What is a dormer?

What is the purpose of a dormer?

A dormer adds headroom and daylight to the top floor under a sloped roof, and adding one is a common way to finish an attic into rooms people can use; local guides from Oregon, Maine, Massachusetts and Washington, DC say so: In Astoria, Oregon, “These rooftop projections provide needed headroom and light to the top story of a house, which typically had low, sloping ceilings and limited light.” Source: City of Astoria, Oregon, Community Development Department / Historic Landmarks Commission, read 2026-10-29. In Kennebunk, Maine, “Few buildings in the District originally had dormers, and most are later additions designed to make upper floors more useful and habitable by providing light and ventilation.” Source: Town of Kennebunk, Maine, read 2026-10-29. In Cambridge, Massachusetts, “The Design Guidelines have been prepared to help property owners, and the design and building professionals working for them, create dormers that will increase the living space of an attic story while retaining the basic design integrity of the building's rooflines.” Source: City of Cambridge, Massachusetts (Board of Zoning Appeal), read 2026-10-29. In Washington, DC, “Adding skylights or dormers to existing roofs is often considered by building owners who wish to finish an attic.” Source: Government of the District of Columbia, Office of Planning, Historic Preservation Office, read 2026-10-29.

How do city planning guides define a dormer?

A dormer is a small part of a roof that sticks up out of a sloping roof, with its own small roof and a window or vent in its front face. Three public guides describe it this way: In Washington, DC, “A dormer is a small projection above a sloping roof consisting of a window or vent and a small roof.” Source: Government of the District of Columbia, Office of Planning, Historic Preservation Office, read 2026-10-29. In Astoria, Oregon, “A dormer is a rooftop projection containing a window.” Source: City of Astoria, Oregon, Community Development Department / Historic Landmarks Commission, read 2026-10-29. In Astoria, Oregon, “It usually has its own roof.” Source: City of Astoria, Oregon, Community Development Department / Historic Landmarks Commission, read 2026-10-29. In San Francisco, “A dormer window is usually a gabled extension of an attic room through a sloping roof to allow for a vertical window opening into the room (Source: Webster’s Third New International Dictionary).” Source: San Francisco Planning Department (Zoning Administrator), read 2026-10-29.

What are the types of dormers?

Dormers are named by the shape of their own roof (gable, hipped, shed, arched or round) and by where their front wall sits: a roof dormer sits up on the roof, while a wall or engaged dormer continues the wall below: In Washington, DC, “Both dormer windows and vents are capped with a variety of roof shapes: typically gable, hipped, shed, or round.” Source: Government of the District of Columbia, Office of Planning, Historic Preservation Office, read 2026-10-29. In Astoria, Oregon, “Dormers come in a wide variety of shapes, including gabled, arched, shed and hipped.” Source: City of Astoria, Oregon, Community Development Department / Historic Landmarks Commission, read 2026-10-29. In Kennebunk, Maine, “There are two types of dormers in the District: roof dormers and wall or shed dormers.” Source: Town of Kennebunk, Maine, read 2026-10-29. In Astoria, Oregon, “Sometimes dormers break the roofline.” Source: City of Astoria, Oregon, Community Development Department / Historic Landmarks Commission, read 2026-10-29. In Astoria, Oregon, “These are called wall dormers.” Source: City of Astoria, Oregon, Community Development Department / Historic Landmarks Commission, read 2026-10-29. In Washington, DC, “A particular type of dormer, commonly found on residential buildings, is called an engaged dormer.” Source: Government of the District of Columbia, Office of Planning, Historic Preservation Office, read 2026-10-29.

Do dormers always have windows?

No: the Washington, DC preservation guidance says a dormer holds a window or a vent, and the guides read for this page describe what each usually looks like: See the Government of the District of Columbia, Office of Planning, Historic Preservation Office passage quoted above. In Washington, DC, “Vents are commonly rectangular, round or half-round in shape.” Source: Government of the District of Columbia, Office of Planning, Historic Preservation Office, read 2026-10-29. In Washington, DC, “Windows in dormers are commonly double-hung or casement and less commonly diamond, round and half-round.” Source: Government of the District of Columbia, Office of Planning, Historic Preservation Office, read 2026-10-29. In Cambridge, Massachusetts, “In general, window area, including trim, should account for at least 50 percent of the dormer's front wall face (Fig. 10).” Source: City of Cambridge, Massachusetts (Board of Zoning Appeal), read 2026-10-29.

What are dormers made of?

