Dog house with windows

Two eyebrow dormers and a gable form part of a face.

Pier House, by Corry pier, Broadford, Skye formerly Campbell's Temperance Hotel, c.1880

A dormer is a structural element of a building that protrudes from the plane of a sloping roof surface. Dormers are used, either in original construction or as later additions, to create usable space in the roof of a building by adding headroom and usually also by enabling addition of windows.

Often conflated with the term "dormer", a dormer window is a window set into the dormer. Like skylights, dormer windows are a source of light and ventilation for top floors, but unlike skylights (which are parallel to the roof surface) they also increase the amount of headroom in the room and allow for more usable space.

A blind dormer or false dormer is a dormer that can only be seen from the outside of the house: it is roofed on the inside, and does not provide any extra space or light. These are often used to make the house appear more impressive.

Dormer is derived from the French dormir, meaning to sleep or from Latin, dormitorium, meaning a place of sleeping. Dormer windows date their origin to 17th-century Paris and are attributed to French architect Francois Mansart, who introduced the Mansard style roof.

The main types of dormer are:

• Also called simply a gable dormer, the front of this dormer rises along a flat plane to a point at the ridge of the dormer roof. It is also known as a dog-house dormer (due to its visual similarity to same).
• This style of dormer is an analogue to the hip roof—its roof is composed of three sloping planes that converge at the ridge of the dormer.
• Flat roof dormer: The roof of this dormer is flat and parallel to the ground with a frontal eave that parallels the main roof eave.
• Shed dormer: This dormer also has a flat roof but the roof slopes downward at an angle somewhat less than that of the surrounding roof. Its front eave line is, again, parallel to the main roof eave line. Shed dormers can provide more attic space and head room than gable dormers, but cannot be the same pitch as the main roof and may therefore require different roof sheeting. Often used in gable-roofed homes, a shed dormer has a single-planed roof, pitched at a shallower angle than the main roof.
• This is a dormer whose face is coplanar with the face of the wall below, breaking the line at the cornice of the building.
• Eyebrow or eyelid dormer: "A low dormer on the slope of a roof. It has no sides, the roofing being carried over it in a wavy line." The bottom of an eyebrow dormer is flat and the top is curved.
• Link dormer: This is a large dormer that houses a chimney or joins one part of a roof to another.
• Bonneted dormer: This is an arched roof dormer, rounded in shape when viewed from front. Popular in Victorian homes, especially in certain areas, like the Southcott-style row-houses called Jellybean Row in St. John's, Newfoundland.
• Nantucket dormer: This is a complicated dormer structure composed of two gable dormers connected by a shed dormer.
• Lucarne: A small dormer in a roof or a spire.