Some maintain that just as something susceptible to tum’a may not be used as sekhakh, so too, the sekhakh may not be supported directly by something susceptible to tum’a, so that no one mistakenly uses as sekhakh things susceptible to tum’a or other invalid material. Accordingly, the sekhakh may not be supported directly by iron poles, beams that are susceptible to tum’a, or other materials that are invalid for sekhakh, such as plastic, which does not grow from the ground. However, even according to this view, it is permissible for the sekhakh to be supported directly by a stone wall, as there is no concern that people will mistakenly conclude that stones may be used as sekhakh.
Although the decisive majority of poskim maintain that the sekhakh can be supported by things susceptible to tum’a, le-khatḥila it is good to show concern for the stringent view and not support the sekhakh with things susceptible to tum’a. Therefore, one who uses a metal frame for the walls of the sukka should not, le-khatḥila, lay the sekhakh directly on the metal, but rather should lay wooden beams across the metal and then place the sekhakh on the wooden beams. However, one who wants to be lenient may place the sekhakh directly on the metal, and the sukka is still kosher, and he may recite the berakha in it.[7]
If one wishes to be strict and not support the sekhakh with material susceptible to tum’a, but he is concerned that an ordinary wind might blow the sekhakh off, he may place heavy wooden beams which are kosher for sekhakh atop the sekhakh. If necessary, he may attach the beams to the sukka even with material that is invalid for sekhakh, such as rope, nails, or plastic zip ties, because these materials are only “supports of the support,” while the supports themselves, the beams, are kosher as sekhakh.
If the concern is that only an unusually strong wind might blow off the sekhakh, then even according to the stringent view, one may tie the sekhakh down using ropes or zip ties, because under normal conditions, the sekhakh will stay put even without them. Their whole purpose is simply to protect the sekhakh from an unusual wind, so they are not considered to be supporting the sekhakh.[8]
Ḥazon Ish (OḤ 143:2) has a unique explanation of the view that prohibits supporting sekhakh with something susceptible to tum’a. In the author’s reading, even if the supports of the supports are susceptible to tum’a, the sukka is invalid, since the sukka could not stand without those indirect supports. Accordingly, one may not reinforce the walls with nails or screws if the sukka would collapse upon their removal. However, even according to Ḥazon Ish, this stringency applies only to items which are not kosher as sekhakh on the Torah level. Therefore, planks more than 4 tefaḥim wide may still be used as support for the sekhakh, even though they may not be used as sekhakh due to the decree against ceiling material. In practice, the rest of the Aḥaronim do not follow this novel view (MA 629:9; Pri Megadim, Eshel Avraham, ad loc. 9; Bi’ur Ha-Gra ad loc. 11; MB ad loc. 25, all based on Ritva, Ramban, and others), nor does common practice. (See Harḥavot.)
[8]. If the sekhakh will withstand a normal wind without any reinforcements, the reinforcements are not considered to be supporting the sekhakh. Therefore, it is not a problem even if they are made of something susceptible to tum’a or that is invalid as sekhakh (Rashi and Mikhtam, Sukka 21b; Ritva, Sukka 19a; Shevet Ha-Levi 6:74; Be-tzel Ha-ḥokhma 5:44). If there is concern that a normal wind would blow the sekhakh off, one should place beams that are kosher as sekhakh on it. If there is still concern that the wind will blow off the beams, he should tie them down or nail them to the walls. In this way, only the supports of the supports are susceptible to tum’a, and this is permitted even according to those Rishonim who forbid supporting the sekhakh with something susceptible to tum’a, as explained in the previous note. Only according to Ḥazon Ish is this forbidden for the Rishonim who adopt the stringent view, and even Ḥazon Ish agrees that, when necessary, one may rely on the majority view of Rishonim that one may support the sekhakh with something susceptible to tum’a. This is proper practice to avoid a situation where an unusually strong wind might blow off the sekhakh on Shabbat or Yom Tov (when it would be forbidden to replace it because of the prohibition of Boneh). If the ropes are only helpful in a case of an unusual wind, they are not considered supports at all.
We should add that the decree against ceiling material is not grounds to forbid driving nails into large beams whose purpose is to prevent the sekhakh from flying off (see MA 627:2, based on Tosafot), because only when the nails are used to reinforce the planks that are the primary sekhakh is this a concern, as explained in AHS 629:32 and SHT 633:6. However, when the nails are only in the beams holding down the sekhakh, there is no reason for concern. (This goes against the stringent ruling in Piskei Teshuvot 629:11 and nn. 26 and 62, that the nails may not pass from the beam to the walls through the mat or slats; see Ha-sukka Ha-shalem pp. 290 and 335, which states that some say that even nailing down the sekhakh itself does not invalidate it.
When tying down the sekhakh is necessary so that a normal wind does not blow it off, rope is preferable to plastic zip ties, because rope is invalid as sekhakh only rabbinically, and most poskim (even those who are stringent) allow supporting the sekhakh with something invalid only rabbinically. See Harḥavot 5:3.
