X0306 4 2020-11-02 0 0001621563 Summit Materials, Inc. SUM 0001635215 Benedict Anne Lee C/O SUMMIT MATERIALS, INC. 1550 WYNKOOP STREET, 3RD FLOOR DENVER CO 80202 0 1 0 0 See Remarks Class A Common Stock 2020-11-02 4 C 0 8250 A 20114 D Class A Common Stock 2020-11-02 4 S 0 800 18.4901 D 19314 D Class A Common Stock 2020-11-02 4 S 0 7450 18.475 D 11864 D LP Units of Summit Materials Holdings L.P. 2020-11-02 4 C 0 8250 0 D Class A Common Stock 8250 53172 D 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)) for shares of the issuer's Class A common stock on a one-for-one basis. Title: EVP, Chief Legal Officer & Secretary /s/ Anne Lee Benedict 2020-11-04