Algae is a living organism that thrives on dust, pollen, and the organic materials that make up the roof. Dampness triggers its growth. Stains are usually more prominent on northern and western exposures and in heavily shaded areas. Algae grows on, under, and behind the tiny granules that cover the shingles and can harbor a number of molds. This results in unsightly stains and streaks on the roof. Roofs with algae stains absorb heat rather than reflect it, resulting in a super-heated attic and higher home cooling costs as well as reducing the natural life span of the shingles.