Lye-based cleaners such as Defy are good roof-cleaning products, states the Family Handyman. While a bleach and water blend is not recommended, a blend of trisodium phosphate and chlorine bleach can be effective, notes Angie's List.
If using a lye-based roof cleaner, mix it with water according to the label's directions and then spray it on the roof. Wait about 20 minutes before rinsing it off with clear water.
If using a bleach and trisodium phosphate blend, spray the substance on the roof, allow it to dry in the sun and then reapply it as needed to clean off algae and mold. This method may require several applications to see results.