Most roof stains are caused by the growth of one of the following organisms:
Algae, which can harbor a number of molds
Lichen Colonies, which are a fungus or
Moss, which can keep your roof from drying normally.
These organisms feed on the organic material frequently found in the mineral-based granules that make up roofing shingles, mainly limestone. This organic matter is the is the food source for these roof-staining organisms. THESE ORGANISMS ARE LITERALLY EATING YOUR ROOF. This means that these stains are more than just unsightly, they can really damage the materials that make up your roof. The portions of the 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.