Net Sales - Summary of Net Sales by Product Line and Geography (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Disaggregation Of Revenue [Line Items]
Net sales	$ 1,318.0	$ 1,197.1	$ 2,894.5	$ 2,612.5
Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	695.0	585.4	1,539.9	1,282.3
Non-Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	329.7	343.7	727.9	755.7
Complementary Building Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	293.3	268.0	626.7	574.5
U.S. [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1,283.6	1,128.0	2,812.0	2,543.4
U.S. [Member] | Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	686.1	569.9	1,517.5	1,266.8
U.S. [Member] | Non-Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	305.9	293.4	671.9	705.4
U.S. [Member] | Complementary Building Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	291.6	264.7	622.6	571.2
Canada [Member]
Disaggregation Of Revenue [Line Items]
Net sales	34.4	69.1	82.5	69.1
Canada [Member] | Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	8.9	15.5	22.4	15.5
Canada [Member] | Non-Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	23.8	50.3	56.0	50.3
Canada [Member] | Complementary Building Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	$ 1.7	$ 3.3	$ 4.1	$ 3.3