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Begin applying the sheets from the bottom of the roof (the eaves) and work your way up to the ridge. The last course at the top may need to be cut if the roof is not in 4 foot increments. It is important here that the sheathing at the eave line be exactly perpendicular to the rafters so the sheets will meet at the centers of the rafters. It is more important that it be perpendicular than that it is flush with the rafter tails. A tapered piece of sheathing can be cut to fill in at the eave if necessary.
Usually, building codes, which can vary, require nails every 6" on the edge and 12" in the field. Snap a chalk line across the sheets to mark the centers of the rafters for a nailing guide. Do not nail the edge rafters where the sheathing meets until the adjoining sheet is in place. This will enable you to move the rafter a bit if needed, so that the sheathing meets in the center of the rafter.
Stagger the joints of each course of sheathing. This can be done efficiently by cutting a panel in half and using these half sheets to start every other course. Special metal plywood clips will add stability to the splices where the sheets meet between rafters
Carefully work your way up to the peak of the roof. Check for alignment and end support as you go. For safety, temporarily nail a 2 x 4 "toe board" horizontally across the lower panel of sheathing to brace yourself against as you add the second and subsequent courses of sheathing.
Sheath one slope of the roof at a time, ripping the top course to the needed width at the ridge. When one slope is completely sheathed, pop a chalk line down any slope edge (as in a hip roof) that needs to be cut at an angle. You may prefer to cut these panels before you nail them in place. Set your circular saw to the correct depth and angle for cutting along the edge and saw off the excess overhang.