Amendment, dated as of September 28, 2023, to the Transition and Consulting Agreement, dated as of September 8, 2022, by and among Summit Materials, Inc. and Brian J. Harris

September 28, 2023

Brian J. Harris

[***]

Dear Brian,

Reference is made to that certain transition and consulting agreement (the “Agreement”), dated as of September 8, 2022, by and among Summit Materials, Inc. (“Summit”) and you (the “Employee,” and together with Summit, collectively, the “Parties”). Capitalized terms used herein but not otherwise defined shall have the meaning as set forth in the Agreement.

The Parties desire to amend the Agreement as provided herein (the “Amendment”), effective immediately.

1.Transition Period. Section 1(a)(ii) of the Agreement is hereby amended to replace the reference to October 16, 2023, with October 6, 2023.

2.No Other Amendment. Except as expressly set forth in this Amendment, the Agreement shall remain in full force and effect without other or further amendment or modification.

3.Counterparts. This Amendment may be executed in one or more counterparts, which will, collectively and separately, constitute one agreement.

Please indicate acceptance of the terms of this Amendment in the space provided below, whereupon this Amendment and acceptance shall constitute a binding agreement between the Parties.

                            

                            Sincerely,

                            Summit Materials, Inc.

                            By: /s/ Chris Gaskill

                            Name: Chris Gaskill

Agreed to and accepted on October 3, 2023:

Brian J. Harris