The black mold-like stains and streaks that sometimes appear on roofs, and in particular, light-colored asphalt shingles, is actually a blue-green algae (Glockenspiel magma). It is most commonly found in climates with warm, humid summers It does not, in fact, do damage to the roofing, but it definitely takes away from the aesthetic appeal of the house.
You could replace all the roofing with new shingles dark enough to disguise the staining, or with shingles laced with copper granules, which are lethal to algae. That is only a good and economical decision if the shingles on the roof are worn out.
The least expensive solution is to spray wash the roof with a 50 percent mix of water and bleach to get rid of the algae. (Avoid using a pressure washer as they are likely to damage the shingles.) Make sure to wet your foundation plantings first, and then rinse everything in clean water when you are done. Plants don't like bleach, and wetting them with plain water first protects them.
You can keep the algae from coming back by inserting 6-inch-wide strips of zinc or copper under the row of shingling closest to the roof peak, leaving an inch or two of the lower edge exposed to the weather. So that whenever it rains, some of the metal molecules will wash down the roof and kill any algae trying to regain a foothold on your shingles.
You can probably see this same principle working on roofs in your neighborhood. Look for chimneys with copper flashing; the areas directly below the flashing will be free of any algae stains.
The strips also work on roofs suffering from moss buildup. Just scrub it off first with a brush, then bleach as above.
Boyd Construction Co
Family owned and operated in the greater Washington DC area since 1932.