Segment Information - Net Sales (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Jun. 26, 2020	Jun. 28, 2019	Jun. 26, 2020	Jun. 28, 2019
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	$ 384,899	$ 493,491	$ 1,288,001	$ 1,414,828
Electrical Raceway
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	285,412	372,895	965,205	1,069,229
MP&S
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	99,487	120,596	322,796	345,599
Metal Electrical Conduit and Fittings | Electrical Raceway
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	105,079	139,342	354,530	404,720
Armored Cable and Fittings | Electrical Raceway
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	60,141	90,891	226,578	263,864
PVC Electrical Conduit and Fittings | Electrical Raceway
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	71,914	78,765	216,143	213,181
Cable Tray and Cable Ladders | Electrical Raceway
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	40,759	50,680	142,108	147,592
Other raceway products | Electrical Raceway
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	7,519	13,217	25,846	39,872
Mechanical Pipe | MP&S
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	59,985	63,502	184,010	186,209
Metal Framing and Fittings | MP&S
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	25,087	29,328	83,620	88,389
Other MP&S products | MP&S
Revenues from External Customers and Long-Lived Assets [Line Items]
Net sales	$ 14,415	$ 27,766	$ 55,166	$ 71,001