How a parapet flat roof is made
Draining the roof
Where a flat roof is the main house roof, it is usually of the parapet type. A parapet roof may also be given to a one or two-storey extension or to a detached garage, as an alternative to the more usual flat roof with aprons (see Flat roof with aprons). In a parapet flat roof, the walls continue above the roof level. The parapet may be only one course of brickwork or masonry or it may be several courses. The flat area consists of softwood planks or sheets of outdoor-grade plywood or chipboard nailed to roof beams and covered with asphalt or roofing felt.
The roof usually has a slight fall towards an opening in one side with a drainage hopper outside it connected to a downpipe that carries away rainwater.
Where the roof meets the house wall or parapet there is a triangular wooden fillet and a flashing of lead, zinc, aluminum or felt. A cracked or displaced flashing is a frequent source of trouble.
Eventually, cracks may develop in the asphalt or felt covering. You can repair minor damage (see Minor repairs to flat roofs).
If the roof lets in water in several spots, it is wisest to re-cover the whole area to prevent further damage to the roof timbers orb the rooms below. Layers of rubberized liquid bitumen reinforced with a mesh fabric will make a strong new cover over both the unsound surface and the flashings.