The last tosfos on 2b, does he mean to define 'kavua schach' as that which does not let any water in, or is that just one example? Can schach be kavua without being waterproof or waterproof without being kavua?
Ari levy , Sydney Australia
1) It seems to me that Tosfos (2a, DH Ki Avid) just uses Sechach, which does not let water in, as one example. My proof for this is that the Magen Avraham (OC 627:2) cites the Agudah, at the beginning of Sukah, in the name of Tosfos that one should not fix planks into the Sukah with nails, even if the planks are less than four Tefachim wide, because this is considered Kavu'a. This is an example of a Sukah into which the rain can enter, but it is invalid because it has been built in a fixed way, with nails. This Halachah, that one should not fix the planks with nails, is cited by the Mishnah Berurah in Sha'ar ha'Tziyon 633:6.
2) The Maharam on Tosfos writes that temporary Sechach means that one should not cover the Sukah in a permanent way with planks in such a way that the rain cannot enter. We learn from the Maharam that if the Sechach is waterproof, it is automatically considered to be Kavu'a.
3) I found that Rav Shlomo Kluger, in his responsa ha'Elef Lecha Shlomo #366, writes that according to Tosfos the Sukah is disqualified only if two problems are present:
(a) rain cannot enter
(b) the Sechach is affixed with nails.
According to this, it does not become "Kavu'a Sechach" merely by not letting water in, but another condition must also be fulfilled before it is termed "Kavu'a": the Sechach must be affixed with nails.
4) Rav Kluger writes that the proof for this is from the Mishnah, Sukah 22a, which states that even if the Sechach is so thick that it is like a house, and one cannot see stars through the Sechach due to the thickness, the Sukah is still valid.
5) According to this, a Sukah is not Pasul until the Sechach is both waterproof and Kavu'a.