How to Shingle Around a Dormer and Keep the Courses Aligned
Step flashing on the cheeks, an apron at the front wall, valleys at the top, and courses that stay in line on both sides.
- Advanced
- 4 to 10 hours per dormer
- Updated
The short answer
Around a dormer, install ice and water shield on the deck and up the walls, flash the front wall with apron flashing, step flash each cheek wall as the courses rise, and form a valley where the dormer roof meets the main roof. Run the main roof courses across the width of the dormer on both sides, so they meet the dormer at the same height and keep the same bond lines.
A dormer is a small roof sticking out of a bigger one, and every side of it is a water problem. The front wall of the dormer meets the main roof along a horizontal line. The two side walls, called cheeks, meet it along sloped lines. The dormer's own roof meets the main roof at two valleys. Each of these needs its own flashing, and they all need to work together.
What makes it difficult is that it all has to line up. The main roof courses have to run past the dormer and come out on the other side in line, the dormer roof courses need to match the pitch and exposure of their own plane, and the flashing pieces have to be woven in at the right moment. A dormer that is done piece by piece, not as a whole, leaks.
This guide covers the shingle work around a dormer with the flashing it needs. The flashing details themselves are in how to flash a dormer and how to install step flashing. The valley work is covered in roof valley types.
Tools
- Roofing coil nailer and compressor
- Hook blade utility knife
- Chalk line
- Tin snips
- Hand seamer
- Tape measure
- Framing square
- Fall arrest harness and anchor
Materials
- Laminated shingles and starter
- Ice and water shield
- Step flashing for the cheek walls
- Apron flashing for the front wall
- Valley metal or valley membrane
- Roofing nails, 1-1/4 inch galvanized
- Asphalt roof cement
Safety first
- Dormers often sit high on a steep roof. Anchor above the work and tie off before stepping onto the slope beside one.
- Flashing pieces are razor sharp. Wear cut resistant gloves.
- Do not stand on the dormer roof or cheek wall while working. Their framing may not carry a person.
Step by Step
Step 1: Waterproof the dormer area first
The corners where the dormer meets the main roof are the most leak-prone part of the roof. Put a wide band of ice and water shield along the bottom of the front wall and up each cheek, turned up the wall by at least 6 inches and lapped over the main roof underlayment.
Lay the underlayment of the main roof over the membrane on the deck, so water runs over each layer. The wall sheathing or building paper must end up over the membrane on the wall.
Step 2: Snap chalk lines across the dormer width
Snap lines on the main roof at the normal interval, and continue them across the face of the dormer on the underlayment below it. This gives you reference marks on both sides. Mark vertical bond lines on each side of the dormer to keep the offset pattern. See how to snap chalk lines for shingle courses.
On a dormer with a front wall and two cheeks, plan the courses so that a course does not end in a thin sliver at the cheek wall.
Step 3: Shingle up to the front wall and install the apron
The lowest wall of the dormer meets the roof at a horizontal line. Shingle up to about 1/2 inch from the wall, then install the apron flashing with a bottom leg lying on the shingles and a vertical leg running up the wall. The wall cladding laps over the vertical leg. The apron goes over the shingles, not under them.
Nail the flashing only on its upper edge on the wall leg, or as the flashing instructions show. See how to flash a headwall for the same principle.
Step 4: Step flash each cheek wall as the courses rise
Run courses on each side of the dormer up to the cheek wall, leaving a small gap at the wall. Place a step flashing piece on each course, nailing it once in the upper roof corner, then cover it with the next course. See how to install step flashing.
Keep the shingle pattern and the flashing pieces in step. The pieces should be at least 3 inches longer than the exposure, and each should lap the one below by at least 3 inches, the BC Building Code minimum head lap.
Watch out: Never nail the wall leg of a step flashing piece. The dormer and the roof move independently, and a nail there tears the flashing.
Step 5: Run courses past the dormer
Courses above the dormer front wall pass behind the dormer. Shingle the uphill area between the dormer and the ridge like the rest of the roof, with a short course or a cut shingle to fit the valley. The courses on each side must stay at the same height, or the roof will look out of true.
Do not try to fit a full course around the dormer. Cut to the valley line.
Step 6: Form the valleys where the dormer roof meets the main roof
Each side of a gable dormer has a valley between the dormer roof and the main roof. The valley can be open metal, a closed cut valley or a woven valley, following the manufacturer's instructions. See how to install an open metal valley and how to install a closed cut valley.
The valley flashing is covered at its bottom by the apron and cheek flashing. Shingle the dormer roof and the main roof to the valley, trim the ends to the line and keep the nails clear of the valley centre.
Step 7: Shingle the dormer roof and finish the ridge
The dormer roof is a small roof of its own. Start with drip edge and starter at its eave, and use rake details at its gable sides. Work up to its ridge, and finish with cap that runs to the main roof.
Where the dormer ridge meets the main roof, the cap finishes against the main roof shingles, and the joint is sealed with cement. Check the detail on the manufacturer's instructions.
Shed Dormers and Gable Dormers
A gable dormer has a peaked roof and two valleys. A shed dormer has a single-slope roof that meets the main roof along a headwall at the top, and the sidewalls are step flashed. On a shed dormer, the flashing at the top of the dormer roof is a headwall detail, and the dormer roof may be low-slope. See how to shingle a low-slope roof for the low-slope rules.
Why Dormers Leak
The usual culprits are missing kickout flashing at the bottom of a cheek wall, reused step flashing, a nail through a wall leg, and caulk in place of flashing. The bottom corner of the cheek, where the wall, the main roof and the dormer eave meet, takes the most water. See how to install a kickout flashing.
Common Mistakes
- No kickout at the bottom of the cheek wall. Water runs down behind the siding and rots the wall.
- Apron flashing under the shingles. It must lap over them, so water runs onto the shingle and not behind the flashing.
- Courses that do not line up on either side. A step in the courses at the dormer is visible from the street.
- Reusing old step flashing. Bent and rusted pieces leak.
- Shingle tight to the wall. It wicks water. Leave a small gap and let the flashing do the work.
- Caulking in place of flashing. Sealant cracks in a few seasons. See roofing sealants.
When to Call a Professional
Dormers are one of the most leak-prone details on a roof, and a roofer who flashes them regularly will get all four joints right. If the dormer sits high on a steep roof, has rotted cheeks or siding, or has a complex valley, it is a job for a roofer.
If water has been getting in at a dormer, inspect the sheathing and framing behind the siding before new flashing goes on. Covering rot with new metal only hides the problem.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Roof Replacement service or the towns we cover.
How to Shingle Around a Dormer: Questions
How do you flash a dormer?
A dormer needs apron flashing at the front wall, step flashing along each cheek wall, a kickout at the bottom of each cheek, and valleys where the dormer roof meets the main roof. All of it is woven into the shingles as they are laid, with the upper layer always over the lower one, so water is carried onto the roof surface.
Do you shingle the dormer or the main roof first?
Shingle the main roof up to the dormer first, installing the apron and step flashing as you go, then form the valleys and shingle the dormer roof. Flashing and shingles are woven together, so they are installed in sequence, not as separate jobs. The underlayment and membrane go on before any shingles.
What goes at the bottom of a dormer cheek wall?
A kickout flashing, which turns water that has run down the step flashing out into the gutter or onto the roof. Without one, water runs behind the siding at the bottom of the cheek and rots the wall. It goes on first at the eave end, before the first step flashing piece.
Why does my dormer leak?
Common causes are a missing kickout, old or reused step flashing, nails through the wall leg of the flashing, caulk in place of metal, or failed apron flashing at the front wall. Dormer leaks are often slow and show up inside walls. A roofer can test each joint and check the sheathing behind the siding.
How do you shingle the valley next to a dormer?
Line the valley with self-adhered membrane and then either an open metal valley or a woven or closed cut valley, as the shingle manufacturer allows. The main roof and dormer courses are shingled to the valley line, with the ends trimmed cleanly and nails kept clear of the valley centre.
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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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