The view from the Tower of the Americas reveals a huge number of flat roofs, mostly on commercial buildings. Since San Antonio does get rain, (except for during the Summer of 2009), flat roofs generally involve a lot of maintenance, a lot of tar and a lot of gravel. Being dark they also don't tend to reflect much of the sun's heat.
A completely white flat roof would be an advantage, and that is what is sold locally by Jaco-Roofing, whose slogan is "White=Green".
The Duro Last Cool Zone prefabricated flat roof white vinyl roof membrane system has a retained reflectivity of 85% or more after three years. The membrane consists of PVC resins, plasticizers, stabilizers, biocides, flame retardants and U.V. absorbents. By prefabricating the membrane under ideal factory conditions they are able to decrease field labor costs and increase quality.
Though the membrane is available in several colors. The white version is being promoted to counteract the urban heat island effect.