1,178. A Combination of Valid and Invalid Materials as Schach

Hilchos Shofar, Succah v’Lulav 5:13

If one combined something that may be used as schach with something that may not be used as schach and used the two together, it is invalid even if the valid material exceeds the invalid material. If a succah is covered with these two materials but they are separated, then if there are more than three handbreadths (about 9”) of the invalid material in one spot - either in the middle of the succah or on one side - it is invalid.

Hilchos Shofar, Succah v’Lulav 5:14

The above halacha applies to a small succah. When it comes to a large succah, if the invalid material is in the middle, the succah is invalid if there are four handbreadths of this material (about 12”); less than that, the succah is valid. If the invalid material is on one side, the succah is invalid if there are four cubits of it (about six feet); less than that, the succah is valid. Therefore, in the case of a house whose roof is open in the center and schach is placed over the opening, or a courtyard surrounded by a portico that is covered with schach, or a large succah that is covered by an unacceptable material close to the walls, if there are four cubits from the end of the valid schach to the wall, the succah is invalid; less than that, we consider the wall as if it were bent. This means that we consider the invalid schach as if an extension of the wall and the succah is valid. This is a law that was transmitted to Moshe at Sinai.