGREEMENT TO GUARANTEE. The Guaranteeing Subsidiary hereby agrees as
follows:

                (a) Along with all Guarantors named in the Indenture, to jointly
         and severally unconditionally guarantee to each Holder of a Note
         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 Notes or the obligations of the Company hereunder
         or thereunder, that:

                    (i) the principal of and interest, premium and Special
                Interest, if any, on the Notes will be promptly paid in full
                when due, whether at maturity, by acceleration, redemption or
                otherwise, and interest on the overdue principal of and interest
                on the Notes, if any, if lawful, and


<PAGE>


                all other obligations of the Company to the Holders or the
                Trustee hereunder or thereunder will be promptly paid in full or
                performed, all in accordance with the terms hereof and thereof;
                and

                    (ii) in case of any extension of time of payment or renewal
                of any Notes or any of such other obligations, that same will be
                promptly paid in full when due or performed in accordance with
                the terms of the extension or renewal, whether at stated
                maturity, by acceleration or otherwise. Failing payment when due
                of any amount so guaranteed or any performance so guaranteed for
                whatever reason, the Guarantors shall be jointly and severally
                obligated to pay the same immediately.

                (b) The obligations hereunder shall be unconditional,
         irrespective of the validity, regularity or enforceability of the Notes
         or the Indenture, the absence of any action to enforce the same, any
         waiver or consent by any Holder of the Notes with respect to any
         provisions hereof or thereof, the recovery of any judgment against the
         Company, any action to enforce the same or any other circumstance which
         might otherwise constitute a legal or equitable discharge or defense of
         a guarantor.

                (c) The obligations of the Guaranteeing Subsidiary under this
         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 this
         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 and Special Interest, if any) or interest on a Note, 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 Note, 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 Notes, to collect interest on the
         Notes, or to enforce or exercise any other right or remedy with respect
         to the Notes, the Guaranteeing Subsidiary shall pay to the Trustee for
         the account of the Holder, 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 of payment, filing of claims with a court in the event of
         insolvency or bankruptcy of


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<PAGE>

         the Company, any right to require a proceeding first against the
         Company, protest, notice and all demands whatsoever.

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

                (g) If any Holder or the Trustee is required by any court or
         otherwise to return to the Company, the Guarantors (including the
         Guaranteeing Subsidiary), or any custodian, trustee, liquidator or
         other similar official acting in relation to either the Company or the
         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 not be entitled to any
         right of subrogation in relation to the Holders in respect of any
         obligations guaranteed hereby until payment in full of all obligations
         guaranteed hereby.

                (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
         6 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 6 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.

                (k) Pursuant to Section 10.02 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
         Guarantor in respect of the obligations of such other Guarantor under
         Article 10 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) 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, reorganization, should the Company
         become insolvent or make an






                                       3
<PAGE>

         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 Notes are, pursuant to applicable law, rescinded
         or reduced in amount, or must otherwise be restored or returned by any
         obligee on the Notes 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
         Note 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.

         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 Note a notation of this Subsidiary Guarantee.

         4. GUARANTEEING SUBSIDIARY MAY CONSOLIDATE, ETC. ON CERTAIN TERMS.

                (a) Except as otherwise provided in Section 10.05 of the
         Indenture, the Guaranteeing Subsidiary may not sell or otherwise
         dispose of all or substantially all of its assets to, or consolidate,
         amalgamate with or merge with or into (whether or not such Guaranteeing
         Subsidiary is the surviving Person) another Person other than the
         Company or another Guarantor unless:

                (b) immediately after giving effect to that transaction, no
         Default or Event of Default exists; and

                (c) either:

                    (1) the Guaranteeing Subsidiary is the surviving Person, or
                the Person acquiring the property in any such sale or
                disposition or the Person formed by or surviving any such
                consolidation, amalgamation or merger assumes all the
                obligations of the Guaranteeing Subsidiary under the Indenture,
                this Subsidiary Guarantee and the Registration Rights Agreement
                pursuant to a supplemental indenture satisfactory to the Trustee
                and completes all other required documentation; or

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<PAGE>

                    (2) the Net Proceeds of such sale or other disposition are
                applied in accordance with the provisions of Section 4.10 of the
                Indenture;

                (d) In case of any such consolidation, amalgamation, merger,
         sale or conveyance and upon the assumption by the successor Person
         (where applicable), by supplemental indenture, executed and delivered
         to the Trustee and satisfactory in form to the Trustee, of this
         Subsidiary Guarantee endorsed upon the Notes and the due and punctual
         performance of all of the covenants and conditions of the Indenture to
         be performed by the Guaranteeing Subsidiary, such successor Person
         shall succeed to and be substituted for the Guaranteeing Subsidiary
         with the same effect as if it had been named herein as a Guaranteeing
         Subsidiary. Such successor Person thereupon may cause to be signed the
         Subsidiary Guarantee to be endorsed upon all of the Notes issuable
         hereunder which theretofore shall not have been signed by the Company
         and delivered to the Trustee. The Subsidiary Guarantee so issued shall
         in all respects have the same legal rank and benefit under the
         Indenture as the Subsidiary Guarantees theretofore and thereafter
         issued in accordance with the terms of the Indenture as though all of
         such Subsidiary Guarantees had been issued at the date of the execution
         hereof.

