It is called “modified bitumen” (pronounced buy-TOO-men) and consists of a tough mat embedded in a special asphalt formula that includes elastic additives. That allows the material to expand and contract with temperature changes. The surface is covered with ceramic granules to provide solar protection.
Modified bitumen or “modified” as it is usually called, is especially popular on small residential jobs. That’s because it doesn’t require a hot asphalt kettle along with the inherent safety hazards and obnoxious fumes.
Modified can be installed several different ways, but one of the most common methods is with a propane torch. That is also the technique responsible for most of the leak problems with this system when the seams are not heated properly. If you overheat the material, too much asphalt flows away from the seam. Not enough heat results in cold seams that are not completely bonded. Either case creates seams that are vulnerable to water intrusion. If the roof ponds water and the roofer didn’t install a protective mid-ply, leaks are nearly inevitable.
Fortunately, this is a fairly simple flat roof repair. You will need these materials:
Flashing cement. Be sure to get the MB (modified bitumen) variety, as the standard type is not 100% compatible.
Membrane. Either 4″ or 6″ is fine, but be sure to get fiberglass rather than cotton, as it lasts a lot longer.
Granules. For a small repair, you might find enough loose ones on the roof or in the gutter to reuse.
Start by CAREFULLY probing the seams with a small pen knife, looking for voids. Ideally, the seams should have about ½” of asphalt bleed-out. Any areas that don’t are suspect.
The seams need to be both clean and dry to get a good flat roof repair. Spread a thin layer of flashing cement directly over the problem area, slightly wider than your membrane. Test the bond you are making by “tapping” the cement with your trowel. If it comes up, leaving bare patches of modified, the surface is still too wet or too dirty.
Next, keeping the membrane taunt, set it in the thin bed of flashing cement. Work it in with your trowel until the membrane is embedded tightly.
Then spread another layer of flashing cement over the membrane. Again, you don’t need a lot. Just enough to cover the scrim is sufficient.
To complete your flat roof repair, broadcast granules into the fresh cement. That will dress up the repair and protect it from the sun.