It says in the Gemara (Sukka 2a) that the reason why you can build a Sukka whoes walls are made out of iron is because since it's lower than 20 amos it can still be made Arai. My question is, why do we say that the rain has to come through the Schach? Why can't we say that although the rain can't come through it still can be made Arai and is Kosher?
The Gemara in Sukah 2a does not mean that a permanent Sukah is Kosher because it can be changed and made Arai. It means that a Sukah under 20 Amos is always and even now called an Arai-form structure since under 20 Amos it does not need to be built Keva. A low metal Sukah is itself an Arai-form Sukah.
However a Sechach ceiling which is not penetrable by rain is now Keva. It cannot be called Arai just because the situation can be changed.
All the best,