How to Build a Roof - Sheathing the Roof

It is assumed here that you will be applying a plywood, particle board or wafer-board sheathing. If you have chosen to use a rigid roofing (metal, wood shakes or shingles, tile, etc.) you can use 1 x 4 slats instead. With wood roofs, slats are a must to avoid rotting the wood shakes.

Plywood particle board or wafer-board roof sheathing is most commonly used, being low in cost and easy to apply. Choose an exterior C/D grade. Thickness will range from 3/8 to 3/4 inch depending upon local code*. When ordering the sheathing, divide the total number of square feet of roof surface by 32 square feet (a 4' x 8' panel) and add an extra 15% for waste. An air compressor with a nail gun will come in handy for nailing large flat areas like this.

Most Common Mistakes:

1. Not staggering the seams of the sheathing.
2. Aligning sheathing with the edge of the roof rather Than perpendicular to the rafters.
3. Attaching the sheathing wrong side up.
4. Inadequate nailing.
5. Panels not meeting in the center of a truss or a rafter.

Construction:

1. Check the rafter ends (tails) to be sure they are all on a straight line. If the walls are crooked, the rafter tails will also be crooked. Try to correct this problem; but if you are unable to, pop a chalk line across the rafter tails and trim them with your saw before attaching the sheathing. This is crucial because this building line will be a very noticeable one when viewed from below.
2. 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 rip 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 that the sheets will meet at the centers of the rafters it is even 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, code, which can vary locally, requires nails every 6 inches on the edge and 12 inches in the field. Pop a chalk line across the sheets to 'mark the centers of the rafters for a nailing guide.

3. 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.
4. 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.
5. 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.

6. Set your circular saw to the correct depth and angle for cutting along the edge and saw off the excess overhang.
7. Repeat this process for each slope of the roof.