Mix the soap and h2o very well to start with then include the bleach. This will protect 200 sq. ft. Spray the black stains initially then the whole roof so the stains have a next dose. DON”T RINSE!!! The bleach degrades into salt which kills microbial growths as well http://roof-cleaning71594.getblogs.net/18687199/the-smart-trick-of-graffiti-removal-that-no-one-is-discussing