Roof shape

What is a dormer?

A window that stands up out of the roof, with a small roof of its own, and the extra valleys, walls and flashing that come with it.

Updated September 12, 2026 · Every figure links to its source · Sources

DefinitionShape

Dormer — a framed structure projecting from the slope of a roof, roofed in its own right and usually holding a window, built to bring light and headroom into the space beneath the roof; the NRCA glossary records the term and the IRC's flashing rules govern every joint it makes with the main roof.

The common types

Dormers are named for the shape of their own small roof. A gable dormer has a pitched roof with a triangular front, and meets the main roof in two valleys. A shed dormer has a single low-pitched plane, sloping the same way as the main roof, and can run most of the width of the house; it adds the most floor space for the least framing. A hip dormer has slopes on three sides. An eyebrow dormer is a low, curved swell in the roofline with no vertical walls to speak of. A wall dormer rises from the exterior wall itself rather than from the roof slope, so its front is a continuation of the wall below.

Each type is a small building with the same needs as a large one: a structure that carries its own roof, walls that are flashed where they meet the main roof, and a roof that drains without pooling.

Why it matters

A dormer creates more roof edge, more valleys and more wall-to-roof joints per square foot than any other feature of a house, and those joints are where roofs leak. The IRC, Section R903.2.1 requires flashing at wall and roof intersections, wherever the roof changes slope or direction, and around roof openings — a dormer is all three at once. The sidewalls need step flashing woven with each course of shingles; the front wall of a shed dormer needs an apron; the valleys of a gable dormer need a valley lining or a properly woven or cut valley; and where the dormer's roof drains onto the main roof beside a wall, a kick-out flashing keeps the water out of the wall below. The directory's entry on kick-out flashing explains that last, most-neglected piece.

The space behind a dormer — where its roof meets the main slope on the uphill side — can form a "dead valley" that drains poorly. A well-designed dormer avoids it; on an existing one, a cricket or a membrane underlayment in that pocket is the usual remedy.

What it costs and who does it

Adding a dormer is a construction project — framing, roofing, siding, a window, interior finish and usually a permit and a structural check of the rafters being cut — priced as a whole by a general contractor or remodeller, with a roofing contractor doing the roof and flashing. Re-roofing an existing dormer is part of the main roof's price, though every dormer adds labour for its valleys, walls and small planes, and a re-roof is the moment to replace the flashing rather than reuse it. The directory's guide to a new roof installation covers the sequence.

Frequently asked questions

Short answers; the sections above carry the detail and the sources.

What is the difference between a gable dormer and a shed dormer?

A gable dormer has a small pitched roof with a triangular face and meets the main roof in two valleys; a shed dormer has one low-sloped plane and can run much of the house’s width, giving more floor space with simpler framing.

Why do dormers leak?

Because they multiply the joints where a roof is most vulnerable: sidewalls that need step flashing, a front wall that needs an apron, valleys that need lining, and a low corner where the dormer roof meets the wall that needs a kick-out flashing. Missing or reused flashing is the usual cause.

Does a dormer need a permit?

Adding one almost always does, because it cuts structural rafters and changes the roof; check with the local building department. Re-roofing an existing dormer is covered by the re-roof permit where one is required.

Find a roofer who does this

Dormer work is flashing work. The directory lists roofing contractors by state and city, with the licence details to check before the deck is opened.

Find a roofing contractor

Sources

Roofing glossary — National Roofing Contractors Association (NRCA)
2021 International Residential Code, Chapter 9: Roof Assemblies — International Code Council (ICC)