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A poorly constructed flat roof is riddled with problems. Unlike slanted roofs that shed water and snow, a flat roof retains pools of moisture and weight of snow, sprouting roof leaks that can damage the interior of the building. To construct a DIY flat roof that endures through rough weather and blistering sun, professional roofers offer basic tips for the project. These tips include framing the roof so that it has a slight incline, covering the roof with plywood decking, taking advantage of new technology in roofing materials, securing the flashing well, and keeping the roof meticulously maintained.
The best DIY flat roof is, technically, not completely flat. All flat roofs have some incline. Small flat roofs may tilt to one side, while large roofs may slope toward the center of the roof — where water flows toward a large central drain. The most basic do-it-yourself design features an inclined flat roof, such as for a garage or a house addition. The slope of the roof should measure at least 0.125 inch (0.3 cm) to 0.25 inch (0.6 cm). To create the slope, the homeowner can cut small wedges of lumber to secure below the horizontal rafters, or cut and nail the rafters at the proper angle directly.
The best DIY flat roof decking material is 0.625-inch (1.6 cm) exterior-grade plywood. This plywood should be placed with a 0.125-inch (0.3 cm) expansion gap between boards and secured with deck screws. Although the gap may seem counterproductive to preventing leaks, the space allows for the natural expansion and contraction of wood, reducing the chances of bulging and cracking.
Professional roofers then install a rigid foam and fiberglass board product called iso board. This material cushions the area between the hard plywood and one of the best DIY flat roofing materials, Ethylene Propylene Diene Monomer (EPDM). It is sold in rolls and resembles the thick plastic sheeting for lining small ponds.
Before installing the EPDM sheets, the roof should be thoroughly swept or blown off with a leaf blower. This will remove stones, twigs and other debris that can puncture the EPDM sheeting. Once the roof is clean, the homeowner will need to pre-measure and cut the EPDM sheets to fit, providing a minimum 6-inch (15 cm) overlap for all seams. The material should be glued down according to manufacturer directions. Projections such as chimneys and skylights must be meticulously flashed to prevent leaky seams.
Constructing a DIY flat roof is a one of the most difficult do-it-yourself projects. The flat roof must be perfectly framed to direct water flow away from the center of the building and avoid destructive leaks. Seams must be sealed well, as all projections need careful sealing. With these tips and careful yearly maintenance, the DIY flat roof can endure for several years.