The Cause:
These stains are caused by a "blue-green" algae, Gloeocapsa magma, which thrives on the limestone additive in your asphalt or fiberglass composite shingles. As this algae grows, it holds moisture allowing the growth to accelerate. Thus, these roof stains get worse year after year. Shade can also provide the ideal environment for lichens & mosses to thrive.

Algae, moss & lichen spores are carried by wind, birds, squirrels, etc. If your roof is currently clean, it likely won’t stay that way especially if other houses in the area are already stained.

The Solution:
We use a sodium hypochlorite-based solution followed by a non-pressure rinse. Sodium hypochlorite is the active ingredient in bleach and chlorine, and when applied to a roof at the right ratios is quite simply the safest, most effective way to clean a roof. It is the recommended method for cleaning roofs by ARMA (the Asphalt Roofing Manufacturers Association), and their technical bulletin on the topic can be viewed here (pdf file). Finally, we treat the entire roof with a zinc based solution to prevent future growth. For the most severe cases we can also install solid zinc flashing to the roof ridges to provide a constant source of zinc ion prevention. NON-PRESSURE CLEANING.