The black or brown staining seen on many roofs that is frequently mistaken for dirt or soot is really a species of algae called Gloeocapsa Magma. This algae is transferred through the air and lands on the exterior surface areas of your house, including your roof, where it feeds and grows. In addition to causing unsightly black streaks, these algae also feed on the limestone filler in shingles. This can cause major damage and wear and tear to your roof. Your roof isn’t just dirty — it’s contaminated!