Sheesh. From This Old House:
"The black mold-like stains and streaks that appear on roofs,
particularly light-colored asphalt shingles, is actually a blue-green
algae (Gloeocapsa magma). Commonly found in climates with warm, humid
summers, it does no damage to the roofing, but it certainly does looks
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. But that would only make sense if the
shingles were worn out.
The less expensive solution is to spray wash the roof with a 50 percent
mix of water and bleach to get rid of the algae. (No pressure washers,
please. They're likely to damage the shingles.) Just be sure to wet
your foundation plantings first, and rinse everything in clean water
when you're done. Plants don't like bleach, and wetting them with
plain water first protects them.
To keep the algae from coming back, insert 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. That way
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. "