Mishnah/Seder Moed/Tractate Sukkah/Chapter 1/7
A roof of boards that hs no plaster on it- R'Yehudah says that Beis Shammai and Baeis Hillel disagree over what must be done to render it valid: Beis Shammai say that one loosens the boards and takes out one board from between every two boards. And Beis Hillel say that one loosens them or takes out one board from between every two boards.
R'Meir says: One takes out one board from beteen every two bards, and he does not loosen them; i.e. it des not help to loosen them.