does anyone know how to get those long black stains off an asphalt roof without replacing it?
"Cleaning a shingle roof that has this algae infestation(those long balck stains) is not that hard. It is DANGEROUS work, but not hard. The trick is to apply standard treated wood deck cleaners to your roof and follow the instructions.
However, you shouldn’t use just any deck cleaner. You must use a deck cleaning product that contains a safe cleaning agent such as oxygen bleach. Sodium percarbonate does the same job as chlorine bleach. However, chlorine bleach is harmful to you, your roof, your gutters and downspouts, and any plants it might come in contact with.
If you decide to clean your own roof, BE CAREFUL. Wet roofs are slippery. These deck cleaners are slippery. You can easily fall off your roof and either die or become permanently crippled. I’m serious. So, if you still want to do this, and not hire a professional, be sure to wear an OSHA-approved safety harness. Also, do not direct the hose spray UP the roof! Water can get under shingles and leak into your house. Always spray down the roof. " -Tim Carter
I think that explains it all!