The guides read for this page say a dormer is usually built and clad to match the house: its front wall in the same siding as the wall below and its roof matching the main roof: In Washington, DC, “The front of the dormer is designed as an extension of the wall below, almost always using the same materials as the wall.” Source: Government of the District of Columbia, Office of Planning, Historic Preservation Office, read 2026-10-29. In Cambridge, Massachusetts, “The wall cladding or siding of a dormer should be the same material and color as the main walls of the building, with the exception of dormers sided with slate shingles.” Source: City of Cambridge, Massachusetts (Board of Zoning Appeal), read 2026-10-29. In Astoria, Oregon, “The dormer roof typically matches or complements the roof of the house.” Source: City of Astoria, Oregon, Community Development Department / Historic Landmarks Commission, read 2026-10-29. In Kennebunk, Maine, “If repair or replacement is necessary the materials should be in-kind, matching the details and finish of the original as closely as possible.” Source: Town of Kennebunk, Maine, read 2026-10-29.

Do dormers need flashing?

Yes: where a dormer's side wall meets the roof, the U.S. Department of Energy says step flashing goes in, and the model code text printed by Douglas County, Colorado sets a 4-inch by 4-inch minimum: “Step flashing should be installed any time the roof intersects with a wall, such as a dormer.” Source: U.S. Department of Energy, Building Science Education Solution Center, read 2026-10-29. “This flashing protects walls from water intrusion.” Source: U.S. Department of Energy, Building Science Education Solution Center, read 2026-10-29. “IRC Section R905.2.8.3 Sidewall Flashing – Base flashing against a vertical sidewall shall be continuous or step flashing and shall be a minimum of 4” in height and 4” in width and shall direct water away from the vertical sidewall onto the roof and/or into the gutter.” Source: Douglas County, Colorado, Building Division, read 2026-10-29. “Step flashing is often made of corrosion-resistant metal.” Source: U.S. Department of Energy, Building Science Education Solution Center, read 2026-10-29.

Why do dormers leak?

Where a dormer's side walls meet the roof, rain running down the roof is channeled along that joint, and the U.S. Department of Energy explains that missing or poor flashing there lets water into the wall: “Heavy rains can pour thousands of gallons of water onto a home’s roof in a single storm.” Source: U.S. Department of Energy, Building Science Education Solution Center, read 2026-10-29. “Where roofs intersect walls, much of this water is channeled off the roof and into gutters.” Source: U.S. Department of Energy, Building Science Education Solution Center, read 2026-10-29. “If sidewall flashing is lacking or inadequate, water can leak into the wall and cause serious water damage.” Source: U.S. Department of Energy, Building Science Education Solution Center, read 2026-10-29. “The flashing is integrated with the roof and wall drainage planes in shingle fashion, so that the top layer of the WRB or flashing laps over the bottom layer.” Source: U.S. Department of Energy, Building Science Education Solution Center, read 2026-10-29. “Where siding is provided on the vertical sidewall the vertical leg of the flashing shall be continuous under the siding.” Source: Douglas County, Colorado, Building Division, read 2026-10-29.

Are dormers structural?

A roof dormer is usually built as its own small structure with its own roof, sitting below the main roof ridge; town guidance from Kennebunk, Maine and a City of Dallas rule describe it that way: In Kennebunk, Maine, “The most common dormer is the roof dormer, which is structurally separate from the building.” Source: Town of Kennebunk, Maine, read 2026-10-29. In Kennebunk, Maine, “These dormers typically fall below the roof ridge and are set back from the eaves.” Source: Town of Kennebunk, Maine, read 2026-10-29. In Dallas, “1. Dormers added must be true roof dormers with separate roof structures.” Source: City of Dallas, Planning and Development, read 2026-10-29. In Dallas, “3. Dormers may not share the same ridge line as the main roof structure, but must be set down from the main roof ridge – clearly delineating the dormer from the main roof.” Source: City of Dallas, Planning and Development, read 2026-10-29.

Sources

• Guide to Dormers & Roofline Additions — City of Astoria, Oregon, Community Development Department / Historic Landmarks Commission; read 2026-10-29.
• Design Guidelines for Roof Dormers (1996) — City of Cambridge, Massachusetts (Board of Zoning Appeal); published 1996; read 2026-10-29.
• Definition of Dormers as It Applies to One-and-One Half Story Buildings — City of Dallas, Planning and Development; read 2026-10-29.
• District of Columbia Historic Preservation Guidelines: Roofs on Historic Buildings — Government of the District of Columbia, Office of Planning, Historic Preservation Office; read 2026-10-29.
• Step Flashing at Roof-Wall Intersections — U.S. Department of Energy, Building Science Education Solution Center; read 2026-10-29.
• Kick-out Flashing For Re-Roofing Detail (prints IRC Section R905.2.8.3, dated 2/20/2015) — Douglas County, Colorado, Building Division; published 2015-02-20; read 2026-10-29.
• Kennebunk Historic Preservation Overlay District Design Guidelines: Roof, Dormers, Chimneys, Balustrades, Cupolas, Cornices and Friezes, Drainage Systems — Town of Kennebunk, Maine; read 2026-10-29.
• Zoning Administrator Bulletin No. 3: Dormer Windows (June 1996, reprinted January 2019) — San Francisco Planning Department (Zoning Administrator); published 1996-06; read 2026-10-29.