939. More about Schach

134:4 We should be careful not to use anything that is susceptible to ritual impurity to support the schach, such as a ladder that has receptacles for the rungs, and tools like an axe or a rake. It is better not even to place such things on the schach to hold it down but if it was already done, or if one has nothing else to use, then it is permitted because supporting the schach with something that is susceptible to impurity does not disqualify it. (Mishnah Brurah 229:24 says that only this particular scenario does not invalidate the schach.)

134:5 One should add schach until there is more shade than light; if there is more light than shade in a succah, it is invalid under Torah law. We must therefore ensure that the succah will have sufficient coverage even after the schach dries. One must also be careful that there is no gap of more than three handbreadths in any one spot. (Editor’s note: three handbreadths is approximately nine inches.) Optimally, there should be some spaces among the schach so that one can see the stars, but if the schach was so thick that one cannot see the stars, the succah is still valid. (This is true even if one cannot see sunlight through the schach – Shaar HaTziyon 631:6.) However, if the schach is so thick that a heavy rain would not get through, then the structure is more like a house and not valid for use as a succah.