STOCKHOLDERS' EQUITY/MEMBERS' INTEREST - Equity Offerings (Details) - shares	9 Months Ended
	Oct. 02, 2021	Sep. 26, 2020	Jan. 02, 2021	Dec. 28, 2019
Common Class A
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)	114,390,595	113,309,385
Exchanges during period (in shares)	1,440,983	196,542
Stock option exercises (in shares)	1,744,256	14,404
Other equity transactions (in shares)	989,116	603,580
Ending balance (in shares)	118,564,950	114,123,911
Summit Materials, Inc. and Summit Holdings, LP
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Beginning balance (in shares)	117,263,765	116,559,042
Exchanges during period (in shares)	0	0
Stock option exercises (in shares)	1,744,256	14,404
Other equity transactions (in shares)	989,116	603,580
Ending balance (in shares)	119,997,137	117,177,026
LP Units | Summit Holdings LP
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Number of LP Units outstanding, beginning balance (in shares)	2,873,170	3,249,657
Number of LP Units exchanged (in shares)	(1,440,983)	(196,542)
Number of LP Units outstanding, ending balance (in shares)	1,432,187	3,053,115
Summit Holdings LP
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Noncontrolling interest elimination (as a percent)	1.20%		2.50%
Summit Materials Inc
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Ownership percentage	98.80%	97.40%	97.50%	97.20%