Segment Information - Net Sales (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 27, 2020	Mar. 29, 2019	Mar. 27, 2020	Mar. 29, 2019
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	$ 455,654	$ 469,309	$ 903,102	$ 921,337
Electrical Raceway
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	339,005	353,119	679,793	696,334
MP&S
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	116,649	116,190	223,309	225,003
Metal Electrical Conduit and Fittings | Electrical Raceway
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	125,609	134,131	249,451	265,378
Armored Cable and Fittings | Electrical Raceway
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	82,615	88,629	166,438	172,973
PVC Electrical Conduit and Fittings | Electrical Raceway
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	71,341	66,183	144,229	134,416
Cable Tray and Cable Ladders | Electrical Raceway
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	51,103	51,138	101,349	96,912
Other raceway products | Electrical Raceway
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	8,337	13,038	18,326	26,655
Mechanical Pipe | MP&S
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	65,694	62,039	124,025	122,707
Metal Framing and Fittings | MP&S
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	30,676	30,450	58,533	59,061
Other MP&S products | MP&S
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	$ 20,279	$ 23,701	$ 40,751	$ 43,235