Below is a Reprint of Technical Bulletin 217-RR-89 from ARMA, the Asphalt Roofing Manufacturers Association
For many years, roof discoloration caused by algae has been observed throughout the United States and Canada. This is commonly referred to as “fungus growth”. The discoloration usually has a brown to black appearance, and may be mistaken for soot, dirt, moss, or tree droppings.
The primary species of algae being observed is Gloeocapsa Magma. This type of algae is contained in and transported through the air, and tends to collect and grow upon roofing structures. Natural pigments produced by this algae may cause a white or light colored roof to gradually turn dark brown or black. The algae discolorations should not be confused with moss or tree droppings, which typically produce only localized discolorations.
This type of roof discoloration has been most widespread in the Gulf States and along the Northwest and Eastern Seaboards. It is not, however, confined to only these regions. Algae growth occurs to varying degrees in all regions of the country, especially those subjected to warm, humid conditions. It should be noted that almost all types of roofing systems are susceptible to algae discoloration. It is, of course, most readily visible upon white or pastel roofs, while it is not so visible upon darker shades of roofing.
This roof algae can be killed and removed to restore your roofs appearance. But the right cleaners and low pressure is a must, so that damage is not done to the roof. Never use high pressure equipment to clean a roof.