Deon MacMillan, Executive Vice President, Chief People Officer and Head of Corporate Communications of Summit Materials, Inc. (the Company), will be leaving the Company effective November 14, 2022. Ms. MacMillan's departure constitutes a termination without cause under the Company's Executive Severance Plan. Effective upon Ms. MacMillan's departure, Karli Anderson, Executive Vice President, Chief ESG Officer and Head of Investor Relations of the Company will assume leadership of the Company's human resources and communications functions.

As of the time of this report, the Company has not entered into any compensation arrangements with Ms. Anderson in connection with the announcement described above.