Segment Information - Net Sales (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 27, 2019	Dec. 28, 2018
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	$ 447,448	$ 452,028
Electrical Raceway
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	340,788	343,215
MP&S
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	106,660	108,813
Metal Electrical Conduit and Fittings | Electrical Raceway
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	123,842	131,247
Armored Cable and Fittings | Electrical Raceway
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	83,823	84,345
PVC Electrical Conduit and Fittings | Electrical Raceway
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	72,888	68,233
Cable Tray and Cable Ladders | Electrical Raceway
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	50,246	45,774
Other raceway products | Electrical Raceway
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	9,989	13,616
Mechanical Pipe | MP&S
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	58,331	60,668
Other MP&S products | MP&S
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	$ 48,329	$ 48,145