                (e) Except as set forth in Articles 4 and 5 of the Indenture,
         and notwithstanding clauses (a) and (b) above, nothing contained in the
         Indenture or in any of the Notes shall prevent any consolidation,
         amalgamation or merger of a Guaranteeing Subsidiary with or into the
         Company or another Guarantor, or shall prevent any sale or conveyance
         of the property of the Guaranteeing Subsidiary as an entirety or
         substantially as an entirety to the Company or another Guarantor.

         5. RELEASES.

                (a) In the event of a sale (including by way of merger,
         amalgamation or consolidation in compliance with Section 5.01 of the
         Indenture) of all the capital stock of the Guaranteeing Subsidiary to a
         Person that is not (either before or after giving effect to such
         transaction) the Company or a Restricted Subsidiary then the
         Guaranteeing Subsidiary (in the event of a sale or other disposition,
         by way of merger, consolidation or otherwise, of all of the capital
         stock of such Guaranteeing Subsidiary) will be released and relieved of
         any obligations under this Subsidiary Guarantee; provided, that the
         sale complies with Section 4.10 of the Indenture. Further, if the
         Company redesignates any Restricted Subsidiary that is a Guarantor as
         an Unrestricted Subsidiary in accordance with Section 4.17 of the
         Indenture then the Guaranteeing Subsidiary will be released and
         relieved of any obligations under this Subsidiary Guarantee. Upon
         delivery by the Company to the Trustee of an Officers' Certificate and
         an Opinion of Counsel to the effect that such sale or other disposition
         was made by the Company in accordance with the provisions of the
         Indenture, including without limitation Section 4.10 of the Indenture,
         the Trustee shall execute any documents reasonably required in order to
         evidence the release of the Guaranteeing Subsidiary from its
         obligations under this Subsidiary Guarantee.

                                       5
<PAGE>

                (b) The Guaranteeing Subsidiary not released from its
         obligations under this Subsidiary Guarantee shall remain liable for the
         full amount of principal of and interest on the Notes and for the other
         obligations of the Guaranteeing Subsidiary under the Indenture as
         provided in Article 10 thereof.

         6. 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 Notes, 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 Notes by
accepting a Note waives and releases all such liability. The waiver and release
are part of the consideration for the issuance of the Notes. 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.

         7. THE LAWS OF THE STATE OF NEW YORK SHALL GOVERN AND BE USED TO
CONSTRUE THIS SUPPLEMENTAL INDENTURE, THE INDENTURE, THE NOTES AND THE
SUBSIDIARY GUARANTEES.

         8. COUNTERPARTS. The parties may sign any number of copies of this
Supplemental Indenture. Each signed copy shall be an original, but all of them
together represent the same agreement.

         9. EFFECT OF HEADINGS. The Section headings herein are for convenience
only and shall not affect the construction hereof.

         10. THE TRUSTEE. The Trustee shall not be responsible in any manner
whatsoever for or in respect of the validity or sufficiency of this Supplemental
Indenture or for or in respect of the recitals contained herein, all of which
recitals are made solely by the Guaranteeing Subsidiary and the Company.

         11. SUBROGATION. The Guaranteeing Subsidiary shall be subrogated to all
rights of Holders of Notes against the Company in respect of any amounts paid by
the Guaranteeing Subsidiary pursuant to the provisions of Section 2 hereof;
provided, however, that, if an Event of Default has occurred and is continuing,
the Guaranteeing Subsidiary shall not be entitled to enforce or receive any
payments arising out of, or based upon, such right of subrogation until all
amounts then due and payable by the Company under the Indenture or the Notes
shall have been paid in full.

         12. BENEFITS ACKNOWLEDGED. The Guaranteeing Subsidiary acknowledges
that it will receive direct and indirect benefits from the financing
arrangements contemplated by the Indenture and this Supplemental Indenture and
that the guarantee and waivers made by it pursuant to this Subsidiary Guarantee
are knowingly made in contemplation of such benefits.

         13. SUCCESSORS. All agreements of the Guaranteeing Subsidiary in this
Supplemental Indenture shall bind its Successors, except as otherwise provided
in Section



                                       6
<PAGE>

2(k) hereof or elsewhere in this Supplemental Indenture. All agreements of the
Trustee in this Supplemental Indenture shall bind its successors.



                                       7
<PAGE>


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



                                     Ainsworth Lumber Co. Ltd.



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


                                     By:     /s/ Michael Ainsworth
                                             ---------------------------------
                                            Name:  Michael Ainsworth
                                            Title:  Executive Vice-President


                                     Ainsworth Engineered Corp.



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


                                     Ainsworth Engineered (USA), LLC



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


                                     The Bank of New York, as Trustee



                                     By:    /s/ Patricia Phillips-Coward
                                            ----------------------------------
                                            Authorized Signatory





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