The simplest, least expensive roof option is an asphalt shingle roof. These roofs require a minimum slope of 3:12 (3 feet vertical rise for every 12 horizontal feet). The simplest design option for a roof is a gable roof with no chimney and only one central plumbing vent penetrating the roof deck (see Figure 6). The simplest framing method is a central ridge beam with rafters extending from the eaves to the ridge. The ridge beam is only actually necessary over the living area. In other areas, the top plate of the central walls can support the rafters. Figure 6 illustrates the simple design concept for this roof.
Figure 6. Roof Design (1-foot grid)
For dimensional lumber made from Douglas Fir, Spruce, or Pine, 2×8 rafters will work, and two 2x12s placed side-by-side will work for the ridge beam over the living area. The rafters should be attached with Simpson hurricane ties to both the ridge beam and the top plate of the outer walls.
After the beam and rafters are in place, fasten the roof decking (7/16 OSB) to the top of the rafters using 8d nails spaced six inches on center. Use OSB clips between sheets of OSB to further strengthen the roof decking. The easiest way to lift the OSB without heavy equipment is to sit them on top of something sturdy (like a bench) and lean them against the tail of the rafters. It is important to stagger each successive row of OSB, progressively working up the slope of the roof, so that the seams in the OSB are offset.
After installing the OSB, use 1” roofing nails to attach flashing, felt underlayment, and shingles. Nail drip edge flashing along the eaves (the lowest edges of the roof). Next, apply 30-pound roofing felt using the fewest nails possible. Make sure that the printed lines are facing up – they will serve as guidelines for installing the shingles. Overlap at least 2 inches horizontally, and 4 inches on the ends of the roll. Make sure that the final piece of felt is centered on the roof ridge and attached on both sides of the roof. Next, install drip edges over the underlayment along the rake ends (the slanted edges of the roof). Begin installing shingles at one of the bottom corners of the roof, with a starter strip of shingles from which the mineral-surfaced portion has been removed. Cut 3 inches from the end of the first strip so that the cutouts on the first regular course will be properly staggered. Nail the starter strip 3 inches above the eaves at 12 inches on center, and make sure the nails you place will be covered by the course that will follow. After the starter strip is installed, proceed with each successive course, ensuring that they are properly staggered to look visually appealing and so that cutouts on two adjacent courses do not line up. For most typical shingles, this involves a 6-inch offset with adjacent courses. You can often minimize waste in trimming the first shingle by starting each row on the side where you finished the previous row and using the leftover piece from trimming the last shingle of the previous row.
The underlayment should be cut close around the vent pipe. When you reach the vent pipe during shingling, cut the top of the downslope piece of shingling to fit around the vent pipe. Install a Neoprene boot with flashing over the vent pipe, and install the upslope piece of shingling over the flashing by cutting it to fit around the boot, and setting the downslope edges of that piece in plastic cement.
When you reach the top of the roof on both sides, cut single shingles out and fold them over the center of the ridge, attaching one nail near the edge of the side that does not have the mineral surface. If there is a prevailing wind direction for your area, start on the side of the ridge that it usually downwind. Continue attaching shingles to the ridge in this way, overlapping them appropriately until you reach the other side of the ridge. Use plastic cement to attach the last shingle, so there are no exposed nails showing.