As with most things, there are different materials to choose between in the sheathing and underlayment of a roof and this is often based on the type of surface that is being used. Most of the time roofing materials are laid over solid sheathing while wood shingles and shakes are more commonly laid over spaced lumber. This is a more ‘open’ sheathing system which allows the air to circulate more freely.
The most common material used for roof decks requiring solid sheathing is plywood and it is easy to see why. It is a durable and strong material which doesn’t warp easily, even during bad and changing weather. Also, plywood often comes in 4 by 8-foot panels which allows for easy installing over rafters. Wood boards come in two sizes: 1 by 6 and 2 by 6 with tongue-and-groove edges for added stability. Houses with exposed beams in the ceilings can have either decks of tongue-and-groove lumber or manufactured fibreboard combining a solid nailing base, thermal insulation and with a finished underside providing a beautiful ceiling. Wood shingles are laid on spaced or ‘open’ sheathing. The boards used are laid horizontally and several inches apart, across either rafters or the existing roof. They usually measure 1 by 4.
Choosing the right wood for sheathing can be tricky for the novice roofer. I would advice to use plywood or really dry and well seasoned lumber. The reason for this is that if a roof is laid over so called ‘green’ lumber, which have an higher than 20 percent moisture content, it can buckle as the sheathing warps as it is drying. Always check the local building codes to find out the permissible thickness for the rafter spans. Quite a few codes permit the use of 5/16-inches plywood on roofs with 12-inch rafter spans. This would be 3/8-inches thickness on 24-inches rafter spans. I would recommend to go for a thicker one as it provides for a sturdier nailing base. Also remember to use exterior grade glues if you are using plywood on roofs. There is however no need to have exterior type veneers.
Use well-seasoned tongue-and-groove planks with a maximum width of 6 inches if you are installing board sheathing as any wider boards could easily warp. Do not use any wood that has a high number of knots or pockets of resin as it can penetrate and cause damage to the actual roof surface and this can cause a delay in the installation and further costs if it has to be redone.