How to Flash a Dormer on a Shingle Roof
Four walls' worth of flashing in one small place: front apron, two cheek walls, and the valleys where the dormer roof meets the main slope.
- Advanced
- 4 to 8 hours
- Updated
The short answer
A dormer is flashed in sections. The front wall gets an apron flashing lapped over the shingles below. Each cheek wall gets step flashing, one piece per course, ending in a kickout at the bottom. Behind the dormer, the dormer roof meets the main roof in valleys, or a headwall if it is a shed dormer. Ice and water shield goes under every joint, and the wall barrier laps over all the metal.
A dormer is a small house on a roof, and every wall of it needs its own flashing. The front wall is a headwall in reverse: the dormer face stands above the roof below it and needs an apron. The two side walls, the cheeks, are sidewalls. Behind the dormer, where its roof or valleys meet the main slope, water collects and is steered around it.
That makes a dormer one of the most leak-prone parts of a roof, simply because so many details meet in a small space. A missing kickout, a short piece of step flashing or a blocked valley shows up as a stain in the room below. Most dormer leaks are not from the dormer roof. They are from a flashing joint.
This guide connects step flashing, headwall flashing and kickouts, and adds the dormer-specific parts. For the shingle side, see shingling around a dormer.
Tools
- Roofing hammer or coil nailer
- Hook blade utility knife
- Tin snips
- Hand seamer or brake
- Flat bar for lifting siding
- Chalk line
- Tape measure
Materials
- Apron flashing for the front wall
- Step flashing for the cheek walls
- Kickout flashing for each lower end
- Valley metal or valley membrane
- Ice and water shield
- Housewrap or building paper and flashing tape
- Roofing nails, 1-1/4 inch
- Sealant for laps and exposed nail heads
Safety first
- Dormers sit mid-slope with a long drop below them. Tie off and use roof anchors. WorkSafeBC requires fall protection at 3 metres (about 10 feet) and wherever a fall could cause serious injury.
- Working on a cheek wall puts your back to the roof edge. Keep your anchor short and set above the dormer.
- Lifting siding and cutting cladding brings sharp edges and dust. Wear cut resistant gloves and eye protection.
Step by Step
Step 1: Strip the roof and inspect the dormer walls
Reroofing is the time to do this properly. Pull everything back to bare deck around the dormer, at least 12 inches out on all sides. Check the sheathing on the dormer walls and the roof deck below them for water damage. Dormers often have rot at the lower corners where a kickout was missing, and sometimes at the front wall sill.
Watch out: If the dormer framing or sill is soft, the repair is carpentry. Do not flash over it.
Step 2: Lay ice and water shield at every joint
Run membrane up each wall 4 inches or more and onto the deck 6 to 12 inches, pressing it into every inside corner. At the front wall, lap the lower strip over the deck. At the corners where walls meet, cut and fold the membrane, then patch with another piece so no corner has a gap. See how to install ice and water shield.
Step 3: Flash the front wall with apron flashing
The front is a wall that stands above the lower roof slope, so it is flashed like a headwall. Use apron flashing with a roof leg of at least 4 inches and a wall leg of at least 4 inches, and lap it over the last course of shingles. Fold the ends around the corners to meet the cheek wall flashing. See how to flash a headwall.
Set a kickout or diverter at each outer corner of the front wall if water will run off the ends.
Step 4: Install a kickout at the bottom of each cheek wall
The cheek walls run down to the eave and drop water right at the roof edge. Without a kickout, that water runs behind the cladding at the lowest corner. See kickout flashing. On a dormer that sits mid-slope, the cheek wall ends on the roof, and the lowest step flashing piece laps onto the apron below.
Step 5: Step flash each cheek wall
Use pieces at least 4 by 4 inches and at least 3 inches longer than the exposure, commonly 10 inches long for laminated shingles. Each piece laps the one below by at least 3 inches, the BC Building Code minimum head lap. Nail only the upper corner of the roof leg. See step flashing for the full sequence.
Shingle right up to the wall leaving about 1/4 to 1/2 inch gap, and do not jam shingles tight to the dormer wall.
