Sources and reasoning:
The Gemara in Sukkah 17a states: Rabbah said, “I found the scholars of the academy of Rav sitting and saying: Airspace invalidates with three [tefachim], while invalid s’chach invalidates with four.”
The Gemara further states in Sukkah 19a, in the discussion of s’chach extending beyond the sukkah, which is treated as part of the sukkah: Rabbi Hoshaya said, “This ruling is necessary only with regard to invalid s’chach measuring less than three [tefachim] in a small sukkah. And what is meant by ‘extending’? Extending from the status of a sukkah.” Rav Hoshaya challenged this: “It should be no different from airspace; does airspace measuring less than three tefachim invalidate a small sukkah?” Rabbi Abba replied: “This combines [with the valid s’chach], and one may sleep beneath it; whereas that combines [with the valid s’chach], but one may not sleep beneath it.”
The Gemara makes clear that invalid s’chach measuring less than four tefachim does not invalidate the sukkah. However, regarding whether one may sleep beneath s’chach measuring less than four tefachim but more than three tefachim, the Rishonim disagree. The view of the Baal HaMaor, 7a, is that one may sleep beneath it; however, the Raavad, in his glosses to the Baal HaMaor there, section 2, maintains that one may not sleep beneath it unless it is less than three tefachim.
Regarding the halachah, the Shulchan Aruch, Orach Chaim 632:1, rules as follows:
Invalid s’chach invalidates in the middle [of the sukkah] when it measures four tefachim; but if it is less than four, the sukkah is valid, and one may sleep beneath it.
However, the Mishnah Berurah cites the dissenting view and writes in se’if katan 3:
There are poskim who maintain that although the sukkah is valid when [the invalid s’chach] is less than four tefachim, nevertheless it is forbidden to sleep or eat beneath that area unless it is less than three tefachim. Therefore, one should be stringent l’chatchilah.
It is important to note that even Sephardim who rule in accordance with Maran the Shulchan Aruch should preferably be stringent not to sleep beneath invalid s’chach measuring less than four tefachim and more than three tefachim, as the Kaf HaChaim writes in se’if katan 4. This is likewise written in Chazon Ovadia, Sukkot, Hilchot Dinei HaS’chach, section 32: invalid s’chach invalidates the sukkah when it measures four tefachim; but if it is less than four tefachim, the sukkah is valid. Nevertheless, one may not eat or sleep beneath it unless it is less than three tefachim.
This response was translated automatically from Hebrew and has not yet been reviewed by a person. Therefore, it may contain inaccuracies and should not be considered a definitive reference.
