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Asphalt / Asphalt Fiberglass / Composite Shingles
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.
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 as indicated on the following map.
Algae Stain / Moss / Lichen Danger Area
 The highly humid areas 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 and mold. Moss and lichen grow on roofs in these areas as well; however, moss & lichen tend to occur on roofs more in the Northeast and Northwest climate areas.
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.
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