Translation:Mishnah/Seder Moed/Tractate Sukkah/Chapter 1/6

Mishnah 6

We may cover a succah with boards; these are the words of R'Yehudah. But R'Meir prohibits them.

If one put a board that is four tefachim wide on a succah, the succah is still valid and one can discharge his obligation with it, provided that he does not sleep beneath the board.