X0306 4 2020-12-18 0 0001621563 Summit Materials, Inc. SUM 0001275621 CANTIE JOSEPH S C/O SUMMIT MATERIALS, INC. 1550 WYNKOOP STREET, 3RD FLOOR DENVER CO 80202 1 0 0 0 Class A Common Stock 2020-12-18 4 P 0 2500 19.64 A 2500 I See Footnote Class A Common Stock 2020-12-18 4 P 0 2600 19.67 A 5100 I See Footnote Class A Common Stock 20947 D The price reported in Column 4 is a weighted average price. These shares were purchased in multiple transactions ranging from $19.63 to $19.641, inclusive. The Reporting Person undertakes to provide to the Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares purchased at each separate price within the range set forth above. Reflects securities held by a trust for the benefit of the Reporting Person, for which the Reporting Person and his spouse serve as trustees. /s/ Anne Lee Benedict, as Attorney-in-Fact 2020-12-22