If, on the floor of the sukka, there is an equal amount of shade and sunlight you have fulfilled the minimum requirement of schach thickness. (ShA 1)
If the overall sukka has more shade than sunlight the sukka is kasher even though there are patches of sparse schach. There is an opinion that if an entire area of 7×7 tefachim is sparse, that part is possul. Therefore, do not sit under that part. (ShA 2 – MB 4)
The following is a list of schach thickness from thin to thick:
You can see the dim stars at night through the schach – l’chatchila.
You can only see the bright stars that are visible before sunset – l’chatchila.
You can’t see any stars – only some cracks by day. This is l’chatchila especially if the thickness makes the sukka more comfortable.
Sunproof like the roof of a house – bedieved
Rainproof – some poskim say possul. Use it only when you have no other option. (ShA 3 – MB 5,6 – ShaH 6)
If you want a thick schach, you can still fulfil the star visibility by making a sparse section in one small area. (MB 5)
A sukka that is slanted from the top to the ground is not kashera. You need to see either a horizontal tefach at the top to represent the roof, or a vertical tefach at the bottom to represent the wall. Besides that, the sukka should also be big enough to contain an imaginary box of 10h X 7l X 7w (tefachim).(ShA 10)
