The most common and most likely answer to this crossword clue is the 12 letter word PLASTERBOARD
I've seen this crossword clue in The Daily Telegraph
P
L
A
S
T
E
R
B
O
A
R
D
Buddy explains!

The straight part
We know that "building material" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case building material = plasterboard.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does PLASTERBOARD mean?
PLASTERBOARD noun- A construction material consisting of a rigid panel of several layers of fibreboard or paper bonded to a gypsum core.
- To fit or reinforce with plasterboard.