The Gemara (in several places, here's one) lists a group of three halachos related to walls that were taught to Moshe at Sinai:
- gud [asik/achis] - extending a partial wall up or down under certain circumstances
- lavud - a 3 tefach gap is considered closed for many purposes
- dofen akumah - you can regard a ceiling as part of the wall next to it
The first two are applied all over the place, such as eiruvin and kilayim. On the other hand, I've never heard an application of dofen akumah outside of sukkah. Are there any?
I'm wondering because from the way they're grouped, it sounds like the three halachos are all generally applicable to walls, not localized to one specific area of halacha like sukkah.