The most common culprit of the black streaks on roofs is a blue-green algae called gloeocapsa magma. Over the past 25 years, this strain of algae has become more hardy and has migrated to less humid environments. It has now become a common sight on asphalt roofs across 80% of the United States. This unsightly growth usually occurs on the north or west side or the shaded areas of the roof. As the algae grows and decomposes, it holds moisture, which accelerates its growth.
and Not-So-Safe Roof Cleaning Products
Use DEFY® Roof Cleaner to quickly and safely clean and restore the appearance of your roof. One gallon of Roof Cleaner Concentrate makes seven gallons and will cover 700-900 square feet, depending on severity of stains and roof texture. Using roofing cleaning products like chlorine bleach or sodium hydroxide will accelerate the corrosion of metal gutters, downspouts, flashings, and roofing fasteners. Sodium hydroxide is the active ingredient found in many paint strippers, oven cleaners, and drain cleaners; it is caustic and high concentrations may cause irreversible damage to asphalt shingle roofs.
Besides the obvious health risks of handling sodium hydroxide and chlorine bleaches, strong bleach solutions will damage almost any hard surface that comes in contact with it; asphalt shingles are no exception.
Roof Cleaner is the first effective roofing cleaning product that does not rely on harsh chemicals like chlorine bleach or sodium hydroxide.
It strikes a perfect balance between aggressive cleaning and not being harmful to the roof or surrounding vegetation. It's made of an optimized blend of detergents, coupled with a unique buffered sodium metasilicate, a chemical made from sand and soda ash, which is recognized as a non-pollutant. For severe stains, especially on lighter colored roofs, multiple applications may be required. In some cases, complete stain removal may not be possible.
DEFY® Roof Cleaner is the first step in a two-step system to clean and protect your roof. To find out more about the roof cleaning tools needed, go to our Roof Cleaning Tools page. Once your roof is clean, the next step is to block the black roof stains from coming back by using DEFY® Stain Blocker for Roofs.
For more details on our roof cleaning system, read our report "Asphalt Roof Cleaning and Stain Prevention".