I've heard many theories in my career. Some think it's caused by granule loss, some say it's jet fuel. It's actually an algae, Gloeocapsa Magma, that feeds on oraganic matter in your roofing shingle. It is a hardy algae with a rigid husk. It can almost always be found on the north side of the roof where there is less sunlight. With the proper combination of cleaning agents it can be safely removed. Powerwashing causes damage and can loosen the roofing granules that protects the underlying membrane.
