The following are some of the basic requirements for a "kosher" Sukkah:
A) Made for the Mitzvah:
A Sukkah must be built anew every year for the purpose of the mitzvah. This requirement, however, applies primarily to the sechach (the roof covering of branches or bamboo), since it is the sechach that makes the Sukkah a Sukkah. Thus, one can leave the walls standing all year, and place the roof covering before the festival. Also, if the Sukkah has been up all year, one can lift up and replace the sechach, which allows the Sukkah to be considered as new.
B) Order of Construction:
One must first erect the walls and only then place the sechach covering. If the sechach is put up before there are "kosher" walls in place, the Sukkah is invalid, until the sechach is removed and re-applied.
C) The Walls:
How many walls
A Sukkah must have at least two full walls plus part of a third wall (the "part" needs to be a minimum of 3.2 inches wide). It is preferable, however, that the Sukkah have four complete walls.
What the walls are made of
The walls of the Sukkah can be made of any material, but they must be be sturdy enough so that they do not move in a normal wind. One can use a pre-existing walls, such as the walls of a garage or one's house, as one or more of the walls. An existing structure that is roofless or has a removable roof can also be made into a Sukkah by covering it with proper sechach (see below).
Size and dimensions
The walls must be at least 32-inches high, and the entire structure (i.e., the distance of the roof-covering from the ground) may not be higher than 30 feet. In length and breadth, a Sukkah cannot be smaller than 22.4 inches by 22.4 inches (large enough to hold a person's head and torso, and a small table). There is no size limit in how large a Sukkah may be.
Gaps in the walls
It is best that a Sukkah have four solid walls (aside from the doorways and windows). However, under certain conditions, incomplete walls will qualify, as follows:
a) If there is a gap between the bottom of the walls and the ground, the bottom of the walls must be less than 9.6 inches from the ground.
b) If the walls are 32 inches high, the roof may be higher (up to the maximum height of 30 feet off the ground), as long as the walls are beneath the roof.
c) There may be gaps of empty space in the walls, as long as these are less than 9.6 inches wide. (Thus a fence made of upright or horizontal slats can be used, as long as the spaces between the slats are less than 9.6 inches inches.)
D) The Sechach (Roof Covering):
What can be used as sechach
The Sukkah should be covered with sechach, a roof covering of raw, unfinished vegetable matter. Common Sukkah roof-coverings are: bamboo poles, evergreen branches, reeds, corn stalks, narrow strips (1x1 or 1x2) of unfinished lumber, or special sechach mats (see below).
Mats made of bamboo, straw or other vegetable matter can be used only if they were made for the purpose or serving as a roof covering (e.g. not for sitting, sleeping or any other use).
An important requirement is that the sechach be severed from source of growth--thus a live trellis, or branches still attached to the tree, cannot serve as roof covering for a Sukkah.
How much sechach
There must be sufficient sechach to provide enough shade so that on a bright midday there is more shade than sun seen on the floor of the Sukkah.
The sechach has to be spread out evenly over the entire Sukkah so that there should not be any spaces more then 9.6 inches apart.
Supporting the sechach
Anything that is directly supporting the sechach should not be made out of materials that are not fit to be used as sechach. Thus, if the sechach is resting directly on the Sukkah walls and the walls are not made out of wood, strips of wood should be placed between the Sukkah walls and the sechach. In larger sukkahs where a framework of beams is needed to to hold up the sechach, wood or bamboo poles should be used, not metal. Nor may the sechach be tied on with wire or fastened with any metal object.
