Recently, JLA had a lunch and learn on roofing products with Tim Kenna from Exterior Building Solutions. We learned about the different benefits of white v. black roofs, how they function and how surrounding areas are effected.
White roofs have been part of a movement called cool roofing. The idea is to use the white roof to reflect sun off the building thus keeping the cooling loads down. While this works great in geographic areas with low heating loads and high cooling loads, it does not work well in places that have high heating loads and low cooling loads. The best location for a white roof is below Zone 4 on the ASHRAE rating; buildings in this area are dominated by cooling loads. The white roof will decrease air conditioning costs of those buildings, and help keep the surrounding area cooler as well. LEED currently gives points for having a white roof, even though a black roof performs better in colder climates.
Some of the biggest downsides to a white roof is moisture build up, hotter to be on, increase to interior heating in the colder months, and they are harder to maintain. The roof temperature is decreased right at roof level due to the sun reflecting, but the temperature goes up the further you are away from the surface. This causes the temperature to be much hotter at 5 feet above then at the surface meaning it feels hotter to be on the roof. A white roof will slowly lose it reflectivity due to the amount of dirt that has accumulated on the roof; this causes the roof to shift from white and move towards being a darker gray color. This cuts down on the reflectivity of the roof which decreases any benefit one would receive. The white roofs have seen an issue with moisture and condensation due to the change in temperature within the assembly. This buildup of moisture mostly occurs in colder climates due to the water vapor not being able to escape from the assembly and sitting under the white roof, which prevents it from evaporation due to not heating up the same was a black roof would.
Black or dark roofs function differently as they absorb the sunlight which in turn keeps the roof surface at a hotter temperature. This allows for the roof to help with heating loads in the building. Tim Kenna suggested that in the colder temperature areas (above zone 4 on the ASHRAE rating) that designers use darker roofs as the benefit is significantly better. Unlike white roofs, there are not as many issues with moisture and condensation.