a roof job is when a chick lays on her back on a bed or coach or other similar surface (preferrably butt ass naked) with her
head hanging over the edge. the dude then straddles her
head, grabs her by her hair and/or ears, shoves his
cock into her mouth and begins to bang for all he's worth - effectively knocking the shingles off the roof of her mouth
SECOND DEFINITION
when a considerably tall man gets a considerably short girl to get on her knees and suck his
cock. the necessity of the short girl to stretch her
head and neck in the upward position to reach the
cock achieves the same effect of the roof of her mouth getting the shit banged out of it. Let's not forget the hair, head, neck grabbing to get the appropriate amount of banging and gagging. :)