The stains on your roof are caused by an air-born algae called gloeocapsa magma that thrives on the limestone filament present in many types of asphalt shingles. Normally dormant during the winter months, this type of algae will start growing on the north or shaded side of the house and tends to spread very rapidly in a humid environment. “Roof algae,” as it’s called, can also drift from house to house.