There are 2 main companies that are referred to as the authorities on roofing one is GAF which stands for the General Aniline & Film. Gaf has many articles about there roofing and one of the best is on how to explain roof algy to a homeowner found HERE. We follow all the recommended methods and cleaning supplies to clean roofs as layed out by GAF.
The second is called ARMA or the Asphalt roofing manufacturing association. There is a great articles on there main web page which goes over what is discoloring and how to treat the blue green algae Here We use the recommend cleaning methods as laid out by ARMA as well on all of our roof cleans.