GOODWILL AND INTANGIBLE ASSETS - Non-Amortized Intangible Assets (Details) - USD ($) $ in Thousands	Dec. 26, 2020	Dec. 28, 2019
Newmark
Non-amortized intangible assets
Carrying value of trade names	$ 11,111	$ 11,111
Webforge
Non-amortized intangible assets
Carrying value of trade names	7,972	9,143
Valmont SM
Non-amortized intangible assets
Carrying value of trade names	8,720	7,966
Ingal EPS/Ingal Civil Products
Non-amortized intangible assets
Carrying value of trade names	7,730	7,454
Shakespeare
Non-amortized intangible assets
Carrying value of trade names	4,000	4,000
Walpar
Non-amortized intangible assets
Carrying value of trade names	3,500	3,500
Convert
Non-amortized intangible assets
Carrying value of trade names	9,137	8,378
Other
Non-amortized intangible assets
Carrying value of trade names	14,828	17,555
Trade Names
Non-amortized intangible assets
Carrying value of trade names	$ 66,998	$ 69,107