Zero Pitch Roof
In contrast to a sloped or pitched roof, a flat roof is horizontal or nearly horizontal. Materials that cover flat roofs should allow the water to run off freely from a very slight inclination as insufficient slope can result in water ponding on the roof. This in turn can result in deterioration of roof covering with water penetration onto roof framing creating fungal decay and conditions favourable to timber pest activity.
Modern flat roofs tend to use a continuous membrane covering which can better resist pools of standing water. These membranes are applied as a continuous sheet where possible, though sealants and adhesives are available to allow for bonding multiple sheets and dealing with structures penetrating the roof surface. Far more expensive flat roof options include sealed metal roofs.
Flat roofs tend to be sensitive to human traffic. Anything which produces a crack or puncture in the surface can quite readily lead to leaks. It is thus not generally advisable to use a flat roof as a living area unless steps are taken to protect the roofing membrane from those using the area.
WIKIPEDIA ENTRY FOR FLAT ROOF.
