Roof Leaks Around Dormers and Walls: Step Flashing 101
A stain that runs down the wall below a dormer window, or along the ceiling where a one-story section meets a two-story wall, is one of the most common leaks in Durham and one of the most misdiagnosed. The homeowner suspects the window. The handyman suspects the siding. The roofer who's seen enough of them lifts the bottom row of siding, looks at the step flashing, and finds either that it's been caulked over, that it's too short, or that it was never there.
This is what's supposed to be behind that wall, what fails, and what fixing it costs.
Where the roof meets a wall
Anywhere a roof slope runs into a vertical wall — the sides of a dormer, the wall of a two-story section above a garage roof, the side of a chimney — water running down the slope hits the wall and has to be turned. On a properly built roof, three layers do that:
- Step flashing. Individual L-shaped pieces of metal, about 5 by 7 inches, one per shingle course. Each piece lies on top of one shingle with its vertical leg against the wall, and the next shingle up covers it. They overlap like the shingles themselves, so water hits metal and is shed onto the shingle below, course by course. On a two-story wall there might be forty of them.
- The water-resistive barrier — housewrap or felt on the wall — lapped over the vertical leg of the step flashing so anything running down the wall lands on the metal.
- Siding, held an inch or more off the roof surface so it isn't wicking water, or on a brick wall, counter-flashing let into a mortar joint and bent down over the step flashing.
Water that gets past all three has to go through the housewrap. Water that gets past one of them ends up in the wall.
How it fails
Caulk instead of counter-flashing. The most common failure on the 1990s houses in Woodcroft and Hope Valley Farms. The step flashing is there, but instead of counter-flashing on the brick or properly lapped housewrap on a sided wall, there's a bead of caulk along the top edge. Caulk lasts five to eight years in Durham sun. Then the top of the step flashing is open to the wall.
Continuous flashing instead of step. One long strip of metal bent into the corner and run the length of the wall, nailed through the face. It looks tidy and it leaks from the first year, because the shingles can't lap over it and water runs behind it at every nail.
Short legs. Step flashing with a two-inch vertical leg instead of four or five. A heavy summer storm splashes over it.
Missing at the bottom. The first piece of step flashing at the eave end of a dormer needs to turn out over the shingle and kick the water into the gutter. Without that kick-out flashing, water runs down the wall behind the siding and rots the sheathing, the rim joist and sometimes the floor. We find this on houses where the corner of the siding is stained and the wood underneath is soft. It's a $400 flashing problem that becomes a $4,000 framing problem over ten years.
Never installed. On some of the 1920s–40s houses in East Durham and around Old West Durham, dormers were added in the 1950s or 60s by whoever was available. The shingles run to the wall and stop. Tar was applied when it leaked. Tar was applied again.
Reused during a re-roof. A roofer who tears off the shingles but leaves the old step flashing in place — because pulling it means disturbing the siding — has just put new shingles against 25-year-old rusted metal with a caulked top edge. This is the single most common reason a two-year-old Durham roof leaks at a dormer.
Finding it
From inside, the trail on the underside of the deck runs along the wall line, not from a single point. Our post on finding a roof leak from inside covers the attic method. From outside, look at the siding above the roof line: staining at the bottom edge, a bead of caulk along the roof, or metal that runs in one long piece instead of overlapping steps.
The repair
Done right, a step flashing repair means lifting the bottom course or two of siding (or cutting a reglet into the brick for counter-flashing), removing the shingles along the wall, installing new step flashing course by course with the shingles, lapping the housewrap over it, adding a kick-out at the bottom, and reinstalling the siding. On a dormer it's half a day. On a long two-story wall over a garage, a full day.
| Repair | Typical Durham price |
|---|---|
| Re-flash one side of a dormer | $450 – $800 |
| Re-flash a full dormer, both sides and the front | $800 – $1,400 |
| Step flashing along a two-story wall, 15–25 ft | $700 – $1,100 |
| Add a kick-out flashing only | $200 – $400 |
| Counter-flashing let into brick, per wall | $400 – $900 |
| Rotted sheathing or framing found behind the wall | $90 – $150 per sheet plus carpentry |
A caulk job on top of the old flashing is $150 and lasts until the second summer.
During a replacement
If you're getting a new roof, this is the moment to do all of it properly, because the shingles are off anyway and the added cost is a few hundred dollars in metal and an hour of siding work per wall. Ask the roofer whether the quote includes new step flashing at every wall and dormer or whether the old flashing stays. The roof replacement page lists what we replace; step flashing and counter-flashing are on it, and reusing them is how the cheap quote got cheap. If you're in the middle of that comparison, the pricing page has the lines a quote should show.
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