In the Southeast US roof stains have become out of control. The hardy algae, Gloeocapsa magma has spread across the region and is reported now on both coasts.
40% of homeowners believe that the Black Roof Stains are actually the result of shingle granules having been washed off, thus exposing the tar. As a result they pay thousands of dollars to replace the roof.
They SHOULDN'T!
The Black Shingle Stains are actually covering the granules, and they can be removed by roof cleaning. Roof Cleaning is largely unknown, and roof contractors would rather it stay that way.

Look around your neighborhood...
Look on the roof...
Do you see black stains?