According to Wikipedia,
“A flat roof is a type of covering of a building. In contrast to the sloped form of a 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.
Traditionally flat roofs would use a tar and gravel based surface which, as long as there was no pooling of water, was sufficient to prevent penetration. However, these surfaces would tend to fail in colder climates, where ice dams and the like could block the flow of water. Similarly, they tend to be sensitive to sagging of the roof reversing the subtle grading of the surface.
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 using copper or tin. These are soldered interlocking systems of metal panels.
Modernist architecture often viewed the flat roof as a living area. Le Corbusier’s theoretical works, particularly Vers une Architecture, and the influential Villa Savoye and Unité d’Habitation prominently feature rooftop terraces. That said, Villa Savoye’s roof commenced leaking almost immediately after the Savoye family moved in. Le Corbusier only narrowly avoided a lawsuit from the family due to the fact they had to flee the country as France succumbed to the German Army in WWII.”
- “Flat roof.” Wikipedia, http://en.wikipedia.org/wiki/Flat_roof (accessed May 15, 2010).