The Cool Roof Rating Council is probably the best source.
For your application you might consider a reflective coating. GAF's Topcoat is one example. However, a ballasted roof can be surprisingly good -- here is an excerpt from the CRRC website:
- The City of Chicago has recently updated its municipal building code to include ballasted roof systems as an accepted cool roofing alternative on low-sloped roofs as of January 1, 2010. This follows a similar decision by the California Energy Commission to recognize ballasted roof systems in its cool roofing standards.
- The latest version of draft ASHRAE Standard 189.1 also lists ballasted roofing as a prescriptive alternative to a cool roof. A ballasted roofing system normally consists of river-rock ballast over a membrane, rigid insulation, and a metal roof deck. Though the solar reflectance is lower than a traditional cool roof, the river rock has high thermal emittance properties and the mass of the ballast acts as a heat sink, resulting in, according to some, a similar overall performance to a cool roof.