A safe process is key when considering Roof Cleaning. The appropriate cleaners need to be doing the bulk of the work. Mildewcide needs to kill the mildew and mold causing black streaks and black patches.
The detergents are allowed an extended dwell time, reapplying to keep the substrate wet. When the mildew and mold has been Killed, then, and only then, will a thorough flush and repeated rinsing will begin.