The flat roof typifies modern and contemporary architecture, however a flat roof is prone to leak. The slope on a flat roof is too small to shed water, and the shallow slope tends to collect leaves and debris that act like dams. A sloped roof is more capable of removing water from the building's exterior, and can be an attractive alternative to the flat roof.
Remove the roofing membrane of the flat roof. Inspect the roof structure for water and pest damage. Replace any rotten joists or plywood decking. Double-check the stability of the structure. If there is any lateral movement, the structure must be reinforced or replaced.
A flat roof is easily replaced by a one-way slope or shed roof. The structure does not need to be reinforced, and the surface area is not that much greater for a shallow sloped shed roof.
Determine the slope of the roof. Roofs with a slope of 1 to 3 inches of fall to 12 inches of run should use rolled roofing. Slopes above 3 inches fall to 12 inches run can use shingles.
Build a stud frame wall at the high end of the proposed roof structure. Anchor the wall to the existing roof deck. The height of the wall is proportional to the roof's determined ratio of fall to run. For example, a roof with a slope of 4 inches of fall to 12 inches of run will require a stud wall that is 2 feet tall, if the roof surface is 6 feet deep.
Cut rafters to the hypotenuse of the roof plane depth and stud wall height. At the intersection with the existing roof decking and stud wall, make bird-mouth cuts and fix the rafters at 16 inches on center using toe-nailing or joist hangers. Plumb cut the ends of the rafters, and place blocking between the rafters. Brace the rafters at the midpoint or third-points if the depth of the roof is too great. Screw exterior grade plywood to the rafters, frame the gaps at the triangular open ends of the roof structure and cover with plywood or beadboard. Using the same finish board, sheath the stud wall at the high end of the roof.
Apply the polyethylene sheeting and roof underlayment, and place the rolled roofing or shingles. Trim the roof assembly with fascia boards and rakes along the edges. Flash any intersecting planes on the exterior of the structure. Paint the exterior wood surfaces. Roofs that are above air-conditioned spaces should be insulated with rigid insulation and blown cellulose insulation.