X0306 4 2015-08-11 0 0001621563 Summit Materials, Inc. SUM 0001635271 Harris Brian James C/O SUMMIT MATERIALS, INC. 1550 WYNKOOP STREET, 3RD FLOOR DENVER CO 80202 0 1 0 0 EVP & Chief Financial Officer LP Units of Summit Materials Holdings L.P. 2015-08-11 4 D 0 100747 D Class A Common Stock 100747 547951 I See Footnote Pursuant to the terms of an exchange agreement, dated as of March 11, 2015, the limited partnership units of Summit Materials Holdings L.P. ("LP Units") reported herein are exchangeable from and after the first anniversary of the closing of the issuer's initial public offering (subject to the terms of the exchange agreement and vesting requirements, including certain vesting events more fully described in the issuer's Registration Statement on Form S-1 (File No. 333-201058) (the "Registration Statement")) for shares of the issuer's Class A common stock on a one-for-one basis. The Reporting Person sold to the issuer the number of LP Units set forth above at a price per LP Unit of $24.784375, which represents the $25.75 price at which the issuer's Class A common stock was sold in a secondary offering, less the underwriters' discount of $0.965625 per share. Total holdings include LP Units acquired in connection with a dividend of LP Units from Summit Materials Holdings GP, Ltd., of which the Reporting Person is a shareholder. The acquisition of such LP Units was exempt pursuant to Rule 16a-9. Reflects securities held by a trust for the benefit of Mr. Harris' family, for which Mr. Harris' spouse serves as trustee. The Reporting Person disclaims beneficial ownership of the securities reported herein as indirectly beneficially owned, except to the extent of his pecuniary interest therein. /s/Anne Lee Benedict, as Attorney-in-Fact 2015-08-13