What Constitutes Schach?
Throughout the generations, schach has traditionally been made from reeds and tree branches (Levush, 629:18; Bikurei Yaakov ibid. s.k. 6; Kitzur Shulchan Aruch 134:3; Ben Ish Chai, First Year: Haazinu, 1). The Ben Ish Chai writes that in his own community it was customary to make schach primarily from tree branches. He finds support for this practice from the gematria of the word sukkah, which is 91, equivalent to the word “ilan – tree.”
The above-listed materials fulfill all three qualifications for schach stated in our Gemara: they grow from the ground, they are no longer attached to the ground, and they are not subject to ritual impurity. Therefore, they are undoubtedly kosher.
Thin Wooden Boards
Recently, it has become common to use thin wooden boards for schach. During the last few decades, doing so has become an accepted practice in Yerushalayim. But objections have been raised against this practice.
The first question involves the rabbinic prohibition against using wooden boards. Since these boards are also used to roof houses, our Sages were concerned that if we allowed people to use boards for their schach, they might mistakenly assume that their homes also qualify as sukkos.
However, the Sages in our mishnah, as explained in the Gemara, only forbade using boards at least four tefachim wide since standard boards used for roofing are that size. (According to the most stringent opiniont, four tefachim equals 12.9 inches or 32 centimeters.) However, the Shulchan Aruch HaRav (O.C. 629:32) notes that building standards have changed since the time of the Gemara. It now is common to build roofs from boards even thinner than three tefachim.
And yet, builders would never use boards that let in rain. Since the boards people use for schach let in lots of rain, many of the most prominent rabbonim in Yerushalayim thought them permissible and used them (Piskei Teshuvos 629, footnote 72). The Chazon Ish once said that since using these boards is accepted in Yerushalayim, it may be practiced there. In other places, however, it is best not to use boards for schach (Orchos Rabbeinu II, p. 218. See Sefer HaSukka p. 281).
Some poskim concede that very thin boards, which are never used for roofing, may be used for schach. The boards commonly used in Yerushalayim are so thin that they would never be used in roofing. Indeed, they are so thin that perhaps they should not even be considered boards at all.
Because these boards are so thin, we need not worry about the Rabbeinu Tam’s ruling that schach may not be water-proof. We also perhaps need not worry about the Mishnah Berurah ruling against boards lest one confuse the sukkah for one’s house. The boards we use are so thin that a person will never confuse them with the permanent roof of his home (Teshuvos Beis Yisroel, O.C. 6).