Net Sales - Summary of Net Sales by Product Line and Geography (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Disaggregation Of Revenue [Line Items]
Net sales	$ 6,943.9	$ 7,105.2	$ 6,418.3
Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	3,099.6	3,079.6
Non-Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1,646.6	1,705.2
Complementary Building Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	2,197.7	2,320.4
U.S. [Member]
Disaggregation Of Revenue [Line Items]
Net sales	6,766.4	6,918.5
U.S. [Member] | Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	3,043.2	3,023.2
U.S. [Member] | Non-Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	1,534.5	1,582.8
U.S. [Member] | Complementary Building Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	2,188.7	2,312.5
Canada [Member]
Disaggregation Of Revenue [Line Items]
Net sales	177.5	186.7
Canada [Member] | Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	56.4	56.4
Canada [Member] | Non-Residential Roofing Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	112.1	122.4
Canada [Member] | Complementary Building Products [Member]
Disaggregation Of Revenue [Line Items]
Net sales	$ 9.0	$ 7.9