Sources and Reasoning:
The Shulchan Aruch, Orach Chaim 630:2, rules as follows:
If the walls of the sukkah consist of two walls adjacent to one another in the shape of a gimel, he makes a wall slightly wider than one tefach and places it within less than three tefachim of one of the walls. He then places a pole opposite that tefach and makes a tzurat hapetach by placing a pole over it and over the tefach. It is kosher even though the pole above them does not touch them. Gloss: If the tefach and the wall reach the sechach, no pole above them is required.
It is explained in the words of the Shulchan Aruch that where there are two walls in the shape of a gimel—that is, in the shape of the letter L—it is sufficient for the third wall to consist of a wall slightly wider than one tefach, positioned within less than three tefachim, together with a tzurat hapetach completing the required width of seven tefachim for the wall.
The reason is as stated in the Gemara, Sukkah 6a: We derive from the words basukkot, basukkot, basukkot that three walls are required—namely, two proper walls and a third wall of even one tefach. That tefach is positioned at a distance of slightly less than three tefachim in order to create the significant measure of a wall of four tefachim, and the remainder is completed with a tzurat hapetach.
Regarding a sliding glass door, since it has a track above and at the sides, it is considered a tzurat hapetach. See Sefer Chut Shani, Hilchot Sukkah, p. 225, which states this.
Furthermore, even if there were only two halachically valid walls arranged in the shape of the letter L, and in addition a fourth wall consisting of four tefachim of a wall, with the remainder being a tzurat hapetach through the sliding glass door, the sukkah would be kosher and no additional fourth wall would be needed at all.
Even if one of the two sliding glass doors remains fixed and only the other is opened and closed, the fixed door is considered a wall of four tefachim. Together with a tzurat hapetach of four tefachim, this constitutes a valid third wall lechatchilah.
This response was translated automatically from Hebrew and has not yet been reviewed by a person. Therefore, it may contain inaccuracies and should not be considered a definitive reference.
