The Density of Sechach
Question: Is there a limit to how dense the sechach may be?
Discussion: There is debate as to whether sechach that allows no rain in is valid.13 Therefore, one should ideally ensure that rain can penetrate the sechach. If one has no other option, one may eat even in a sukkah that rain cannot penetrate.14
Sechach that allows no sunlight through is acceptable.15 It is preferred that one be able to see the stars at night in at least one place in the sukkah, or that one be able to at least see holes in the sechach during the daytime.16
Question: How sparse may one make the sechach?
Discussion: The following points must be considered:
The sechach must, in total, provide more shade than sunlight.17
There must be no open space of three tefachim wide (24 cm18) crossing the length of the sukkah. An open space of three by three tefachim should be avoided as well. [However, depending on the particulars in both cases, other parts of the sukkah may remain valid; in a case of need, a competent halachic authority should be consulted.]
If there is an open space of less than three tefachim, the sukkah remains valid. However, if the open space spans the entire length or width of the sukkah, or is big enough to fit the majority of one’s head or body, one may not eat or sleep under this space.19
If there is a seven by seven tefach area (56 by 56 cm20) that has more sun than shade, one may not sit in that area.21
If such an area splits the sukkah into two, the sukkah may be invalid.22
13. Mishnah Berurah 631:6, see, also, Shulchan Aruch 629:18, with Mishnah Berurah 49 there.
14. Mishnah Berurah 631:6.
15. Shulchan Aruch 631:3.
16. Mishnah Berurah 631:5.
17. Shulchan Aruch 631:1.
18. This assumes the calculation of R’ Chaim Ne’ah, whose measurements are the more stringent in this case.
19. Shulchan Aruch and Rema 632:2.
20. This assumes the calculation of R’ Chaim Na’eh, whose measurements are the more stringent in this case.
21. Rema 631:2, Mishnah Berurah 631:4.
22. Mishnah Berurah 631:4; see Hilchos Chag Bechag (I, p. 202), asserting that this applies specifically to a swath of seven tefachim, contrary to Bikkurei Ya’akov (631:12), who opines that even three tefachim is a problem.