GOODWILL AND INTANGIBLE ASSETS - Carrying Amount of Goodwill (Details) - USD ($)	3 Months Ended	12 Months Ended
	Sep. 26, 2020	Dec. 26, 2020	Dec. 28, 2019	Dec. 29, 2018
Goodwill [Line Items]
Goodwill, Gross			$ 478,111,000	$ 434,454,000
Accumulated impairment losses			(49,247,000)	(49,247,000)
Carrying amount of goodwill
Balance at the beginning of the period		$ 428,864,000	385,207,000
Acquisitions		6,138,000	42,133,000
Impairment	$ 0	(12,575,000)
Foreign currency translation		7,895,000	1,524,000
Balance at the end of the period		430,322,000	428,864,000
Engineered Support Structures Segment
Goodwill [Line Items]
Goodwill, Gross			228,634,000	204,735,000
Accumulated impairment losses			(18,670,000)	(18,670,000)
Carrying amount of goodwill
Balance at the beginning of the period		209,964,000	186,065,000
Acquisitions		0	21,870,000
Impairment		(12,575,000)
Foreign currency translation		3,689,000	2,029,000
Balance at the end of the period		201,078,000	209,964,000
Utility Support Structures Segment
Goodwill [Line Items]
Goodwill, Gross			130,594,000	123,618,000
Accumulated impairment losses			(14,355,000)	(14,355,000)
Carrying amount of goodwill
Balance at the beginning of the period		116,239,000	109,263,000
Acquisitions		1,100,000	7,889,000
Impairment		0
Foreign currency translation		3,641,000	(913,000)
Balance at the end of the period		120,980,000	116,239,000
Coatings Segment
Goodwill [Line Items]
Goodwill, Gross			93,747,000	80,937,000
Accumulated impairment losses			(16,222,000)	(16,222,000)
Carrying amount of goodwill
Balance at the beginning of the period		77,525,000	64,715,000
Acquisitions		0	12,374,000
Impairment		0
Foreign currency translation		562,000	436,000
Balance at the end of the period		78,087,000	77,525,000
Irrigation Segment
Goodwill [Line Items]
Goodwill, Gross			25,136,000	25,164,000
Accumulated impairment losses			0	$ 0
Carrying amount of goodwill
Balance at the beginning of the period		25,136,000	25,164,000
Acquisitions		5,038,000	0
Impairment		0
Foreign currency translation		3,000	(28,000)
Balance at the end of the period		$ 30,177,000	$ 25,136,000