Several kinds of wallboard are appropriate for outside application, but the types most frequently used for siding are exterior grade plywood and asbestos-cement board. The most popular size is 4 by 8 feet; it takes less labor to cover a wall with units of this size than to apply smaller units.
Wallboards are suitable for covering old as well as new outside wall surfaces. The old siding and trim may be removed or left in place, but, in either case, the surface should be covered with waterproof sheathing paper before the wallboard is applied.
If the old siding is left in place, it may be necessary, to use beveled strips of approximately the same thickness as the butts of existing siding to provide an even bearing for the new wallboard.
Trim around door frames and window sills will have to be extended to meet the new finish surface. Joints between plywood or asbestos-cement panels should be made watertight by calking, and panel strips and fastenings should be sound and secure. It is usually desirable to apply a decorative finish to exterior wallboards.