All of the conditions for microbial development are present on an inadequately kept roof. Algae is a bacterium that feeds on asphalt and limestone roofing in parts of the roof that are damp, chilly, and shaded. Algae takes on a blue-green hue when exposed to little light. Algae and fungus grow together in a symbiotic relationship. Fungus is difficult to remove off the roof and can recur due to humidity and rain. Mold and algae growth can harm the health of those who live in the house, in addition to creating structural damage. Mold growth is a complex issue that, if left ignored, can quickly spread to other areas of the building.