Apron flashing is the metal that covers the transition between a roof slope and a roof penetration such as a dormer or chimney. Properly installed this flashing will protect the most vulnerable areas of a roofing system.
Apron flashing is usually applied as part of a two piece roofing system. The apron is the lower L-shaped piece that covers the roofing material and extends up the vertical surface of the roof penetration. The second part of the system is a counter flashing that extends from the vertical surface and covers the vertical leg of the apron.
The apron metal is first measured and cut to extend approximately 3 to 4 inches past either side of the roof penetration, 3 to 4 inches up on the vertical leg, and long enough on the lower flange to cover the roofing material at least as far as the exposure line. We usually install apron with a minimum of 4 inches on the roof. Longer in high wind areas.
The corners are cut diagonally, one bent around the vertical surface and the other flattened against the roof surface.
Securing the apron depends on the roofing material being used. For an asphalt shingle roof with solid decking, a couple nails in the upper corners and nails spaced evenly across the front of the apron is enough to hold the flashing in place. A small dab of roofing sealant over the nail head will prevent water seepage. A good galvanized nail long enough to penetrate the wood decking will suffice. Avoid using electoplated roofing gun nails as their thin coating will rust faster. With an average lifespan of 25 - 35 years, an asphalt roof will need to be replaced long before the nails have a chance to rust out or create a leak.
Do not nail the flashing to both the roof surface and the penetration surface. The expansion and contraction of the flashing material along with the movement of the different roof components, especially chimneys and masonry walls, will loosen the fasteners and lead to early failure.
For a better looking installation, hem the lower edge of the apron for a finished and stiffer edge. For longer lasting roofing materials and to avoid face nailing or exposed fasteners, the apron can be secured with clips nailed under the flashing and bent over the lower edge, or an apron with a hem can be hooked onto a blind cleat nailed to the roof.