Cover - shares	3 Months Ended
	Mar. 28, 2020	Apr. 27, 2020
Document Information [Line Items]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 28, 2020
Document Transition Report	false
Entity File Number	001-36873
Entity Registrant Name	SUMMIT MATERIALS, INC.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	47-1984212
Entity Address, Address Line One	1550 Wynkoop Street
Entity Address, Address Line Two	3rd Floor
Entity Address, City or Town	Denver
Entity Address, State or Province	CO
Entity Address, Postal Zip Code	80202
City Area Code	303
Local Phone Number	893-0012
Title of 12(b) Security	Class A Common Stock (par value $.01 per share)
Trading Symbol	SUM
Security Exchange Name	NYSE
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Central Index Key	0001621563
Amendment Flag	false
Current Fiscal Year End Date	--12-28
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Class A Common Stock
Document Information [Line Items]
Entity common stock, shares outstanding (in shares)		114,110,597
Class B Common Stock
Document Information [Line Items]
Entity common stock, shares outstanding (in shares)		99
Summit Materials, LLC
Document Information [Line Items]
Entity File Number	333-187556
Entity Registrant Name	SUMMIT MATERIALS, LLC
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	26-4138486
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Central Index Key	0001571371
Amendment Flag	false
Current Fiscal Year End Date	--12-28
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1