How to Determine Roof Sheathing

Determine the sheathing’s thickness by the rafters’ spacing, which is typically 24 inches on centers. Use five-eighths inch plywood or OSB for 24-inch spacing. Move up to three-quarters inch if rafters are spaced wider than 24 inches or if roof loads, weight of snow accumulations in severe climates, are extreme.

Measure the roof’s dimensions with a tape measure or use a house plan to get the actual length of the roof from end rafter to end rafter, including any overhang, and down the slope from peak to eave or side overhang. Get the actual run, not just half the roof’s width. Allow for the difference in length of rafters for the pitch or slope. A slope of 5-inches per foot with an 18-inch eave has a run of 174 inches or 14 1/2 feet.

Multiply with a calculator the run times the length; 14.5 times 30, for example on that roof 30 feet long, or 435 square feet. Divide that by 32, the square feet in a 4-by-8-foot sheet of sheathing. Multiply that number, 13.6, times two to get the number of sheets needed, 27.2. Add about 10 percent for trimming or other waste and order 30 sheets to cover both sides of that roof.

Estimate sheathing needs for other roof styles with the same basic method. Figure for only one slope on a shed or pent roof. Allow more trim and wastage on a hip or gambrel roof because different angles on those roofs will change the coverage pattern.