Flat roofs are built up of layers of roofing felt and tar. Leaks usually occur at low spots or where the roofing felt has been damaged. In most cases, the leak is directly below the damaged spot and the damage to the roofing felt is easy to see.
If there’s still water pooled in the leak area, mop it up or soak
it up with rags, and let the surface dry. Brush off any gravel. Look for cracks in the felt or for large blisters where the top layer has separated.
To mend a blister:
Use sharp utility knife to slice blister open down middle. Cut should penetrate to full depth of blistered layer but should not reach sound roofing felt beneath it.
Lift cut edges of blister. If there’s water inside blister, press from edges in toward center to squeeze out water from between roofing layers. Soak up all the water you can with rags; then prop edges up to let layers dry. In cold weather or if layers are thoroughly saturated, use propane torch with flame-spreader nozzle to dry out felt (be sure to wear safety goggles). Carefully move flame back and forth over inside layers of blister. Roofing felt and tar are very flammable, so don’t let layers get hot enough to burn or bubble.
If there’s water under a large area of the roof, the problem is more than a simple blister; water may be running in from an adjoining pitched roof surface. In this case, it’s best to call a professional roofer.
Spread thick coating of roof cement on bottom edges of loose felt and firmly press down sides of blister.
Close blister permanently with row of 6d galvanized roofing nails along each side of slit, then spread roof cement over entire blister, making sure nail heads are well covered.