Fictional walls TB Sukkah 17 and 18

The rabbis have a very good and practical imagination when it comes to the building of a sukkah. Previously in the Gemara we learned about the two principles ascending walls, gud asik (גוּד אָסִיק), and descending walls gud akhit (גוּד אָחִית). As long as there is part of an actual physical wall, but it doesn’t reach to the ground or doesn’t reach to the roof of the sukkah we can consider the wall continuing up or down as the case may be and completing a kosher wall.

On dappim TB Sukkah 17 and 18 that the principal of lavud (לָבוּד) not only applies to walls but also to the sekhakh. The principle of lavud says that if there is a gap less than three tefakhim, one may consider the gap filled in and nonexistent. The example in massekhet Eruvin is a series of polls in a line less than three tefakhim apart from one another creates a “solid” wall. Our Gemara differentiates between airspace and unkosher sekhakh from the edge of the wall. “Rabba said: I found the Sages of the school of Rav, who were sitting and saying in the name of Rav: Space without roofing renders the sukka unfit with a measure of three handbreadths of space. However, unfit roofing renders the sukka unfit with a measure of four handbreadths.” (Sefaria.org translation) If the airspace is less than three tefakhim, we treat the area as if it is filled in. (Shulkan Arukh, Orekh Hayim, 632:2)

If the nonkosher sekhakh is less than four tefakhim from the wall we apply the principle of a “curved” wall (דּוֹפֶן עֲקוּמָּה). “as both Rav and Shmuel said that in this case, the Sages in the mishna touched upon the principle of curved wall. In other words, the fact that this house is a fit sukka is unrelated to the minimum measure of unfit roofing. It is fit due to the principle of curved wall.” (Sefaria.org translation) Imagine the wall moving in a diagonal direction in no more than four tefakhim to reach the kosher sakhakh. There are some commentators like Rabbi Nisan Gaon who explained that the wall curves to the kosher sekhakh. Most commentators however explain that we consider the nonkosher sekhakh as if it is part of the wall and not part of the roof.

Rava and Abaye disagree about the last imaginary wall, the edge of the roof descends and seals (פִּי תִקְרָה יוֹרֵד וְסוֹתֵם). “If one roofed a portico that does not have posts on its open side, Abaye said: The sukka is fit, and Rava said: The sukka is unfit. The Gemara elaborates: Abaye said: The sukka is fit, as we say that the edge of the roof descends and seals the opening. The edge of the roof itself is considered as though it were a small partition that extends downward and forms a wall. Rava said: This sukka is unfit, as we do not say that the edge of the roof descends and seals. (פִּי תִקְרָה יוֹרֵד וְסוֹתֵם)” (Sefaia.org translation) of course the edge has to be at least one tefakh. The Gemara later limits the disagreement between Rava and Abaye. Rava accepts the principle of the edge of the roof descends and seals when the edge was made for the purpose of the interior of the structure. If the edge was part of an exterior wall of a porch that has no polls supporting it, the edge cannot descend and seal the interior space of a sukkah. “and Rava could have said to you: Rav stated his opinion only there, with regard to a portico in the field, because the partitions formed by the descent of the edge of the roof are partitions established for the portico. However, here, in the case of a sukka, where the partitions formed by the descent of the edge of the roof are not partitions established for the portico, no, Rav would not say that the edge of the roof descends and seals.” (Sefria.org translation)