If you've chosen to build a backyard shed yourself rather than buying a premade model from a hardware store, you're going to have to decide on a shed roof design. Like homes, sheds can have roofs that are traditional pitched (A-shaped), shallow pitched, gambrel style, almost flat (often called a shed roof), or salt-box style to name several, and each has its advantages and disadvantages.
Designing a Roof to Maximize Storage
Because the most common reason for constructing a shed is to provide extra storage space, being able to utilize any available area inside the shed and immediately beneath the roof would add to the overall storage area of the structure. This would rule out a flat (shed-style) roof and a shallow-pitched traditional roof, which would give little if any overhead storage (these types of roofs are also prone to water leakage). If you live in an area that experiences large amounts of snowfall, flat and low-pitched roofs are prone to collapsing under heavy accumulation. One point in favor of flat and shallow-pitched roofs, however, is that they are easier to maintain and replace.
The Gambrel-Style Roof
Constructing a gambrel style roof for your shed can put your carpentry skills to the test. The gambrel shape, while providing the most available storage space, requires constructing trusses, which then must be positioned along the side walls of the shed and fastened before the roof underlayment is attached. Constructing the trusses requires cutting somewhat complex angles and fastening the trusses themselves together with metal or homemade plywood gussets before installation. If you are working alone, installing the finished trusses can be a daunting job.
The Pitched Roof
The classic traditional pitched roof provides a compromise to the gambrel style. It is easier to construct, provides adequate storage space beneath the roof, is excellent for shedding rain and snow, and can be erected working alone. Leaving the actual framing technique to install such a roof aside, once the roof rafters are fastened in place, the underlayment of the roof can be installed using sheets (4 feet by 8 feet by 1/2 inch) of exterior plywood or glued particle board rated for exterior use (which is less expensive). Because it can be a struggle if you are working alone to move the sheathing into position on the roof before fastening it in place, a slightly more expensive but much easier way to install the underlayment is to use pine boards (3/4 by 10 foot or 12 by 6 foot, 8 foot, 10 foot). Most lumberyards have stacks of pine boards that are graded #2 or #3 and are affordable. Be sure to leave a 1-inch space open along the roof's ridge for a vent to allow air inside the shed to escape to the outside for adequate ventilation.
Weather-proofing your Shed Roof
Now that the shed roof is sheathed, it's time to make it weather-tight. Before adding on the shingles, use felt paper to cover the roof surface, beginning at the bottom edge and moving upward, overlapping the felt paper by 5 to 6 feet for each new course. You can choose to shingle the shed roof with asphalt or fiberglas shingles or cedar. Unless you feel very strongly about the latter, asphalt is easier to install and can be rated to last 30 years or more. Fiberglass is more expensive but lasts even longer.
Remember to install an aluminum drip edge along the lower edge of the sheathing before beginning to install shingles, and also remember to install the first course of shingles with a double row, the bottom shingles which lay against the felt paper being turned upside down (exposed or finished face of shingle facing toward roof ridge) onto which the first finished row of shingles is fastened. When you reach the slot opening along the ridge of the roof, add the ridge vent material before capping the shingles. This venting aids in preventing mold and mildew from forming inside the shed. If you choose not to use a ridge vent, you must add a vent on each gable end of the shed just under the peak for proper air movement.