How to Sheath a Roof

Determine the amount of sheathing you should buy by calculating the square footage of your roof. The square footage on rectangular sections of your roof can be determined by multiplying the width by the height. The square footage of triangular sections of your roof can be determined by multiplying the width by the height, and dividing by two. If you have trapezoidal sections of roof, add together the length of the top and bottom of the trapezoid, multiply this length by the height, then divide by two. Add together the total square footage of all sides of your roof to determine the total square footage of your roof. Sheathing plywood comes in 4-by-8-foot panels, providing 32 square feet of coverage each. Divide the square footage of your roof by 32. This is the minimum amount of sheathing you will need to cover your roof. Increase this number by 15 percent to give yourself a little extra and accommodate any mistakes you may make during the sheathing process.

Snap a chalk line over the tails of the rafters, and use this chalk line to check that all the tails are even with the chalk line. Saw the excess off any tails that poke out farther than the other tails so they fall along a straight line.

Start from the eaves of the roof and work up toward the ridge. Lay the first panel of sheathing along the bottom of the roof so that the vertical edge of the panel is flush with the gable and the horizontal edge is flush with the eave. The panels should be laid so they are wider than they are tall. Line up the vertical end of the panel opposite the end over the gable so that it falls in the center of the rafter. The rafters should be spaced so that a standard size sheathing panel will fit flush with the edge and centered over the rafter at the other end. Use a roofing nail gun to nail the panels to the rafters. Along the vertical edges of the panels, nails should be inserted up the rafters every 6 inches. Inside the panel, nails should be inserted up the rafters every 12 inches.

Lay the next panel of sheathing on the roof so that the edge of the panel is centered over the rafter that was half covered by the previous panel, and the opposite end of the panel is also centered over a rafter. Nail the panel into the rafters with the same spacing indicated in Step 3. Continue to lay the panels over the rafters until you reach the end of the course.

Use a circular saw to cut a 4-by-8-foot panel into two 4-by-4-foot panels. Use a 4-by-4-foot panel to start the second course of panels, so that the seams of panels will be staggered on the roof. Nail the second course of panels into the roof with the same method used in Steps 3 and 4. For the third course of panels, start with a standard 4-by-8-foot panel. Alternate the size of the first panels for each course until the roof has been completely covered.