Metal flashing is required at junctures between the roof and sidewalls, at roof edges, in valleys, and around skylights, plumbing stacks, and other penetrations. We apply the flashing at the dry-in stage, placing it over the 30-pound felt and sealing it with cold-applied asphalt cement and a 4-inch or 6-inch cotton mesh reinforcing fabric.
At sidewalls, we clean up the existing flashing and inspect it for holes to see if it's still serviceable. If it has deteriorated, we replace it - even though this may require stripping off a few inches of stucco or siding to expose and remove the metal. Of course we have to replace and blend in any siding we remove.
In our part of Florida, nobody uses step flashing at wall-to-roof intersections. Instead, we use long lengths of 4-inch-by- 5-inch L-flashing, sealing the bottom leg to the felt with asphalt cement. That's enough to keep the roof dry temporarily. When the 90-pound felt or modified membrane is hot mopped on top, the flashing joint is fully sealed.
Roof edges receive drip edge. Again, the upper edge of the flashing is sealed to the 30-pound felt with asphalt roofing cement.
Valleys get special treatment. We lay down two layers of 30-pound felt - the usual horizontal courses, plus a course running down the center of the valley. Then we place a 16-inch-wide piece of metal flashing along the valley on top of the felt, to protect the felt against pinching or wear as the roof components inevitably expand and contract from temperature changes in service. Finally, the flashing is sealed to the tar paper with cement. Just before the hot mop goes on, we apply a cold-rolled asphalt primer to all the metal flashings to ensure a good seal.
Most reroofs include skylight replacement. We prefer self-flashing skylights with an integral flashing built into the frame, such as the dome skylights from Texas-based Maxim (maximskylights .com). These units mount directly on top of the 30-pound felt and bolt down through the felt and roof sheathing into the framing. The worker applies the cold asphalt cement to the underside of the skylight, sets the skylight on the felt, and nails the flange down with ring-shank nails.
We also replace all the vent pipe flashing and install new goose-neck attic exhaust vents. Again, these elements are cemented down to the asphalt felt and fastened through the felt and sheathing into the framing.
The Hot Mop
Once the dry-in is complete and all skylights, vents, and flashings have been installed and sealed, we can go home for the day. The next day we do the hot mop.
Actually, hot-mopped felt isn't the only alternative for this second stage of the job. Some roofers use a peel-and-stick modified-bitumen membrane. It's an easier method because there's no kettle of molten asphalt at 475F to deal with - you just peel off the release sheet and roll out the membrane. But our workers, who have applied both systems, trust the hot-mopped approach more because the molten asphalt fuses everything together.
There are a few cases where we use modified bitumen - on the flat roof of a bump-out, for example. In that situation, customers may choose a reflective white-coated membrane to keep the roof cooler on sunny days.
It takes at least three men to apply the hot-mopped asphalt. One man runs the mop, spreading the viscous asphalt over the 30-pound felt. Co-workers then roll out the 90-pound felt or SBS modified bitumen over the hot asphalt and nail the sheet along the top edge with ring-shank cap nails. Those fasteners will be covered by the next course's 4-inch overlap, so there are no exposed fasteners in the finished installation.
The kettle has to be constantly monitored, both for safety and to make sure that the asphalt stays at the right temperature, so we assign workers to this task. Fire code requires that we keep two extinguishers handy.