Much of this is taken from last years posts, however, I reworked it and added some additional material.
As now is succah building time I would like to post some common halachic issues that come up when building a succah.
When I was growing up, canvas succahs were very prevalent and in fact, last year when I was in the US for Succos I still saw quite a few. These kinds of Succahs (as well as other non-wood succahs) have a number of halachic problems which I would like to address. For those who aren't familiar, a canvas succah is made of a frame of metal pipes on which canvas is hung to create the walls.
There are 2 issues with this kind of succah:
1. The walls move which may invalidate the wall completely which would invalidate the succah. The gemara, shulchan aruch etc. write that if a wall moves 3 tefachim (about a foot) in the wind then the wall is considered invalid. Every canvas succah that I have seen (including the one I grew up in) has walls that move 3 tefachim in the wind and therefore many poskim are not happy with them.
There is an easy solution to the problem which I used for many years. The solution is based on 2 halachos related to walls.
1. לבוד - this means that if you have 2 objects within 3 tefachim of each other the halacha considers it as if the intervening space is filled in.
2. A wall only needs to be 10 tefachim (40 inches) high. Once it is 10 tefachim high we look on it as if it extends up to the sky.
Given these 2 halachos we can construct walls of string for a canvas succah. What we do is tie string/rope from 1 pole to another. We space the string around 8.5 inches apart so that they are within 3 tefachim of each other. With 5 or 6 strings like this we have a wall of greater then 10 tefachim. It works because starting from the bottom, the bottom string is within 3 tefachim of the ground and therefore the intervening space is considered to be filled in. Each subsequent string is placed less then 3 tefachim above the previous one again using לבוד so that we look upon the space as solid. We repeat this until the top string is above 10 tefachim. We have created a wall that is halachically kosher and in actuality serves as the wall of the succah. We repeat this for all the walls.
There is 1 very big caveat that needs to be mentioned. The acharonim (Magen Avraham סי' תר"ל ס"ק א) say that a wall created just by לבוד one way (e.g. strings across the wall like I suggested) is called a מחיצה גרועה a weak (for lack of a better translation) wall. Therefore they say that if you are creating walls like this you need to have 4 walls even though normally, a succa does not require 4 walls (like for a reshus hayachid on Shabbos), rather it only requires 3, and really only 2 plus a tefach.
This means that if you want to be machshir your canvas succah using string and לבוד you have 2 choices:
1.you must do this on all 4 walls.
2.In addition to the strings across, you put string going up and down less then every 3 tefachim the length of the wall, you end up with a real mechitza. The reason being that the strings go both ways (you end up with squares less then 3 tefachim in length and width, שתי וערב) and there is no big gap of space. Therefore you no longer need 4 walls.
There is 1 other point to keep in mind. the סכך has to be placed after you create the walls of strings, otherwise it is a problem of תעשה ולא מן העשוי. If the סכך was already put down, then you need to move the scach around to avoid this problem.
I did this for years and it worked well.
2. מעמיד הסכך על דבר טמא - the gemara has 1 opinion that you are not allowed to support the סכך on something that cannot be used for סכך. There is a machlokes harishonim whether we pasken like this opinion. The Mishna Berura mentions that l'chatchela a person should try to be machmir and the contemporary poskim also say that a person should try to be machmir. This problem applies to any non-wood succah (canvas, fiberglass, etc.) as well.
To get around this the minhag evolved to place wood poles on top of the metal walls and then rest the סכך on top of the wood poles. This makes the wood the מעמיד of the scach and the metal a מעמיד דמעמיד. There are 2 problems with this approach:
1. If the סכך would not fall without the wood then the wood is not considered a מעמיד. In other words if your succah is 6 feet wide and your סכך is 6.5 feet wide, if you just rest the סכך on wood poles it doesn't help, if you took away the wood the סכך would not fall it would rest on the metal. Therefore the metal is called the מעמיד of the סכך. The way to get around this is to make sure that the סכך would fall if you remove the wood, namely, move the סכך to one side so that it doesn't overlap the other wall (it is just very close). In that case, the סכך is truly being held up by the wood.
2. A number of acharonim point out the following. The wood that is used to hold up the סכך is in and of itself kosher סכך. therefore, why should we consider the wood a מעמיד of the סכך, rather it should just be considered סכך which is resting on the metal. I have not seen a good answer for this claim.
The Chazon Ish has an unbelievable chumra. The Chazon Ish understands (based on the Ramban in the milchamos) that even if you have a wooden succah, if the walls are held up by metal screws, that metal is considered to be a מעמיד of the סכך because if you took that metal out the walls would fall down and so would the סכך. In other words, if you have something that is mekabel tumah holding up any part of your succah such that without this piece the סכך would fall down (e.g. the walls would fall down causing the scach to fall) the Chazon Ish considers this to be מעמיד the סכך with a davar hamekabel tumah and no good. Basically according to the Chazon Ish you cannot use any metal to build your succah.
Almost no one holds like this Chazon Ish, it makes building a succah an absolute nightmare, you need to use wooden screws, etc. I remember in KBY everyone was amazed that the posek held from this Chazon Ish.
The bottom line is that with a wooden succah you avoid almost all of these problems and are yotze the mitzva. It is not difficult to build and therefore I highly recommend it.
The מקראי קדש raises the following issue which is halacha l'maase for me. He quotes an opinion that if you use the wall of a tall building as one of your walls (taller then 20 amos), even if the scach is below 20 amos (supported by something else) the wall is פסול because it is taller then 20 amos. The מקראי קדש diagress and brings a number of sevaros.
1. The fact that the wall continues up is irrelevant, for your purposes you could take it away and your succah will not be affected.
2. The gemara (succa 2a) comments that you can build your succah out of metal (e.g. a permanent structure) even though a succa has to be a דירת עראי. If so why is taller then 20 amos no good? The answer is that taller then 20 amos can only be built as a דירת קבע, the halacha requires that you build the succah in a way that it could be built as a דירת עראי. therefore, the building wall even though it is above 20 amos, since the schach is below is kosher because it could be built as a דירת עראי.
The acharonim point out that you only say דופן עקומה when the wall reaches the scach. However, if the wall does not reach the schach you don't say דופן עקומה and you don't have a wall. This is very relevant especially in the situation I described above, if you are using לבוד to make walls and you have an overhang, you can't use דופן עקומה if the wall doesn't reach the scach and therefore you won't have a wall.