The ugly, dark stains that are so prevalent are caused by a very hardy “blue-green” algae, Gloeocapsa magma, that thrives on the asphalt shingled roofs in much of the country, especially in the highly humid areas that are indicated in green on the map. These are areas most likely to be impacted by stain causing roof algae, Gloeocapsa magma, as well as other stains caused by mildew, mold, moss and lichen.
Growth is more prevalent on the north and west facing surfaces of the roof (and highly shaded areas) due to the reduced amount of sun received by these surfaces. After a rain, these roof surfaces tend to not dry completely thus providing an ideal area for algae growth. As algae grows on the roof, it holds even more moisture which accelerates the growth. The results is ugly roof stains that continue to get worse and worse over time
Algae spores are carried by wind, birds, squirrels, etc. If your roof is currently clean, it likely won’t stay that way - if other houses in the area are already stained.
There are other types of stains found on roofs. Green roof stains are often caused by mold and mildew. Lichen & moss also cause stains and growths on roofs.
The Design of Asphalt Shingles has Changed
Years ago, stains on asphalt shingled roofs were not a significant problem. Today it is a big problem because the design of asphalt shingles has changed over time.
Asphalt shingle roofs have been in use since the late 1800s. Early versions were made from felts containing cotton or wool fibers. In the early 1940s, cellulose fibers derived from recycled paper or wood began being used to manufacture asphalt shingle mat. Since the late 1970s, the primary material has been inorganic fiberglass mat (over 80% of the current market). The mat is coated with asphalt that is then covered with a protective layer of U.V. resistant, colored ceramic granules. In the manufacture of fiberglass asphalt shingles, limestone is used as a void filler between individual mats and also provides resistance to fire and weather. Unfortunately, limestone is a favorite food for the blue-green algae, Gloeocapsa magma, that cause the dark stains on these fiberglass asphalt shingle roofs. The algae used to starve to death when it landed on your roof - now it thrives on your roof.
According to the Asphalt Roofing Manufacturing Association, asphalt shingle roofs represent 80 to 85% of the current residential roofing market. When the popularity of these shingles is considered, plus the current use of limestone filler, it is easy to understand the reason for the wide spread occurrence of algae caused roof stains - especially in areas with high humidity.
Algae also Damages the Roof & Shortens Roof Life
It is bad enough that algae causes ugly roof stains, unfortunately it also damages the roof The algae takes root on the shingles dislodging the granules which protect & color the roof. It holds water to the roof's surface and can cause decay & rot of the roof's underlayment. Not only is your roof being discolored by the algae, but it is shortening the life of the roof leading to earlier than necessary roof replacement.
