Some halachos of building a succah

This post, Some halachic points regarding building a succah , covers many of the main issues.

I would like to add an addtional point.

When do you say דופן עקומה?

R' Akiva Eiger has a famous teshuva where he quotes the Ran that you cannot put together 2 הלכות למשה מסיני to make a wall. Therefore the Ran (and others) hold that you you can only say דופן עקומה when the walls reach the סכך. If the walls don't reach the סכך, then you need the halacha of גוד אסיק to complete the wall and the halacha of דופן עקומה. This is 2 הלכות למשה מסיני which according to the Ran doesn't work.

This idea of 2 הלכות למשה מסיני can't be used together is only if the 2 are not independent. However, if they are independent then there is no issue. In the case of the walls not reaching the סכך they are not independent, you can only say דופן עקומה after you complete the wall with גוד אסיק. However, the following case would be good even according to the Ran. You have דופן עקומה and then you have a 2 טפח gap in the סכך. here you can say both halachos as the דופן עקומה stands independently of the לבוד and the לבוד is independent of the דופן עקומה.

The Mishna Berura quotes this chumra (he says it seems like the Shulchan Aruch holds this way). However, he comments that if the wall is within 3 טפחים of the סכך everyone agrees that it would be good. At first glance the MB is very difficult, why should לבוד be any different then גוד אסיק, they are both הלכות למשה מסיני and you can't say דופן עקומה without the לבוד. The מקראי קדש answers that maybe the pshat is that you can't be מכשיר a wall with 2 הלכות למשה מסיני of walls. However, לבוד works everywhere and is not a halacha of walls. Therefore לבוד would work even in conjunction with another הלכות למשה מסיני.

This is an important MB, without it we would almost never say דופן עקומה. In all of the succahs that I see the סכך doesn't actually rest on the walls it usually rests on some supports which rest on the walls. The סכך is generally a few inches on top of the wall (especially if you have a non-wooden succah where you have an issue of מעמיד). Therefore we are always relying on לבוד to fill in that little gap. Without the MB you would never be able to say דופן עקומה because you are relying on 2 הלכות למשה מסיני.