Net Sales - Summary of Net Sales by Product Line and Geography (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disaggregation Of Revenue [Line Items]
Net sales	$ 1,872.1	$ 1,549.3	$ 4,766.6	$ 4,161.7
Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	981.6	832.1	2,521.3	2,114.6
Non-Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	486.7	418.4	1,214.9	1,174.0
Complementary Building Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	403.8	298.8	1,030.4	873.1
U.S. [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1,802.8	1,504.3	4,614.9	4,047.6
U.S. [Member] | Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	955.9	814.7	2,473.4	2,081.6
U.S. [Member] | Non-Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	446.4	393.4	1,118.4	1,098.8
U.S. [Member] | Complementary Building Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	400.5	296.2	1,023.1	867.2
Canada [Member]
Disaggregation Of Revenue [Line Items]
Net sales	69.3	45.0	151.7	114.1
Canada [Member] | Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	25.7	17.4	47.9	33.0
Canada [Member] | Non-Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	40.3	25.0	96.5	75.2
Canada [Member] | Complementary Building Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	$ 3.3	$ 2.6	$ 7.3	$ 5.9