The bottom Tosfos on Sukkah 2a (DH Ki Avid) talks about how it must be that the scach of a sukkah can't be rain proof, because if it was, then when the Rabbanan say that rain on sukkos is a bad sign (Taanis 2a) wouldn't be true because we could just put up rain proof scach. But I don’t understand Tosfos, for I can put up a tent inside my Sukkah (as long as there isn't a there isn't a one tefach wide flat area - Sukkah 10a), and I can eat and sleep inside the tent and also abrogate the reason it’s a "bad sign.”
it must be that the scach of a sukkah can't be rain proof because if it was then when the rabbanim say that rain on sukkos is a bad sign wouldn't be true because we could just put up rain proof scach. But I dont understand tosfos, for I can put up a tent inside my sukkah (as long as there isn't a there isn't a one tefach wide flat area.
Note that Tosfas is talking about the Schach, i.e. the Succah itself - the tent is not the Succah. Presumably, Tosfos understands that if the Succah itself is what is judged in the criteria of being exempted, and holds one which will never come to exemption is not a kosher Succah.
Another, slightly different possibility is that Tosfas holds that everything is taken into account for the exemption, but holds that it's still a bad sign if you're only there because of a reason external to the Succah, whilst if the reason you're okay is because of the Succah itself it cannot be seen as a 'rejection' of the Succah. To riff of the example of Chazal, the first case is like wearing a waterproof face covering, the second is like the master not throwing the cup of water you just fetched in your face at all.