Siman 628

It is a decree of the verse that a sukkah beneath a sukkah, even though the upper one is also kosher, nevertheless the lower one is invalid because of a sukkah beneath a sukkah, as it is written "you shall dwell in sukkot," and "sukkot" is written lacking the letter vav, to tell you: you shall dwell in one sukkah, and not in a sukkah beneath a sukkah. Therefore, a sukkah beneath a sukkah: the upper one is kosher, and the lower one is invalid. And we do not say: just as the upper one invalidates the lower one, so too should the lower one invalidate the upper one because of a sukkah on top of a sukkah. For it is not so, as the Torah invalidated a sukkah beneath a sukkah, and not a sukkah on top of a sukkah. And the reason for the matter is: for the essence of the sukkah is the sechach, and the Torah invalidated two roofs of sechach, and not two floors. And the lower sechach is like a floor to the upper one, for he treads upon it.

It is a simple matter: when does the upper one disqualify the lower one? When it is covered as required, its shade being greater than its sunlight; and that it has walls, and that it has the minimum measure of a sukkah. For if not so—this is not a sukkah, such that it would disqualify the lower one due to "a sukkah beneath a sukkah." And furthermore, the sages said (10a) that the lower one must be able to support the pillows and blankets of the upper one. For if not so, what kind of sukkah is the upper one, since it is not fit for use? However, even if it cannot support them comfortably, but only with difficulty—it also disqualifies, for a sukkah is a temporary dwelling, and regarding a temporary dwelling, even with difficulty is considered use. And likewise, the upper one must be at a height of ten tefachim from the lower one, for in less than this it does not have the status of a sukkah, as it is not fit for use at all.

And if there are not ten tefachim between them, or if it cannot support the pillows and blankets of the upper one—the lower one is kosher, if it is covered according to its law, its shade being greater than its sunlight, even if the sakh of the upper one is higher than twenty amot, which is invalid in a sukkah, as I will write in siman 633. And even though in siman 626 it was explained that if there is invalid sakh above the kosher sakh—it is invalid, that is when the invalidity is in the essence of the sakh. But here the sakh is kosher, but it stands above the place of its validity—it is not considered as actual invalid sakh, which would invalidate the lower one because of invalid sakh. However, this is specifically when the lower one does not need the combination of the upper one. But when it needs it, such as when it itself has its sunlight greater than its shade, and only with the combination of the upper one is its shade greater than its sunlight—it is certain that the upper one must also be within twenty.

One who makes his sukkah on top of a wagon, or on top of a ship, even though it is a temporary structure—nevertheless, the sukkah is valid, for a sukkah is a temporary dwelling. However, we require in any event that it be able to stand in a common wind of the land. For if not so, it is nothing, as the wind will come and uproot it. But if it can stand in a common wind of the land, even though it cannot stand in a common wind of the sea, for at sea the wind is stronger, and a common wind of the sea is considered an uncommon wind of the land, nevertheless the sukkah is valid even if the ship is traveling on the sea, for a common wind of the sea is not present at all times.

One who makes his sukkah on the back of a camel, or at the top of a tree, since in both cases it is forbidden to use them on Yom Tov, nevertheless the sukkah is kosher. However, one may not ascend to it on Yom Tov. For the prohibition of using animals and that which is attached to the ground is of rabbinic origin, and according to the Torah it is a proper sukkah: even if we say that one requires a sukkah fit for seven days, and even if the sukkah itself stands on the ground, and the reeds of the sechach are supported by the tree – it is forbidden, as a decree lest he place his objects on the sechach. Therefore, in a place where it is not their way to use the top of the sechach – it is possible to permit in such a case (Magen Avraham, subsection 6). And if the entire sukkah is not on the tree, but rather part of it is supported by the tree and part of it by something else, if it is in a manner that if the tree were removed it would remain standing and not fall – one may ascend to it on Yom Tov. And if not – one may not ascend to it on Yom Tov.