A dormer is a window set vertically on a sloping roof, carrying its own small roof that may be flat, arched, hipped, pointed, or ornamented. From the street, dormers read as the eyes of a dwelling; inside, they bring daylight and cross-ventilation to attic rooms that would otherwise stay dark and stuffy. Because a dormer interrupts the roof plane and transfers its weight to the structure below, the framing and flashing need planning before the first shingle is lifted. Homeowners who understand the load path make better decisions when they review a dormer framing and structural plan with a contractor. This article covers the main dormer types, what each one costs, and where they tend to fail.
What a Dormer Does for a House
Dormers became common when homeowners wanted to finish attic space without adding a full story. A dormer adds three things at once: a vertical window where the roof slope would otherwise force a low, awkward skylight, headroom along the new wall, and a way to move air through the top of the house. The architectural character matters just as much. Dormers appear on Colonial, Colonial Revival, Cape Cod, Craftsman, Tudor, Greek Revival, Prairie, Shingle-style, and many modern homes, and each style observes its own proportions. The full range of dormer design and architecture choices starts with deciding what the window will do before you pick its shape.
Light, Headroom, and Egress
Attic conversions are the most common reason to add a dormer. A finished attic room needs a window that meets the building code for emergency escape and rescue, and a dormer is often the only way to fit one into a steep roof. Most jurisdictions require a clear opening of at least 5.7 square feet with the sill no higher than 44 inches above the floor. Dormers also create the vertical wall space where a bed or a desk can stand, which is why a 4-foot-wide dormer can make a 10-foot-wide attic room feel usable.
Cross-Ventilation Benefits
Two dormers placed on opposite roof slopes create a natural draft. The stack effect pulls cool air in through the lower window and pushes warm air out through the higher one, and in summer this passive flow can shave several degrees off the attic temperature before the air conditioner runs.
- Brings vertical daylight into rooms under the roof
- Adds usable headroom along the new wall plane
- Provides a code-compliant egress opening for finished attics
- Creates cross-ventilation when paired with a second opening
- Adds visual rhythm to long, plain roof planes
Dormer Types and Their Roof Forms
Dormers are classified by the shape of their own roofs, and that shape drives both the look and the construction cost. The gable dormer has a pitched roof with its own small gable end. The shed dormer uses a single continuous slope and often stretches wide across the roof. Hipped dormers carry three sloping planes to a ridge, arched dormers curve the header over the window, and eyebrow dormers sweep a low curved roof over a short window.
Gable and Shed Dormers
The gable dormer is the most common choice: a small peaked roof that echoes the main roof and a vertical front wall with a window. It suits Colonial, Cape Cod, and Tudor houses because its proportions repeat the house gable. Shed dormers trade the miniature roofline for interior space, and because one long opening costs less per square foot of floor than several small ones, they dominate attic conversions.
Hipped, Arched, and Eyebrow Dormers
Hipped dormers carry three sloping roof planes that meet at a ridge, which gives them a softer profile than a gable and better wind resistance. Arched dormers bend the header over the window in a smooth curve, a signature of Shingle-style and Prairie houses. Eyebrow dormers are the rarest form, with a low curved roof that sweeps over a short window set almost at the roof surface.
Contractors have argued for years over whether a water-damaged unit is worth rebuilding in place or should come off entirely. The question of whether a failing dormer can be saved usually comes down to the condition of the side walls, the rafters, and the flashing, and an honest inspection settles it faster than a gut feeling.
| Dormer type | Roof form | Interior gain | Relative cost |
|---|---|---|---|
| Gable | Pitched mini-roof with gable end | Single window bay | Moderate |
| Shed | One continuous slope | Wide, deep headroom | Low to moderate |
| Hipped | Three slopes meeting at a ridge | Moderate | Moderate to high |
| Arched | Curved header over the window | Single window bay | High |
| Eyebrow | Low curved sweep | Short window bay | High |
Pros and Cons of Adding a Dormer
Dormers can add beauty and curb appeal, and they match well with Colonial, Colonial Revival, Cape Cod, Craftsman, Tudor, Greek Revival, Prairie, Shingle-style, and some modern homes. They also carry real drawbacks, and the decision comes down to whether the added space justifies the roof surgery.
