Find fault with roofing material (5)

I believe the answer is:

slate

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'roofing material' is the definition.
(I know that slate is a type of roofing material)

I can't explain the rest of the clue.

(Other definitions for slate that I've seen before include "Rubbish" , "list of candidates" , "Roofing tile" , "Blast" , "Scold; hard rock" .)

I've seen this clue in The Globe and Mail.

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