Many homeowners are surprised to discover that their once clean asphalt roof shingles have been slowly covered with a form of roof Algae or Mold that appears as a light discoloration and spreads down the roof toward the gutters. The black roof stains are caused by a form of algae that has been around for years, typically found in the woods on the north side of trees, or on exposed rocks; but now this black algae is termed a roof shingle algae because of the annoying discoloration on an asphalt shingle roof.