Flashings at intersections
by Joan P. Crowe, AIA
To prevent roof system leaks, a common building configuration that requires special attention is the point where asphalt shingles intersect a continuous side wall.
NRCA, manufacturers, architects and designers typically suggest individual pieces of metal flashing, or step flashings, be installed at these interfaces at the end of each shingle course. Step flashings seal a roof system edge to a wall to prevent water infiltration. The back leg of each step flashing should be installed behind the cladding material to direct water away from behind a wall, past the face of cladding and onto a roof.
NRCA suggests a diverter be installed as the bottom step flashing. A diverter, which directs water away from a wall covering and off a roof, can be constructed out of a sheet-metal section that is weather-tight. A diverter requires a different configuration from the L-shaped profiles used in other step flashings to keep water from a wall.
For example, the underlayment and, if applicable, ice-dam protection should be installed up to a backup wall and turned up about 3 inches (75 mm). At the end of the first asphalt shingle course, a diverter should be installed.
Step flashings then should be installed at every shingle course and against the vertical surface. NRCA suggests using a 7- by 8-inch (180- by 200-mm) step flashing for a standard asphalt shingle. These dimensions ensure a 2-inch (50-mm) minimum step flashing headlap, 4-inch (100-mm) extension onto each underlying shingle and 4-inch (100-mm) leg up the vertical surface to prevent leaks. The figure shows proper step flashing and diverter installation.
Once a roof system and related flashings are installed, wall cladding should be installed. A 1-inch (25-mm) minimum gap between the bottom of the wall cladding and roof covering should be maintained.
An important issue to consider is the type and minimum thickness of the metal being used to fabricate step flashings and diverters. The decision of what to specify should be commensurate with the anticipated service life of an asphalt shingle roof system.
NRCA suggests flashings be constructed from 26-gauge (0.019-inch- [0.48-mm-] thick) galvanized or prefinished galvanized steel; 0.032-inch- (0.81-mm-thick) aluminum or prefinished aluminum; 26-gauge (0.018-inch- [0.46-mm-] thick) stainless steel; 16-ounce (0.022-inch- [0.56-mm-] thick) copper; 16-ounce (0.026-inch- [0.66-mm-] thick) lead-coated copper; or 4-pound (0.062-inch- [1.57-mm-] thick) lead.
In some regions, particularly those with mild climates, other metal types or metals of lesser thicknesses successfully may be used.
For more information about flashings and asphalt shingle construction, refer to The NRCA Roofing and Waterproofing Manual, Fifth Edition, which is available from NRCA InfoExpress by calling (847) 299-9070 or (800) 323-9545; faxing (847) 299-1183; or accessing NRCA's Web site, www.nrca.net.
Joan P. Crowe is an NRCA manager of technical services.
Copyright © 2004 National Roofing Contractors Association
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