Roof boards are used for sheathing under materials that require solid and continuous support, such as asphalt shingles, composition roofing and metal-sheet roofing, must be laid closed.
Closed roof sheathing may also be used for wood shingles. Closed roof boards should preferably be of dressed and matched or shiplap material, 8 inches in width and 1 inch in thickness. Where rood boards are re-used from concrete dormwork, they should be thoroughly cleaned and redried before use.
Boards should be nailed with two eightpenny nails at each bearing, and joints should be made over the center of the rafter. To obtain good framing anchorage, it is desirable to use long boards, especially at roof ends.
Where end-matched boards are used, the joints may be made between rafters, but in no case should the joints of adjoining boards be made over the same rafter space, and each board should bear on at least two rafters.
Spaced Sheathing Installation
When wood shingles are applied to nailing strips, which may be 1 by 3 or 1 by 4 inch strips, they should be spaced the same distance apart on centers as the shingles are to be laid to the weather. This form of construction is commonly used, paricularly in damp climates.
Plywood Roof Sheathing
When plywood sheathing is used, it should be laid with the face grain perpendicular to the rafters. Sheathing-grade unsanded plywood is generally used. Joints should be made over the center of the rafters.
For wood or asphalt roof shingles with a rafter spacing of 16 inches, 5/16 inch plywood is recommended. For slate and tile shingles, 1/2 inch plywood for 16 inch rafter spacing, and 5/8 inch plywood for 24 inch rafter spacing should be used. Sheathing should be nailed to rafters with sixpenny nails, spaced 6 inches apart on edges and 12 inches elsewhere.
If wood shingles are used and the plywood sheathing is less than 1/2 inch thinkness, then 1 by 2 nailing strips, spaced according to shingle exposure dimensions, should be nailed to the plywood. Plywood roof sheathing, unless of the exterior type, should have no surfaces or edges exposed to the weather.