Sukkah 12a and Sukkah 12b, how we know schach must be made of materials not susceptible to ritual impurity, why bundles of straw or wood cannot be sechach, with a rabbinic problem and a Biblical one of ta’aseh ve-lo min ha-asui, what kind of opening makes a piece of wood susceptible to ritual impurity, and whether a foul odor makes something invalid as schach.
