The sheets are about 27cm * 12.5 cm. it says they cover up to 32 sq. inches, which I guess is about 200 cm2.
A small building wouldn't take so much, of course, but I used about 9 sheets on those two modules, coz they are so huge. So that gets pretty expensive, but as I said, for me, well worth it.
I think people here have got a more warhammery effect by using cut up card, etc. If you go for that exaggerated warhammer feel then that would be good, but if you want a roof tile that is more to scale, then each tile would be only a few mm wide, so that would be a real pain to cut up.