Step 6: Flash the back: valleys or a headwall
On a gable dormer, two valleys run from the dormer ridge down to the main roof. Open metal or woven shingle valleys both work. See roof valley types. On a shed dormer, the dormer roof ends against the main roof and needs flashing like a headwall in reverse: apron flashing at the upper end over the dormer shingles.
Where the upslope side of the dormer meets the main slope, water from above runs into the back of the dormer. A small cricket (saddle) behind wide dormers sends it around. See build a chimney cricket for the same idea.
Step 7: Lap the wall barrier over all the metal
Dormer walls are often clad in siding or stucco, each of which has its own clearance and detail. See flashing a roof to a wall by cladding type. At the corners where two walls meet, make sure the barrier laps correctly and corner boards do not touch the roof.
Step 8: Test with water and check the attic
Test the lower joints first, then the cheeks, front and valleys, one at a time, so if you find water you know where it came from. See how to trace a leak from the attic.
Where Dormers Usually Leak
- At the lower end of a cheek wall, where a kickout is missing.
- At the corners where the front apron meets the step flashing.
- In the valleys, where needles and moss collect and back water up.
- At the top of the cheek wall where the dormer roof meets the wall.
- Through cladding that touches the shingles and wicks water in.
Shed Dormers and Wide Dormers
A shed dormer has a single sloped roof and usually flashes at the top against the main roof like a headwall, with step flashing on the ends. A wide dormer sits across a lot of slope, so water coming down the main roof above it concentrates on the top edge. A cricket or a diverter behind it is worth building so water flows around both sides instead of pooling.
Fraser Valley Notes
Cedar and fir debris collects in dormer valleys and behind wide dormers, and wet debris holds moisture against metal and shingle edges. Clear it each fall and after windstorms. Wind-driven rain hits dormer faces hard, so give the front apron and the barrier laps their full width.
Common Mistakes
- Leaving out kickouts at the lower cheek corners, rotting the dormer walls from behind.
- Using one long strip on a cheek wall instead of step flashing.
- Short step flashing pieces that leave gaps between pieces.
- Skipping membrane in the valleys and corners where water concentrates.
- Siding touching the roof so it wicks water and traps debris.
- Ignoring the back of the dormer where water from the main slope pools.
When to Call a Professional
Dormers combine several flashings and usually a change in framing and cladding. If you are not sure how they tie together, or the dormer has had repeated leaks, have a roofer trace the leak rather than adding sealant.
If the dormer framing is soft or the walls have visible rot at the corners, involve a carpenter. A roofer can often coordinate both.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Roof Repair service or the towns we cover.
How to Flash a Dormer: Questions
How do you flash a dormer?
Flash each wall separately. The front wall gets a continuous apron flashing lapped over the shingles below. Each cheek wall gets step flashing, one piece per shingle course, ending in a kickout at the bottom. Behind the dormer, flash the valleys or headwall where its roof meets the main slope. Ice and water shield goes under every joint and the wall barrier laps over the metal.
Why do dormers leak?
Most leaks are at flashing joints, not the dormer roof. Common causes are missing kickouts at the lower cheek corners, short or reused step flashing, reverse laps at the front apron corners, clogged valleys and siding that touches the roof and wicks water. Sealant applied in place of lapped metal tends to fail and hide the problem.
What flashing goes on the side of a dormer?
Step flashing goes on each cheek wall, one piece per shingle course, each at least 4 by 4 inches and long enough to lap the piece below by at least 3 inches. It is nailed once in the roof leg and covered by the next shingle course and the wall's building paper. A kickout flashing goes at the bottom end of each cheek wall.
Do dormers need a cricket?
A wide dormer, or one directly below a long roof plane, benefits from a cricket or diverter on its upslope side. It splits the water and sends it around both sides instead of letting it pool behind the dormer. Narrow dormers with valleys on both sides usually shed water without one. A roofer can judge it for your roof.
Can you reuse dormer flashing on a new roof?
It is rarely a good idea. Old step flashing is often rusted, too short for modern shingle exposure or nailed through the wall leg. A reroof is the one time all the flashing is exposed, so replace it. The exception is masonry counter flashing in good condition, which can sometimes be kept.
Related Guides
All Flashing Details guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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