Choose a clear, sunny day to repair the flat roof. Sweep any foreign matter, such as dirt, gravel and leaves, off the entire roof with a broom.
Inspect the roof for rust patches, which occur with flat metal roofs. Scrub the rust off the roof with a wire brush, and then evenly spray the area with metal primer. Let the primer dry overnight.
Rinse the flat roof thoroughly to prepare a clean surface for repairing the leaks. Spray the entire roof with water, and then allow the roof to fully dry for 24 hours.
Make sure the temperature is above 65 degrees before repairing the roof. Put on full-length clothing, protective glasses and rubber gloves.
Spread elastic crack sealer over the damaged regions of the flat roof using a stiff-bristled paintbrush. Evenly coat the crack or split with the flexible compound, extending it 2 inches beyond the patched regions.
Cover the wet sealer with reinforcement fabric. Press the fabric into the coating using a trowel.
Brush the elastic crack sealer over the reinforcement fabric using the paintbrush. Fully cover the fabric with an even coat of sealer, and then let the coated area cure for 48 hours.
Apply elastomeric roof coating on a sunny, dry day in temperatures over 65 degrees. Using a three-quarter-inch nap roller, cut in along the edges of the roof and around roof penetrations with the coating.
Coat the entire flat roof with the elastomeric product using the nap roller. Apply the elastomeric coating in smooth passes, working back and forth across the expanse of the flat roof. Use 1 gallon of coating per 50 to 75 square feet of porous roofing, such as roll roofing, cement and previously coated roofing, while metal roofs require a thinner coat at 1 gallon per 80 to 100 square feet. Let the first coat of elastomeric coating cure for 48 hours.
Apply the second and final coat of elastomeric coating to the flat roof using the same previous techniques. However, apply the passes in the opposite direction to ensure even coverage. Let the final coating cure for 48 hours.