MISHNA: A sukka that is meduvlelet and whose shade exceeds its sunlight is fit. A sukka whose roofing is thick like a house of sorts, even though it is so thick that the stars cannot be seen from within it, is fit.

GEMARA: The Gemara asks: What is the meaning of meduvlelet? Rav said: It means an impoverished sukka, i.e., a sukka whose roofing is sparse, although at no point in the roofing is there a gap of three handbreadths. And Shmuel said: It means that the roofing is aligned with one reed ascending and one reed descending. There are two layers of roofing, with each reed on the upper layer situated directly above the space between each reed on the lower level.

The Gemara explains: Rav taught the first clause in the mishna as one halakha, and Shmuel taught that clause as two halakhot. Rav taught one halakha: The halakha of a sukka meduvlelet. And what is a sukka meduvlelet? It is a sparse sukka. Nevertheless, as long as the shade exceeds the sunlight the sukka is fit. And Shmuel taught two halakhot. What is a sukka meduvlelet? It is a disordered sukka. And he teaches two halakhot: A disordered sukka is fit, and one whose shade exceeds its sunlight is fit.

Abaye said: They taught that a sukka with two layers of roofing is fit only in a case where there is not a gap of at least three handbreadths between the top and bottom layers. However, if there is a gap of three handbreadths between them it is unfit. Rava said: Even if there is a gap of three handbreadths between them, we say that the two layers of roofing are not considered joined only in a case where there is not the width of a handbreadth in its upper roof. However, if there is the width of a handbreadth in its upper roof, even if the gap between them is three handbreadths, the sukka is fit, because we say that the principle: Lower and cast down the upper level of the sukka roofing down to the level of the lower sukka roofing, applies here.

Rava said: From where do I learn to say that when there is a handbreadth of width in its roof, we say that the principle lower and cast applies, and when there is not a handbreadth in its roof, we do not say that the principle lower and cast applies? Rava learns this from the halakha of impurity imparted by a corpse, as we learned in a mishna: With regard to the boards of the ceiling of the first floor of the house and of the second story that do not have plaster on them, so that each of the boards is considered a separate entity, and the boards of each are aligned so that the boards of the ceiling of the second story are directly above the boards of the house: If there is a source of ritual impurity imparted by a corpse beneath one of the lower boards, any object that is directly beneath that board is rendered impure by means of a tent over a corpse. However, any object that is above the board or off to the side remains pure.

If the source of impurity is in the airspace of the second story between the lower and upper boards, any object between the two boards is impure; however, any object beneath the lower board or above the upper board or off to the side remains pure. If the source of impurity is atop the upper board, any object aligned with the source of impurity even up to the heavens is impure. However, if the upper boards are spaced between the lower boards, if the source of impurity is beneath any of the boards within the house, any object that is beneath any of the boards is impure, as the legal status of the roof is as though the upper boards were lowered to the level of the lower boards, and the result is one continuous ceiling. If the source of impurity is above them, i.e., above the top boards, any object aligned with the source of impurity even up to the heavens is impure.

And it is taught in the Tosefta elaborating on this mishna: In what case is this statement said? It is only when these boards have the width of a handbreadth, and between them is the space of a handbreadth. However, if the boards are close together and there isn’t even the space of a handbreadth between them, then if the source of impurity is directly beneath one of the boards, only objects in the space beneath it is impure, while an object between the two layers of boards and atop them remains pure. Apparently, when there is a handbreadth in the upper layer, we say lower and cast the upper sukka roofing down to the level of the lower sukka roofing. And when there is not even a handbreadth in the upper layer, we do not say lower and cast the upper sukka roofing down. Indeed, conclude from here that this is the halakha.

The Gemara relates: Rav Kahana sat in the study hall and stated this halakha of Rava, that in a case where the upper sukka roofing is a handbreadth wide, even if the gap between the two layers of roofing is greater than three handbreadths, they are considered attached. Rav Ashi said to Rav Kahana: Is it so that wherever there is not the width of a handbreadth, we do not say lower and cast?

Isn’t it taught in the Tosefta with regard to the halakhot of the merging of alleyways that one of the means of rendering carrying in a closed alleyway permitted on Shabbat is by placing a beam one handbreadth wide over the entrance of the alleyway within twenty cubits but no less than ten handbreadths off the ground? With regard to a cross beam that projects from this wall of an alleyway but does not touch the other opposite wall, and similarly, with regard to two cross beams, one projecting from this wall and one projecting from the other opposite wall, and they do not touch each other, if there is a gap of less than three handbreadths between the beam and the wall, or between the two beams respectively, one need not bring another cross beam to render the alleyway fit for a person to carry within it. This is because they are considered joined based on the principle of lavud. However, if there is a gap of three handbreadths, one must bring another cross beam.

However, Rabban Shimon ben Gamliel, who holds that the principle of lavud applies to a gap of up to four handbreadths wide, says: