Sukka 9 a, b

A new Mishna teaches us of about sukkot that are built more than 30 days before the Festival.  Beit Shammai deems these sukkot unfit, while Beit Hillel deems them fit.  However, Beit Shammai concedes that the sukka can be fit if it is modified before the Festival.  The modification can be a handbreadth of roofing, or any measure that stretches along the length of the sukka.  The Gemara looks to clarify and find proofs for their assertions, including questioning when we prepare for Sukkot and whether or not the wood used for the sukka is sanctified.

Rav Yehuda quotes Rav to prove that we must be stringent in how we practice the mitzvot.  He uses the example of tzitzit that are woven with sub-standard thread.  In fact, he asserts that the thread for tzitzit must be found, woven and tied with the intention of fulfilling the mitzva of tzitzit.   Thus the construction of the sukka must be intentional from start to finish, according to Rav Yehuda.

Amud (b) begins with another Mishna: a sukka built under a tree is like a sukka built in a house: unfit.  A sukka built on top of another sukka might be unfit; however, Rabbi Yehuda says that the lower sukka is fit as long as there are no residents in the upper sukka.

The Gemara moves into a discussion of why a sukka under a tree is unfit.  First off, we are supposed to step out of our homes and into the sukka, which is why sukkot cannot be inside of our homes.  And if a tree is like a home, then the sukka under a tree is automatically unfit - there is no need for other proofs.  But of course the rabbis do not stop there.  They question the shade/sunlight of the tree, the placement of the tree's branches, when the tree was planted, the existence of unfit roofing (ex. grapevines), and other mitigating factors.  The rabbis are mindful of the fact that at least four by four cubits of the roof must be fit.

Regarding the instance of a sukka atop another sukka, the rabbis argue about whether we are commanded to reside in one sukka or in two sukkot; the wording of the mitzvah might be unclear.  Rabbi Yirmeya suggests that there are times when both upper and lower sukkot are fit, when both are unfit, when the upper alone is fit and when the lower alone is fit.

When are both fit?  When the lower sukka benefits from the greater measure of shade in the upper sukka AND when the upper sukka is less than twenty cubits high.  

When are both unfit?  When the upper sukka is more than twenty cubits high, though the shade is greater than the sunlight in both sukkot.

There is a picture of this 'piggybacking' sukka in the Koren Talmud.  I have to wonder how often people built one sukka on top of another sukka.  Perhaps the hot weather and small courtyards encouraged people to become creative in their construction.  Or perhaps this seemed like the most appropriate interpretation of 'living in sukkot', plural dwellings.  Whatever the reason, it is tough for me to imagine dwelling in a double sukka for the week of the Festival.