Dormer

A dormer is a structure that projects up and out from the plane of a sloping roof, with its own small roof and, usually, a vertical window in its face. Dormers turn awkward attic space into usable rooms by adding headroom and admit daylight and ventilation. The word derives from the Middle French dormeor, a sleeping room, because dormers often lit attic bedrooms.

Common types

• Gable (gable-fronted) dormer: a small gable with two sloping planes; the most common type, sometimes called a doghouse dormer.
• Hipped dormer: three sloping planes meeting at a short ridge.
• Shed dormer: a single roof plane sloping in the same direction as the main roof but at a shallower pitch, giving maximum floor area.
• Flat-roof dormer: a nearly level roof with slight fall; widely used in UK loft conversions, where large box dormers are common.
• Eyebrow dormer: a low, wide dormer with a gently curved roof that flows into the main roof without vertical sides.
• Wall dormer: the face of the dormer is flush with the wall below, breaking the eave line.
• Bonneted, Nantucket, link and blind dormers are less common variants; a lucarne is a slender dormer on a Gothic spire.

Construction and weatherproofing

Dormers interrupt the roof and create several vulnerable junctions: step flashing or soakers along the dormer cheeks (sides), an apron flashing below the window, and valleys where the dormer roof meets the main roof. The roof framing around a dormer opening needs trimmers to carry loads from the cut rafters.

History

Early dormers included lucarnes ventilating English Gothic spires. Dormer windows have been used in British houses since the 16th century, and François Mansart's 17th-century mansard roofs in Paris helped popularise them. In many countries, adding a dormer can require planning permission or must follow permitted development rules.