The dark streaks you see on your roof is a bacteria called gloeocapsa magma. Gloeocapsa Magma is a species of cyanobacteria. Cyanobacteria are an ancient line of photosynthesizing bacteria that photolyze water-generating oxygen gas. You must treat these types of bacteria and algae with the appropriate combination of chemicals to ensure a 100% kill ratio. This cleaning method more closely resembles a pest control application, rather than pressure washing.