How is a Sukkah built?
One first assembles three or four walls and then places sechach [a covering], taken from something that grows, as a roof. Any type of material may be used to build the walls of the sukkah, provided that they are strong enough to withstand a normal gust of wind. Hence, if one uses canvas or sheets as walls for his sukkah, he must make sure to bind them tightly to the frame so that a normal gust of wind will not make them billow or dislodge them.
Even if they are secure enough that the wind will not dislodge them, if they sway they are not considered to be walls.
If there is a space three handbreadths wide between the bottom of any walls and the ground, the sukkah is not fit for use.
If one first constructed a frame, placed sechach on the roof, and then built the walls, the sukkah is unfit for use. A sukkah is only considered fit for use if the walls were built before one placed the sechach on top. However, if the wood or poles used for the frame are at least a tefach [a handbreadth] wide, one may place the sechach on top even before the walls are added.
If one placed sechach on the roof before building the walls, one can still make the sukkah fit for use. One should build the walls, lift the sechach from its place and then let it down. In this way, it is seen as if he has only now placed the sechach on top of the sukkah.
If one built two complete walls, a third wall which is more than a tefach wide, and a doorway between the walls, the sukkah is fit for use. Nevertheless, it is preferable to build four full walls.
Not all materials can be used for sechach. The material used as sechach must be something of plant origin that is now detached from the ground. It may not have been previously used for another purpose. It must never have acquired the status of a utensil nor have been capable of becoming ritually impure. Thus, branches which have been cut from a tree, strips of wood, straw, and similar materials can be used for sechach. Boards which were previously part of a crate may not be used for sechach since they originate from a utensil. Similarly, one may not use materials which are edible and if one does so, the sukkah is not fit for use.
Boards that are four tefachim wide cannot be used for sechach even if they had never been used previously, for their width makes them look like the ceiling in a home.
If one built his sukkah under a tree whose branches cover the sukkah, the sukkah is unfit for use even if one also uses proper sechach. The branches are considered to be part of the sechach since they also cover the sukkah; and since sechach can only be taken from something that is detached from the ground, the sukkah is unfit.
The Sages did not approve of using malodorous materials for sechach. Similarly, one should not use branches whose leaves fall off easily since they spoil the sukkah and may cause the person to leave the sukkah. Moreover, if the leaves fall off easily, the sechach may become so scanty and diminished that the sukkah will become unfit for use, for the halachah states that there must be more shade than sunlight in the sukkah.
Regarding bundles of stalks: If there are twenty-five stalks in the bundle, they may not be used for sechach as long as they are bundled together. If, however, one placed the bundle on the roof of the sukkah [with the intention of using them as sechach] and then unbound them, they may be used as sechach.
A mat of stalks or straw which was made specifically to be used as sechach may be used even though it has other uses as well. Since it was specifically intended for use as sechach, it is not susceptible to ritual defilement and is therefore fit for use.
One must place a sufficient amount of sechach so that the resultant shadow inside the sukkah is greater than the sunlight. If the amount of sechach is not sufficient to make the shadowed area of the sukkah greater than the sunlit area, the sukkah is unfit for use.
On the other hand, one should not place an excessive amount of sechach on the sukkah, i.e., an amount that prevents one from seeing the larger stars in the night sky through the sechach. However, even if this amount of sechach is used, the sukkah is nevertheless fit for use provided that the sechach is not so dense that rain cannot penetrate.
One may not derive any benefit from the materials used to construct the sukkah and the sechach during the eight days of the Festival.