Document And Entity Information - shares	3 Months Ended
	Dec. 31, 2020	Jan. 31, 2021
Cover [Abstract]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Dec. 31, 2020
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q1
Trading Symbol	BECN
Entity Registrant Name	BEACON ROOFING SUPPLY, INC.
Entity Central Index Key	0001124941
Current Fiscal Year End Date	--09-30
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Common Stock, Shares Outstanding		69,407,397
Entity Shell Company	false
Entity Current Reporting Status	Yes
Entity File Number	000-50924
Entity Tax Identification Number	36-4173371
Entity Address, Address Line One	505 Huntmar Park Drive
Entity Address, Address Line Two	Suite 300
Entity Address, City or Town	Herndon
Entity Address, State or Province	VA
Entity Address, Postal Zip Code	20170
City Area Code	571
Local Phone Number	323-3939
Entity Interactive Data Current	Yes
Entity Incorporation, State or Country Code	DE
Document Quarterly Report	true
Document Transition Report	false
Title of 12(b) Security	Common Stock, $0.01 par value
Security Exchange Name	NASDAQ