The idea is to avoid stepping into the pure bleach you just sprayed onto the infected roof! again, start at gutter area and work your way up to top. Let bleach stand and do it’s job then using a water hose spray off the bleach, you may not notice much of a difference until roof dries if the roof does not have that much black fungus on it. If it’s coated with fungus you will see a big difference quickly! If the first spray coat did not get it all then allow roof to dry somewhat and then respray again. While you are waiting for the water on that side of the roof to dry, go to the other side of the roof (left side) and begin applying your bleach the same way you did before. Repeat these steps until roof is clean. 2 Coates is usually all you need to apply unless roof is really bad.