Clean asphalt roof shingles is most often required after a case of blue-green algae called Gloeocapsa Magma on the shingles.
The problem with this strain of algae has been that over the last 25 years it has migrated to less humid environments from its main areas of existence in the South of United States.
So much in fact, that the algae has been found at some 80% of the United States.
The growth of the algae usually happens on the north or west sides as well as shaded areas of the roof.
What's even worse, as the algae grows and decomposes, the remains hold moisture, further accelerating its growth.
One of the factors that have caused the algae to expand is the fact that some 30 years ago asphalt shingles manufacturers started using fiberglass mat compositions.
Previously these manufacturers had used an organic mat made of felt or other organic base composed of various cellulose fibers.
Fiberglass mats also use crushed limestone or marble dust, which, unfortunately, is a food source for Gloeocapsa Magma algae.
There are two main types of roof shingles cleaners that the professionals use to clean roofs: chlorine bleach based cleaners and non-chlorine bleach cleaners made from sodium hydroxide (lye).
Both products have their pros and cons.
For one, bleach solutions will lighten the stains to the point they will not be visible, but not completely remove them. Sodium Hydroxide products are often more effective at removing the algae, but require more time and are harder to rinse from the roof surface than chorine bleach.
Because the products used in the cleaning products are toxic, the work needs professional touch. Here's one professional video ad about how the work is done: