How to roof a dormer: shingle the dormer roof itself first, then integrate the three wall-to-roof joints with flashing. Run self-adhered ice-and-water membrane up the valleys and along the sidewalls, install step flashing piece by piece as you lay each shingle course up the sidewalls, set continuous apron (headwall) flashing where the dormer face meets the main roof, and add a kickout flashing at the bottom of each sidewall where the roof edge meets the wall. The order that prevents leaks is membrane first, then bottom-up flashing woven into the shingle courses, never caulk as a substitute for a lapped metal detail.
By The Roofing Brief Team · Last reviewed: September 2026
A dormer is one of the most leak-prone details on a shingle roof because it adds two sidewalls, a headwall, and often two valleys to an otherwise simple slope. Every one of those transitions has to shed water by overlap, not by sealant. This guide covers the dormer types you are likely to meet, the numbered install sequence, the flashing details at each joint, and the spots where dormers most often leak.
Dormer types and how each is roofed
The roofing method changes slightly with the dormer’s shape. The four common residential types:
- Gable dormer. A peaked roof with two slopes meeting at a short ridge, the most common type. It is shingled like a miniature gable roof, finished with hip-and-ridge shingles along its short ridge, and tied to the main roof with two valleys plus sidewall step flashing.
- Shed dormer. A single flat plane that slopes in one direction, common where headroom matters. If the pitch sheds water it can be shingled like a small main roof; very low-slope shed dormers may call for a membrane instead (see the low-slope note below). It meets the main roof at a headwall across its back.
- Hip dormer. Three sloping faces meeting near a point. It is handled like a gable dormer until you reach the hips, which are then capped with hip-and-ridge shingles.
- Eyebrow dormer. A curved, low-radius form whose slope approaches near-flat at its crown. Because water barely sheds at the top of the curve, it is prudent to cover the whole eyebrow with self-adhered membrane and use a flexible roofing material that can follow the curve.
What you need before you start
Materials for a typical asphalt-shingle dormer: matching field shingles and a bundle of hip-and-ridge shingles (for gable and hip dormers), synthetic underlayment, self-adhered ice-and-water membrane for valleys and sidewalls, step flashing shingles (commonly 5-inch by 7-inch bent pieces), continuous apron/headwall flashing, one kickout flashing per lower sidewall corner, roofing nails, and a compatible sealant for terminations only. Confirm flashing metal meets your local code: the IRC calls for corrosion-resistant metal not less than 0.019 inch thick (No. 26 galvanized) for valley and similar flashings.
How to roof a dormer, step by step
- Dry-in the main roof and dormer deck. Install underlayment across the main slope and the dormer roof. Run self-adhered ice-and-water membrane up both dormer valleys and up the sidewalls where the dormer meets the main roof, lapping the membrane onto the wall sheathing several inches so the flashing later sits over a waterproof layer.
- Shingle the dormer roof itself. Set a starter course at the dormer eave over the drip edge, then lay field shingles up the dormer plane. On a gable or hip dormer, work both slopes up to the ridge or hips.
- Cap the dormer ridge or hips. Finish a gable dormer’s short ridge, or a hip dormer’s hips, with hip-and-ridge shingles before you tie into the main roof, so the dormer plane is watertight on its own.
- Weave the valleys into the main roof. Where the dormer roof meets the larger slope, form the valley (woven, closed-cut, or open metal per your spec) as you shingle the surrounding main-roof courses. Keep valley metal or membrane continuous and centered on the valley line.
- Step-flash the sidewalls. As you lay each main-roof shingle course past a dormer sidewall, slip one step-flashing piece over the top of that course and against the wall, so the next course covers it. Each piece overlaps the one below in a stair-step, and every piece nails to the roof deck, not to the wall. Tuck the vertical leg behind the dormer’s siding or wall covering.
- Set the apron (headwall) flashing. Where the dormer face or the back of a shed dormer meets the main roof above it, install continuous apron flashing so its roof leg laps over the shingles (aim for roughly 4 inches of lap) and its wall leg runs up behind the siding. A hemmed edge and a stiffening crease help it clamp down and resist wind-driven rain. Lap multiple apron pieces by several inches.
- Install kickout flashing at the sidewall bottoms. At the lowest point of each sidewall, where the roof edge meets the wall, add a kickout flashing to fling water out into the gutter or onto the roof instead of behind the siding. This is a code requirement, not an upgrade.
- Finish the main roof and terminations. Continue the main-roof courses past the dormer, integrate the top valley terminations, and seal only the metal terminations and exposed nail heads. Do not rely on sealant to close a joint that should be lapped.
Flashing the three joints in detail
Dormer leaks almost always trace back to one of three flashed joints. Understanding what each does prevents the common mistakes.
Step flashing (the sidewalls)
Step flashing is a stack of individual bent pieces, one per shingle course, each lapping the exposure of the course below so water sheds onto the roof and away from the wall. Because it is woven into the courses, step flashing has to go in as you shingle, not afterward. Nailing it to the roof deck rather than the wall lets the wall and roof move independently without tearing the flashing.
Apron or headwall flashing (the face)
Apron flashing is a single continuous piece that covers the horizontal joint where a vertical surface meets the roof below it. Its roof leg lays over the shingles to shed water; its wall leg goes up under the siding. Where an apron run meets step flashing at a corner, the apron ends should lap under the step flashing and tie into a kickout, so water is handed off cleanly from one detail to the next.
Valley flashing (dormer-to-main-roof)
A gable or hip dormer creates two valleys against the main slope. Whether you run an open metal valley or a closed-cut shingle valley, the key is a continuous waterproof layer (metal and/or self-adhered membrane) centered under the valley, with shingles trimmed back from the center line so runoff stays in the channel. Roofing crickets use the same principle upslope of chimneys; a dormer valley is the analogous detail alongside the dormer.
Where dormers leak most
The transitions, not the field shingles, are the usual culprits. Common entry points include:
- Missing or reverse-lapped step flashing. Flashing installed after the fact, face-nailed, or lapped the wrong way lets wind-driven rain track behind it.
- No kickout at the sidewall bottom. Without a kickout, concentrated runoff from the sidewall pours behind the siding and rots the wall sheathing, often unseen for years.
- Apron flashing sealed instead of lapped. Caulk bridging a headwall gap fails as it ages; the roof leg must physically lap the shingles.
- Valley debris and short trims. Shingles trimmed too close, or a valley that traps leaves, can back water up under the courses.
- The dormer window and trim. Water can enter at the window head, sill, or siding laps above the roof, then appear as a roof leak.
Low-slope shed dormers
Shed dormers sometimes end up below the minimum slope where asphalt shingles perform. Manufacturers and the IRC generally treat slopes under 4:12 as needing extra protection (such as a double underlayment layer), and very low slopes may fall outside shingle territory entirely and call for a membrane roof. Check the shingle manufacturer’s minimum-slope instructions and your local code before shingling a shallow shed dormer, and add ice-and-water membrane along the headwall junction either way.
Related guides
- Roof drip edge: types, placement, and code
- Roofing cricket: what it is and where your roof needs one
- Roofing felt vs synthetic underlayment
- Roofing terminology with pictures
Frequently asked questions
Do you shingle the dormer roof or the main roof first?
Shingle the dormer roof itself first and make it watertight, then tie it into the main roof as you carry the main-roof courses past it. The valleys and sidewall step flashing are woven into the main-roof courses, so those integration steps happen while you shingle around the dormer.
What flashing does a dormer need?
A typical gable dormer needs step flashing along both sidewalls, apron (headwall) flashing across the face, valley flashing or membrane where its roof meets the main slope, and a kickout flashing at the bottom of each sidewall. A shed dormer needs apron flashing across its headwall plus sidewall step flashing and kickouts.
Is kickout flashing required by code on a dormer?
Yes. The International Residential Code requires a kickout flashing wherever the eave of a sloped roof meets a vertical sidewall, which includes dormer sidewalls, to divert water away from the wall. Local codes and effective dates vary, so confirm the version your jurisdiction enforces.
Why do dormers leak so often?
A dormer multiplies the number of wall-to-roof transitions, and each one relies on correctly lapped flashing rather than the field shingles. Missing kickouts, reverse-lapped or face-nailed step flashing, and sealant used in place of a lapped apron are the most common failure points.
Can you shingle a shed dormer, or does it need a membrane?
It depends on the slope. If the shed dormer is pitched steeply enough for the shingle manufacturer’s minimum (commonly 2:12 with special measures, 4:12 for standard application), it can be shingled with added underlayment. Below that, a membrane roof is generally the appropriate choice. Check the manufacturer instructions and local code.