The Benefits
- Curb appeal and a more balanced roofline from the street
- Added living space that counts toward appraised square footage
- Natural light that reduces the need for artificial lighting
- Better air movement through the upper floor
The Drawbacks
- Every roof penetration is a potential leak point if the flashing fails
- Structural work on rafters and headers needs a permit in most areas
- Costs run well above skylights for a similar amount of light
- Poor proportions can hurt the facade more than help it
On Colonial and Greek Revival houses, proportion is the difference between an elegant dormer and a boxy one. The Greek Revival dormer proportions fix common mistakes such as windows set too high or roofs pitched too steeply for the era, and the same geometry lessons apply to any classical exterior.
Framing, Flashing, and Planning the Build
Cutting rafters to install a dormer changes the way the roof carries its load. The opening needs a header sized for the span, doubled trimmers on each side, and cripple studs above and below the window. A wide shed dormer can interrupt several rafters at once, which is why engineers often require a review before the carpenter cuts.
How a Dormer Carries Its Load
The dormer’s own weight, the window, and any snow load on the dormer roof all travel down through the header into the trimmers and then to the bearing walls below. If the opening lands between floors, the load path continues through the wall framing. Moving a dormer even a foot sideways can change which studs carry the load, so the plan has to be fixed before the roof is opened.
Flashing That Keeps Water Out
Water runs down the main roof and meets the dormer wall, and the joint between them is the classic failure point. The dormer roof should shed onto the main roof, not the reverse, and the side walls need step flashing woven into each course of shingles.
Step Flashing and Head Flashing
Step flashing tucks a small piece of metal under each shingle course where the dormer wall meets the roof, and head flashing caps the top of the wall where the dormer roof ties in. Ice and water shield under the shingles near the junction adds a second layer of defense in cold climates.
- Measure the attic floor area and the available roof slope
- Confirm setback and egress requirements with the building department
- Have an engineer or architect review the rafter cuts and header sizing
- Choose the window size and the dormer type together, since they set each other
- Budget for flashing, insulation, and interior finish, not just the window
For the most space per dollar, a wide shed dormer retrofit typically outperforms a row of individual gable dormers, because one long opening costs less per square foot of added floor than several small ones.
Matching the Dormer to the House Style
The dormer should answer the house, not fight it. The same window in a different roof gets judged by how well it repeats the massing, the pitch, and the trim of the original building.
Traditional Styles
Cape Cod houses traditionally carry small gable dormers with six-over-six windows set flush in the wall. Tudor houses favor steeply pitched dormers with decorative half-timbering, and Craftsman houses use low-pitched shed dormers with heavy brackets. A dormer that matches the period trim reads as original even when it is new.
Eyebrow and Curved Dormers
The curved profile of an eyebrow dormer window suits Shingle-style and Craftsman houses, where the roof reads as one continuous surface. Framing the curve takes bent headers and flexible sheathing, and the flashing at the low ends is where most leaks start.
Modern and Minimal Dormers
Contemporary houses use flat-roof dormers with large fixed glass, sometimes wrapping the corner of the roof plane. The geometry is simpler than a pitched dormer, but the wider glass spans require stiffer headers and careful interior light control.
Installation Costs and Hiring the Right Crew
Dormer costs vary more with the roof than with the window. A simple shed dormer on a one-story house might run from $2,500 to $7,500, while a full gable dormer with siding, trim, and insulation commonly lands between $6,000 and $20,000 depending on the roofing material and the number of stories. The price per square foot of added space drops as the dormer gets wider, which is why retrofits favor long shed units.
Who Should Build It
Hire a window specialist and a master carpenter rather than a roofer alone. The window specialist handles the rough opening and the flashing sequence, and the carpenter manages the structural cuts. A roofer who only re-covers the junction rarely catches framing errors, and the mistake shows up later as a cracked wall or a leak.
Specialty shapes push the price higher. Building an arched dormer with curved headers and copper roofing takes a fabricator who can bend the header and a roofer who can seam the metal, and the combined labor usually runs double the cost of a straight-roof dormer of the same width.