1,179. Alternating Strips of Valid and Invalid Schach

Hilchos Shofar, Succah v’Lulav 5:15

A small succah, as used in the previous halacha, means one whose area is no larger than seven handbreadths by seven handbreadths (about 21”x21”). A large succah means one whose area is big enough that seven handbreadths by seven handbreadths of valid schach will remain aside from the invalid schach.

Hilchos Shofar, Succah v’Lulav 5:16

If one used both things that are acceptable and things that are unacceptable as schach, placing them alongside one another leaving no place of three handbreadths without invalid schach, then if the valid schach exceeds the invalid schach, it is valid. If there is exactly the same amount of each material, it is invalid even though there is no one spot with three handbreadths of invalid schach. This is because the invalid schach is considered the same as empty space.