The Sukkah walls should all be assembled before the Schach is put on.
The Schach should be detached from their place of growth prior to being laid out over the Sukkah. Schach bundles containing more than 25 pieces should be unbundled prior to being laid out over the Sukkah. [If this was not adhered to, the validity of the Schach is subject to a number of variables.]
If the Sukkah has a rain cover, one should ensure that the Sukkah is uncovered when the Schach is laid out. Similarly, when placing (or replacing) the Schach on a Sukkah mobile, one should do so in a place which is not covered by a roof or tree.
If necessary, one may lay the Schach beneath overhanging tree branches before cutting those branches.
The Schach must either be laid within thirty days of Sukkos, or must be laid expressly for the sake of the Mitzvah of Sukkos. [If these criteria were not met, one may correct it in retrospect by either lifting and replacing all of the Schach, or by adding new Schach over an area at least 8cmx8cm, or by adding any amount of Schach that extends across the entire width or breadth of the Sukkah.]
The Schach should be laid on the Sukkah by a Jewish male over Bar Mitzvah who is obligated to eat in the Sukkah. After the fact, if it was laid out by anyone else, a Jewish male over Bar Mitzvah should pick up and place down at least one piece.
It is Chabad custom to use lots of Schach, but not to the point that it is watertight. A small hole is left somewhere in the Schach for the stars to be seen. At the very least, the Schach should create more shade than sunlight.
There should be no gap of airspace in the Schach exceeding 24cmx24cm. [A larger gap is sometimes acceptable, but that depends on many factors. Furthermore, the Mitzvah of Sukkah is in any case not fulfilled beneath it, so it is best avoided.]
The Schach must be sturdy enough to remain intact when an ordinary gust of wind blows. [If in an enclosed place, the Schach must still be sturdy enough to withstand the wind were it an exposed place.] Otherwise, they must be secured in an acceptable manner (see the section “Supporting the Schach”). This is of special concern when using mats.
If Schach blows out of place on Yom Tov or Shabbos, one may not fix it, since the Schach is Muktzeh. However, if the Sukkah is no longer Kosher (or its status is questionable), one should consult with a Rov as to whether a Goy can fix it.
