A study sponsored by the Single Ply Roofing Industry to evaluate the long-term reflectivity performance of cool roofs.  Samples were exposed for three years to both identify changes in reflectivity and the causes for those changes.

Long-term field exposure of a variety of single ply membranes has led to the development of a database of information that was employed to validate a calculation tool that can be used to estimate the economic benefits of deploying cool roof strategies. Understanding how the reflectance of these membranes changed with time and the causes of the changes has been explored. The financial impact of these changes was also estimated, and savings in predominantly cooling and some moderate climates justify periodic washing of cool roof membranes.

Roodvoets, David, Miller, William, and Desjarlais, Andres.  2004. Long Term Reflective Performance of Roof Membranes.