4. Maximum amount of Sechach-Is there a maximum amount of Sechach that may be placed on the Sukkah?
Allowing the stars to be seen: Initially, the Sechach must be thin and light enough for the large stars to be visible through the Sechach at night. However, Bedieved, even if the Sechach was as thick as the roof of a house to the point that no rays of the sun penetrate the Sechach and can be seen from it, nevertheless the Sukkah remains valid so long as rain is able to penetrate through the Sechach as will be explained next. [The Chabad custom is to place a lot of Sechach on the Sukkah and to make a hole in the Sechach to allow one to see the stars.]
How much of the Sechach must be able to be penetrated by rain and allow star viewing?
Some Poskim rule that so long as the stars are visible from one area within the Sukkah it is initially valid. [Practically, this is the Chabad custom.] However, if rain cannot penetrate in an area of four Tefachim then that area is considered invalid Sechach for all purposes. Other Poskim, however, rule that so long as there is a 7×7 area that allows penetration of rain, the entire Sukkah is Kosher.
Is rain protective Sechach made in a way that it drains the water to outside the Sukkah valid for use?
This matter is debated amongst the Poskim. Some Poskim rule that it is valid, as practically it contains areas of space from which the stars can be seen, and mosquitos and wind can penetrate, and is hence not similar to the roof of a house. Accordingly, one is encouraged to buy such Sechach in areas that it rains in order to be able to live in the Sukkah even during rain. Other Poskim, however, rule that such Sechach is invalid as practically it does not allow rain to penetrate and is henc similar to a home. Practically, it is not to be used.
The Chassidic meaning behind allowing the stars to be seen:
The Sukkah represents the drawing of an Oar Makif down below to the Jew dwelling inside. The purpose of this Oar Makif is to be drawn internally into the Jew. This is represented by the stars which represent a glimmer of the Makif light.
 Admur 631:5
 Admur ibid; Michaber 631:3; Rambam Sukkah 5:21; Rosh 2:3; Yerushalmi 2:3; M”A 631:1
Other opinions: Some Poskim rule that it is permitted to make the Sechach even very thick, as even if stars cannot be seen through it one can see the rays of the sun from many areas. [Darkei Moshe 631 in name of Maharil, brought in M”B 631:5]
 Admur ibid; Michaber ibid; Mishneh Sukkah 22a and Beis Hillel 22b; Bach 631; Taz 631:2
 Sefer Haminhagim p. 65; Hisvadyus 1990 1:164 that so was the custom of the Rebbe Rayatz and the Rebbe Rashab; Lishmoa Ozen p. 20-21 in name of Rav Issaac Homlir that so was the custom of the Alter Rebbe in Liozna; So was also the custom of the Rebbe Rashab [Lubavitch Vechayaleha p. 31]; This was likewise the custom of the Rebbe. [Otzer Minhagei Chabad p. 366]; See Hamelech Bemisibo 2:192; Likkutei Sichos 9:231
 Hisvadyus 1990 1:164
 Admur ibid; M”A 631:2; Bach 631 and 635; Levush 631; Tur 631 in name of Rabbeinu Tam; Hagahos Maimanis 5:6 Tes; Mordechai Remez 732; Mateh Moshe 904; Elya Raba 631:4; Chayeh Adam 146:18; M”B 631:6; See Beis Yosef 631 and 635
Other opinions: Some Poskim rule that the Sechach remains valid even if it is very thick and rain cannot penetrate it, and that in a time of need one may rely on this opinion. [Implication of Michaber ibid, brought in Birkeiy Yosef 631:2, brought in Shaareiy Teshuvah 631:1; Mamar Mordechai 631:1; Bach 631 in name of Tur; Shivlei Haleket; M”B 631:6]
 M”B 631:6
 The reason: The reason for this is because if the Sechach prevents penetration of rain it is not considered a Sukkah, as a Sukkah is defined as a form of roofing that protects only from shade and not from other matters. However, if it is also able to protect from rain then it is not considered a Sukkah but rather is similar to a house. [Admur ibid; See also M”B 631:6]
 See Piskeiy Teshuvos 631:2
 Peri Megadim, brought in M”B 631:5
 See Hisvadyus 1990 1:164
 Kenad Renana O.C. 67; Pischa Zuta 631:7;Piskeiy Teshuvos 631:2
 See Piskeiy Teshuvos 629:17
 Shraga Hameir 6:111; Ruling of Rav Elyashiv, Rav Chaim Kanievsky and other Rabbanim of today, brought in Piskeiy Teshuvos 629 footnote 81; Teshuvos Vehanhagos 2:310
 Kaneh Bosem 2:26 and that so agreed the Minchas Yitzchak; Lehoros Nasan 5:43
 Shevet Halevi 7:60; Kinyan Torah 5:66
 Likkutei Torah Vayikra 1st Mamar