Roof sheathing comes in grid-marked 4-by-8-foot sheets, and should be installed perpendicular to the frame. Sheathing should be a minimum of 19/32-inch thick. Never attach roof sheathing with staples; 8d ring-shank nails should be used instead. The typical thickness range for sheathing is 3/8 to 3/4 inch.
Several factors must be considered when determining appropriate roof sheathing thickness, such as the span of roof joists and snow load for the area. Tables with important factors can be found in a book called "Structural Design in Wood," written by Judith J. Stalnaker and Earnest C. Harris. It is important to check the local building codes for your area as well.
Types of Sheathing
Typical types of wood used for roof sheathing are oriented strand board, known as OSB, and plywood, the most popular being OSB. Sheets of 7/16-inch thick, with no edge support, can be used in an area where the snow load is 30 pounds per square foot. With edge support and the same snow load, 3/8-inch OSB can be used. Edge support is typically provided by tongue-and-grove sheathing panels or panel clips.
One 4-by-8-foot sheet of OSB or plywood is 32 square feet. Take the roof's total square footage and divide by 32. You should be prepared to have about 15 percent waste, so multiply your figure by 1.15. This will give you the total number of sheets to buy.
Install sheathing perpendicular to roof trusses one slope at a time. Start the sheathing at the eaves and work your way up to 1 inch from the peak. Nails, size 8b, should be installed every 6 inches on edges and every 12 inches in the center area. Be sure to stagger the vertical seams; every other course will start with a 4-by-4-foot sheet of OSB or plywood.