Roof flashing is required whenever there is a change of materials, at seams where gaps occur, or where a material is penetrated or interrupted, especially at openings. Flashing is usually made of metal but any waterproof material used to channel or divert rainwater can be described as flashing. Some of the common metals used for flashing are copper, aluminum, stainless steel, zinc and lead. 16 oz. copper is the popular choice for many types of flashing applications because it is light and malleable.
Flashing is typically installed in thin strips about 10 inches wide. Larger sheets would be susceptible to thermal expansion when applied to exterior surfaces or exposed to a large temperature gradient. Flashing is installed with the upper layer overlapping so that water is always directed downward over an unsealed joint. There are many joints and seams built into the average shingled roof that require flashing. A dormer would need flashing where it intersects the roof plane. The perimeter of the base of a chimney and the entire area of a gabled “cricket” must have flashing. Flashing is also required at ridges, hips, valleys, and where there is a change in roof pitch, as in a gambrel. The steps listed below will explain the proper way to install metal flashing onto a typical asphalt shingled roof.
Ridges and Hips
One of the methods used to flash both ridges and hips uses flashing under the shingles that straddle the ridge (or hip). The flashing is cut into segments about a foot long and placed under each pair of shingles where they overlap.
The open valley is a commonly used method of flashing a valley in a shingled roof. An unbroken strip of metal flashing 20 inches wide is nailed into the valley along its full length. The flashing should cover the same amount of roof on each side. Shingles are installed overlapping the flashing on each side so that a 6 inch strip along the central crease is left exposed.
Exterior Wall and Roof Intersections
Flashing is required in places such as where a gable intersects an exterior wall. The flashing consists of short segments bent in half at a 90 degree angle. The segments are installed at the intersecting planes overlapping from above with one half turned up under the siding material and the other half inserted under the roof shingles.
Each pipe vent that projects through the roof has a pre-fabricated flashing component made of sheet metal 16 inches square. The cylindrical pipe penetrates the plane of the sheet metal at the angle of the roof pitch through an elliptical opening with a fitted collar. The sheet metal is inserted under the upper shingles, rests flat on the roof plane and overlaps the shingles below.
Chimneys and Dormers
Flashing at chimneys consists of base flashing fitted into the shingles at each course, and stepped cap flashing installed onto the brickwork extending into the mortar joints. The base flashing and cap flashing are interwoven along the 2 sides. A solid continuous piece of cap flashing covers the lower edge or the front of the chimney and overlaps the base flashing resting on top of the shingles. The same pattern is used at the rear of the chimney, except that the base flashing is inserted up under the shingles. Dormers are flashed in much the same way, but without the need for cap flashing.