Exhibit 4.12

Execution Version

ELEVENTH Supplemental Indenture

Eleventh Supplemental Indenture (this “Supplemental Indenture”), dated as of November 9, 2017, among Columbia Silica Sand, LLC, a South Carolina limited liability company, Columbia Aggregates, LLC, a South Carolina limited liability company, Northwest Aggregates, Inc., a Colorado corporation, and Northwest Ready Mix, Inc., a Colorado corporation (each, a “Guaranteeing Subsidiary”), each an indirect subsidiary of Summit Materials, LLC, a Delaware limited liability company (the “Issuer”), and Wilmington Trust, National Association, a national banking association, as trustee (the “Trustee”), Transfer Agent, Registrar and Paying Agent.

W I T N E S S E T H

WHEREAS, the Issuer, Summit Materials Finance Corp., a Delaware corporation (together with the Issuer, the “Issuers”), and the Guarantors have heretofore executed and delivered to the Trustee an Indenture (the “Indenture”), dated as of July 8, 2015, providing for the issuance of 6.125% Senior Notes due 2023 (the “Notes”), as supplemented by that First Supplemental Indenture, dated as of July 17, 2015, as further supplemented by that Second Supplemental Indenture, dated as of October 7, 2015, as further supplemented by that Third Supplemental Indenture, dated as of November 19, 2015, as further supplemented by that Fourth Supplemental Indenture, dated as of February 3, 2016, as further supplemented by that Fifth Supplemental Indenture, dated as of April 5, 2016, as further supplemented by that Sixth Supplemental Indenture, dated as of May 25, 2016, as further supplemented by that Seventh Supplemental Indenture, dated as of September 23, 2016, as further supplemented by that Eight Supplemental Indenture, dated as of March 30, 2017,as further supplemented by that Ninth Supplemental Indenture, dated as of June 21, 2017,and as further supplemented by that Tenth Supplemental Indenture, dated as of September 6, 2017;

WHEREAS, the Indenture provides that under certain circumstances a Guaranteeing Subsidiary shall execute and deliver to the Trustee a supplemental indenture pursuant to which such Guaranteeing Subsidiary shall unconditionally guarantee all of the Issuers’ Obligations under the Notes and the Indenture on the terms and conditions set forth herein and under the Indenture (each, a “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 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.  Each Guaranteeing Subsidiary acknowledges that it has received and reviewed a copy of the Indenture and all other documents it deems necessary to review in order to enter into this Supplemental Indenture, and acknowledges and agrees to (i) join and become a party to the Indenture as indicated by its signature below; (ii) be bound by the Indenture, as of the date hereof, as if made by, and with respect to, each signatory hereto; and (iii) perform all obligations and duties required of a Guarantor pursuant to the Indenture.  Each Guaranteeing Subsidiary hereby agrees to provide an unconditional Guarantee on the terms and subject to the conditions set forth in the Indenture, including, but not limited to, Article 10 thereof.

(3)Execution and Delivery.  Each Guaranteeing Subsidiary agrees that its Guarantee shall remain in full force and effect notwithstanding the absence of the endorsement of any notation of such Guarantee on the Notes.

(4)No Recourse Against Others.  No past, present or future director, officer, employee, incorporator, member, partner or stockholder of the Issuers or any Guaranteeing Subsidiary shall have any liability for any obligations of the Issuers or the Guarantors (including any Guaranteeing Subsidiary) under the Notes, any 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 by accepting Notes waives and releases all such liability.  The waiver and release are part of the consideration for issuance of the Notes.

(5)Governing Law.  THIS SUPPLEMENTAL INDENTURE, AND ANY CLAIM, CONTROVERSY OR DISPUTE ARISING UNDER OR RELATED TO THIS SUPPLEMENTAL INDENTURE, WILL BE GOVERNED BY, AND CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK.

(6)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.  This Supplemental Indenture may be executed in multiple counterparts which, when taken together, shall constitute one instrument.  The exchange of copies of this Supplemental Indenture and of signature pages by facsimile or PDF transmissions shall constitute effective execution and delivery of this Supplemental Indenture as to the parties hereto and may be used in lieu of the original Supplemental Indenture for all purposes.  Signatures of the parties hereto transmitted by facsimile or PDF shall be deemed to be their original signatures for all purposes.

(7)Effect of Headings.  The Section headings herein are for convenience only and shall not affect the construction hereof.

(8)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 each Guaranteeing Subsidiary.

(9)Benefits Acknowledged.  Each Guaranteeing Subsidiary’s Guarantee is subject to the terms and conditions set forth in the Indenture.  Each 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 its Guarantee are knowingly made in contemplation of such benefits.

(10)Successors.  All agreements of each Guaranteeing Subsidiary in this Supplemental Indenture shall bind its Successors, except as otherwise provided in this Supplemental Indenture.  All agreements of the Trustee in this Supplemental Indenture shall bind its successors.

[Signatures on following page]

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

COLUMBIA SILICA SAND, LLC
By:	/s/ Christopher B. Gaskill
Name:	Christopher B. Gaskill
Title:	Assistant Secretary
COLUMBIA AGGREGATES, LLC
By:	/s/ Christopher B. Gaskill
Name:	Christopher B. Gaskill
Title:	Assistant Secretary
NORTHWEST AGGREGATES, INC.
By:	/s/ Christopher B. Gaskill
Name:	Christopher B. Gaskill
Title:	Assistant Secretary
NORTHWEST READY MIX, INC.
By:	/s/ Christopher B. Gaskill
Name:	Christopher B. Gaskill
Title:	Assistant Secretary
WILMINGTON TRUST, NATIONAL ASSOCIATION,
	as Trustee
By:	/s/ Joseph P. O’Donnell
	Name:   Joseph P. O’Donnell
	Title:     Vice President

[Signature Page to Eleventh Supplemental Indenture]