I get Shmuel's opinion fine, but Rav Huna's 1 tefach opinion and Rav Chisda and Rabah bar Rav Hunas' 4 tefach opinions seem to be needlessly deviating from the pshat of the Mishna.
Why do they feel the need to start looking for alternative definitions of the word Sukah other than the obvious 10 tefach one?
Martin Ross, Manchester, UK
Rashi points out a fundamental idea in the beginning of the Masechta: "It is called a 'Sukah' because of the Sechach." Applying this to the Mishnah here, it is not a deviation from the Peshat to say that "Sukah" refers to another layer of Sechach, as this indeed is the primary meaning of the word "Sukah." This is all the more applicable when we are discussing when the Sechach of a Sukah becomes invalid for having Sechach Pasul on top of it, as the Mishnah just discussed the case of a Sukah under a tree which clearly does not have to have walls to cause the Sukah underneath it to be invalid.
(Indeed, the Sefas Emes notes that while the others clearly do not require Hechsher Sukah, as they are discussing the case of Sukah Tachas Sukah as though it is Pasul Sechach, it is unclear whether Shmuel requires Hechsher Sukah (i.e. including the walls, etc.) to be called a Sukah Tachas Sukah.)
Accordingly, each opinion tries to understand how much separation is called another Sechach/Sukah that will cause a Pesul, as explained by the Gemara.
All the best,
Yaakov Montrose