Rif Succah 5b {Succah 10a continues; 10b}

HIDE/SHOW IMAGE
5b

{Succah 10a continues}
And the halacha is like Rav Chisda and Rabba bar Rav Huna, for we deduce carefully from a Mishna like them -- for it teaches, "Rabbi Yehuda says, 'If there are no inhabitants in the upper one,' " and we explain that this means "if it is unfit for dwelling." We may then deduce that according to the first Tanna, it is even when it is not fit for dwelling. And what is this? When there is less than 10 {handbreadths, and} the bottom is invalid. Thus it is not in accordance with Shmuel and Rav Huna.

Mishna:
If he spread upon it {=the schach} sheets because of the sun or below it because of the debris, or if he spread it upon a four-poster bed {kinoph}, it is invalid. But one may spread over two-post beds {naklitei mitta}.

{Succah 10b}
Gemara:
To explain kinophot -- they are four, and when he spreads a sheet apon them, they make a succah within the succah, and therefore it is invalid.
And naklitin are two, ane when he spreads upon them it does not make a tent