A dormer is a roof structure, often containing one or more windows, that projects vertically beyond the plane of a pitched roof. Dormers are commonly used to increase the usable space in a loft and to create window openings in a roof plane. If your shed will be placed in tight quarters with another building or near trees where it may be difficult for natural light to reach the windows, having a dormer will provide an alternative way for sunlight to filter into the building.