I had the black streaks removed professionally from my asphalt-shingled house roof just last week. The stains were identified as an algae called 'gloeocapsa magma.'
The house is 42' long with a 5/12 pitch. The contractor had this weird looking sprayer nozzle that easily reached the peak from the ground. He give it two applications (and a rinse) and I watched the green stuff run out of my gutter outlets. It works almost immediately. The roof looks like new now.
He said the bleach he uses is several times stronger than what you'd get from the store.
