How do you properly install flashing on a roof?
Short answer. Proper flashing is installed shingle style, so every piece laps over the one below it and water never runs into a seam. That means step flashing one piece per course at walls, counterflashing over it at masonry, apron flashing below chimneys and walls, kickout flashing where a roof edge meets a wall, and metal tucked behind housewrap or siding instead of caulked to its face.
Rules that apply to every flashing
- Lap with gravity: upper pieces over lower pieces, always. A reverse lap is a guaranteed leak.
- Mechanical first, sealant last: sealant is backup, never the barrier.
- Fasten high: nail the top edge of the metal, where water does not flow.
- Right metal: the International Residential Code requires metal flashing to be corrosion resistant and at least 0.019 inch thick (26 gauge galvanized). Avoid putting copper against galvanized steel or aluminum.
Where each kind goes
- Step flashing at sidewalls: an L-shaped piece at least 4 by 4 inches for each shingle course, tucked behind the housewrap and siding and lapped by the next shingle.
- Counterflashing at masonry: set into a cut mortar joint or reglet, covering the top of the step flashing.
- Apron or headwall flashing where a slope runs into a wall or the front of a chimney.
- Kickout flashing at the bottom of a roof to wall line, steering water into the gutter instead of behind the siding.
- Valley lining where two slopes meet.
- Drip edge at eaves and rakes, under the underlayment at the eaves and over it along the rakes.
Common mistakes
Reusing rusted metal, face-nailing step flashing, running one long L-flashing up a wall instead of individual steps, skipping kickouts, and relying on caulk at chimneys. Each looks fine on day one and shows up as a stain years later.
More: roof flashing explained and drip edge explained.