How much, of a sukkah covered in skach, can be coverd by the roof and still be valid? Further how much wood planks can be places under and above the skach and still be kosher? Does it matter that some of the planks have some nails nailed or staples stapled into them?
If the the roof that is protruding above the schach is less than 3 tefochim ( 24 cm) similarly if the width of the plank above or under the schach is less than 3 tefochim then one can even sit under it and is considered a kosher succah. But if the roof is protruding more than 3 tefochim or the plank is wider than 3 tefochim ) one cannot sit under If the roof protrudes more than 4 Amos (approx. 2 meters) it disconnects the adjacent wall from the succcah. A plank with a few staples or nails in it is still considered kosher scach.
O:CH 632:1 & Mishnah Berurah 3