Daf Yomi - Sukkah 17 - Highlights

The Mishna rules that if the s'chach is distanced from the walls three tefachim or more, the sukkah is disqualified. If there is ineligible s'chach , the sukkah is valid, providing that it not more than four amos away from the walls.

If there is an area of open space on top of the sukkah more than three tefachim, the sukkah is disqualified. There is a debate regarding the amount of ineligible s'chach that would invalidate a sukkah when it is situated in the middle of the sukkah. The students of Rav maintain that the sukkah is disqualified if it has more than four tefachim of ineligible s'chach. They hold that the amount is only four amos when the invalid s'chach is on the side of the sukkah because then we can apply the principle of dofan akumah, bending the wall, however when the s'chach is situated in the middle of the sukkah, it will be disqualified if it is more than four tefachim. Rava maintains that the amount is always four amos.

If there is less than three tefachim of open space and adjacent to that is less that four amos of ineligible s'chach, the sukkah is valid according to Rava because the open space and the invalid s'chach cannot combine with each other . The reasoning for this is because anything that has two different measurements do not combine with one another.

The Gemora cites a Mishna in Keilim that lists different measurements for various materials as to when they will become susceptible to become tamei. The Mishna concludes that they can combine with each other to create the amount needed to become tamei. The Gemora explains that this is only because they would have the same measurement regarding becoming tamei through a zav sitting on them.

Rabbi Meir (who maintains that beams are ineligible for s'chach) admits in a case where you place valid s'chach in the space between each beam that the sukkah is valid, providing that the space is equal to that of the beam. The Gemora asks on the students of Rav who hold that the sukkah is disqualified with a beam of four tefachim, how can this sukkah be valid? The Gemora answers that the sukkah is precisely eight amos and by alternating between the beams and the valid s'chach, there will be eight tefachim in the center of the sukkah with valid s'chach.