Most roof stains are caused by the growth of algae (which can harbor a number of molds), lichen colonies (fungus) or moss that feed on the organic material frequently found in the mineral-based granules that make up roofing shingles, mainly limestone. This organic matter is the food source for these roof staining organisms. These organisms are literally eating your roof. The portions of a roof that receive the least amount of sunlight are more likely to have algae, lichen colonies, or moss growth. This explains why some homes may have roof stains only on one side of the roof, typically the northern and/or western exposure or in heavily shaded areas.
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.
Thick moss growth with its shallow root system keeps the roofing materials damp for extended periods of time. This moisture promotes wood rot and causes erosion of the shingles. Moss typically starts to grow at the top of the vertical slots between the shingles. These areas of asphalt shingles are the last place water evaporates from a roof. Moss, like lichen colonies, is quite damaging to shingles and some of the granules underneath the moss may already be gone as a result of this organism.
Lichens are a fungus, usually of the class Ascomycetes, that grow symbiotically with algae, resulting in a composite organism that characteristically forms a crust-like or branching growth on shingle granules. Lichens put out root-like tentacles that penetrate deep into the shingle’s organic, oil base for nourishment, eventually ruining them by creating dark spots, or pock marks where they grow.