Question:
What is the maximum distance allowed between the wall of a Sukkah and the Sechach? Can the wall be several inches away from the Sechach and still be considered a valid wall?
Answer:
Although a wall that is at least ten Tefachim high is a valid wall even if the Sechach is much higher than the wall in vertical distance, nonetheless, the wall must be within three Tefachim (23.5 cm) of horizontal distance from the Sechach. If there is a horizontal gap of three Tefachim or more consisting of empty air between the wall and the Sechach, the wall cannot be joined to the Sechach through the rule of Lavud and may not be considered a valid wall for that section of the Sukkah, and could potentially invalidate the Sukkah if it no longer contains three valid walls.
This rule applies specifically when there is empty space (avir) between the wall and the Sechach. If, however, the gap contains invalid Sechach (Sechach Pasul) rather than empty air, different rules apply and the wall may still be connected to the Sechach through the principle of Dofen Akumah for up to four Amos.
Conclusion:
A wall must reach to within three Tefachim horizontally of the Sechach in order to be connected to it through Lavud. Therefore, when building or inspecting a Sukkah, one should ensure that any gap between the wall and the Sechach is less than three Tefachim unless a different halachic mechanism, such as Dofen Akumah, applies.
Sources:
Michaber O.C. 630:9; Shulchan Aruch O.C. 632:2; Michaber O.C. 630:9 regarding the requirement that a wall be within three Tefachim of the Sechach when separated by empty air; Mishneh Sukkah 17a as explains Rashi; Rava in Sukkah 17a and 19a
