The Order of the Sukkah’s Construction
Question: Should the sechach and walls be put up in any particular order?
Discussion: The sechach must be placed down only once the walls are already in place. The reason for this is that in order for a sukkah to be valid, the sechach must be put down lechatchilah in a valid state. Sechach that was placed down in an invalid state, and was later validated, remains invalid — it must be removed and placed down again in a valid state. This is the principle of תעשה ולא מן העשוי — “You shall make [the sukkah,27 initially, in a valid state]; it shall not be already made [in an invalid state].”28 Thus, in order for the sechach to be considered valid when it is first placed down the sukkah must already have walls.29
27. [That is, the sechach, which is the essential feature of the sukkah.]
28. See Shulchan Aruch 626:2.
29. Note, however, that this does not require the walls to be at their full height from the outset. For this purpose, it is sufficient if there are walls of a tefach high (10 cm), within three tefachim of the sechach (24 cm), at the time that the sechach is laid down (see Rema 635, Mishnah Berurah 635:9). Therefore, for example, it may be sufficient if the sechach is laid down on a frame, on a beam that is 10 cm high. Should this be relevant, a halachic authority should be consulted for particulars.