May one use wood boards as Sechach?
Wooden planks: All wooden planks which are slightly wide, similar to a table, and is thus fit to support things, is Rabbinically invalid for Sechach. These planks may not even be used to support the Sechach.
Boards of less than 3 Tefach but are used for roofing: Furthermore, today in which even boards of less than 3 Tefach [24 cm] wide are used for roofs of houses, even boards of less than 3 Tefach wide are invalid to be used for Sechach due to the above decree.
Narrow boards which are not commonly used for a roofing: However boards which are so narrow that they are not at all used for a roofing are permitted to be used for Sechach, even if they are wide enough to hold fruits and bread. Nevertheless when such boards are used one must verify that rain is still able to penetrate the Sukkah. For this reason the custom became to completely avoid using even the valid boards for Sechach due to worry that one may come to set it there in a way that the rain will not be able to penetrate.
May the boards be placed with their width side up: Even if one places a wide board with its width facing upwards, and thus its width over the Sukkah is less than an amount which invalidates, it is nevertheless invalid.
 The reason: This is due to that they are similar to a hole made to hold items and were thus Rabbinically deemed able to contract impurity. [ibid]
 The reason for this is because: They are similar to the roofing of a house and if they were to be allowed people may come to likewise eat inside of a house. [ibid]