Translation:Mishnah/Seder Moed/Tractate Sukkah/Chapter 1/10
If one distanced the s'chach three tefachim from one of the walls, [ thus leaving three tefachim of open space between it and the wall,] the succah is invalid.
In the case of a house whose roof was breached and the owner placed s'chach over the opening, if there is a distance of four amos from wall to the s'chach, it is invalid as a succah. And so is the law in a case of a courtyard surrounded by porches over wich s'chach was placed. similarly in the case of a large succah wose s'chach is surrounded by material that may not be used for s'chach, if there is a space four amos wide beneath the ineligible material, the succah is invalid.