Meguiar's All Purpose Cleaner (APC), comes in a gallon sized container, color green. Should be able to pick it up at an auto parts store. Cost is around 15 bucks, and it is heavily concentrated to begin with. Dilute 10:1 for a mild solution with water for average spots and contamination, 4:1 for heavier spots. If you are afraid to use a brush on the roof, try using a soft bristled toothbrush or a sponge to aggitate the solution on the stain. Vacuum out with a wet/dry vac.