What Causes Ugly Roof Stains ?
The dark stains are caused by a hardy algae, Gloeocapsa Magma, that thrives on asphalt shingled roofs in much of the country, especially more humid areas. Algae spores are carried by wind, birds, squirrels, etc. from roof to roof. Growth normally starts on the north side as well as shaded areas of 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.
Years ago, this staining was not a big problem. But asphalt shingles are now manufactured using limestone as the base filler. Unfortunately, limestone is a favorite food of this strain of algae. The algae used to starve to death when it landed on your roof – now it thrives on your roof!