POWER OF ATTORNEY

        Know all by these presents that the undersigned, does hereby
make, constitute and appoint each of Anne Lee Benedict and Jennifer Rose, or any
one of them, as a true and lawful attorney-in-fact of the undersigned with full
powers of substitution and revocation, for and in the name, place and stead of
the undersigned (in the undersigned's individual capacity), to execute and
deliver such forms that the undersigned may be required to file with the U.S.
Securities and Exchange Commission as a result of the undersigned's ownership of
or transactions in securities of Summit Materials, Inc. (i) pursuant to Section
16(a) of the Securities Exchange Act of 1934, as amended, including without
limitation, statements on Form 3, Form 4 and Form 5 (including any amendments
thereto) and (ii) in connection with any applications for EDGAR access codes,
including without limitation the Form ID. The Power of Attorney shall remain in
full force and effect until the undersigned is no longer required to file Forms
3, 4 and 5 with regard to his or her ownership of or transactions in securities
of Summit Materials, Inc., unless earlier revoked in writing. The undersigned
acknowledges that Anne Lee Benedict and Jennifer Rose are not assuming any of
the undersigned's responsibilities to comply with Section 16 of the Securities
Exchange Act of 1934, as amended.

                                        By:
                                             /s/ Michael J. Brady
                                             ----------------------
                                             Name: Michael J. Brady

                                        Date: March 4, 2015
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