a. Wood Sheathing. Wood sheathing may be nailed on horizontally or diagonally (see Figure 2-21 ) however, diagonal application adds much greater strength to the structure. If the sheathing is to be put on horizontally, start at the foundation and work toward the top. If it is to be put on diagonally, start at the corner of the building and work toward the opposite wall.

b. Gypsum Board. The long edges of the 4 by 8 boards are tongue-and-grooved. Gypsum board can be nailed (together with the wood siding) directly to the studs. Gypsum sheathing is fireproof, water resistant, and windproof. It does not warp or absorb water and does not require the use of building paper (see Figure 2-22 ).

c. Plywood. Plywood is highly recommended for wall sheathing because of its weight, strength, and structural properties. Plywood is most commonly used because it adds a lot more strength to the frame than using diagonally applied wood boards. It comes in 4-feet-wide and 5- to 8-feet-long sheets, 1/4 to 3/4-inch thick. Install the sheets with the face grain parallel to the studs (see Figure 2-23 ). It is usually applied vertically from the floor to the ceiling. When plywood is correctly applied (with flush joints), the joints do not need to be concealed. However, to improve wall appearance, joints may be covered with moldings. These may be battens fastened over the joints or applied as splines between the panels. Less-expensive plywood can be covered with paint or covered in the same way as plastered surfaces. Figure 2-24 shows how to fit plywood on rough or uneven walls.