What Causes the Ugly Roof Stains?
Black or very dark roof stains are caused by a hardy type of algae, Gloeocapsa magma (often referred to as blue-green algae), which grows on the roof and feeds on the limestone filler in asphalt shingles. Often the algae originates in standing water as well as in lakes and ponds. Wind, birds and other wildlife carry the algae spores to roofs from the water sources and then from roof to roof. Most of the stains begin on the north facing side of the roof where the lack of sun keeps the roof from drying out completely. The algae normally grows for quite some time before stains are noticed – 5 to 10 years is not uncommon. As the algae grows, it holds more moisture on the roof thus accelerating the growth process. Green roof stains are normally caused by mold or mildew. Other stains are caused by fungus, lichen and moss.
