A dormer is a window set vertically in a structure projecting from a sloping roof.
Dormer windows are used to create a useable area in the roof of a building. They do this by doing two things.
First of all they let a good source of light and ventilation into an otherwise dark space. Secondly, unlike skylights they increase the amount of headroom in the loft to allow the room more useable space.
The main types of dormer window are:-