How is roof flashing always installed?
Short answer. Flashing is always installed so that every piece higher on the roof laps over the piece below it, the same way shingles do, so water running downhill lands on top of the next layer instead of behind it. Base flashing goes under the wall covering and over or woven into the shingles, and counterflashing or siding laps down over the base flashing.
The lapping rule, piece by piece
- Step flashing: each piece sits on top of the shingle below it and under the shingle above it, and the next piece up overlaps it. The wall leg goes under the housewrap and siding.
- Counterflashing on brick: set into a mortar joint above and lapped down over the step flashing, so water off the brick lands on the step flashing.
- Apron or headwall flashing: on top of the last course of shingles at the base of a wall, with siding or counterflashing lapped over its vertical leg.
- Pipe boots: the upper half of the flange tucks under the shingles above; the lower half lies on the shingles below.
- Drip edge: at the eaves it goes on the deck first with underlayment over it; at the rakes the underlayment goes first and the drip edge on top.
- Valley metal: goes under the shingles, and pieces joined end to end lap upper over lower.
Reverse laps are the classic mistake
A reverse lap is any spot where a lower piece sits on top of a higher one, so water runs straight into the seam. Common examples are step flashing laid on top of the shingles all the way up a wall, housewrap tucked behind the flashing instead of over it, and counterflashing slipped behind the step flashing instead of over it. Caulk can hide a reverse lap for a while, but once it cracks, the water goes exactly where the lap sends it.
Why it matters to you
Most of these laps disappear under finished shingles and siding, so you cannot check them after the job. That is why it is worth asking for photos during installation, especially at chimneys and walls. See roof flashing explained and drip edge explained.