What sort of roof - EPDM rubber? And what's shape is it in now? Just plain old dirt, or is there tre sap or other nasty stuff stuck to it? Any green or black mold in the pores of the material, crevices, etc.?
Basically you just want a strong household detergent and some amount of bleach, depending on the mold (if any). Simple Green works well, as does Purple Power, Greased Lightning and similar detergent cleaners. Add bleach to the wash water at the rate of anywhere from 0.25-1.0 cups per gallon, depending on how much mold there is. Use a medium bristle scrub brush, preferably with a long handle (easier on the back).