Succah 1:5

Bundles of straw, wood or reeds may not be used for the roof of a succah but if the bundles are untied, they are valid. Any material may be used for the walls.

Succah 1:6

Rabbi Yehuda says that a succah may be roofed with wooden boards but Rabbi Meir invalidates such a succah. If a board four handbreadths wide (about one foot) is over one of the walls, the succah is valid but one may not sleep under the board.