Burden with building material (6)
I believe the answer is:
lumber
I believe this is a double definition.
'burden' is the first definition.
(thesaurus)
'building material' is the second definition.
(lumber is a kind of building material)
'with' is the link.
(Other definitions for lumber that I've seen before include "Unwanted furniture" , "Move in a heavy way, with partly treated wood" , "Clumsily move - junk" , "Sawn wood ready for use" , "Stored furniture; (US) timber" .)