The black stuff on your roof isn’t dirt, it’s a living organism called Gloeocapsa Magma, a form of cynobacteria.
It spreads fast and can shorten the lifespan of your roof. It can actually spread from roof top to roof top. If it’s on your roof, it can spread throughout the neighborhood.
Gloeocapsa Magma works by embedding itself into your roof. It absorbs moisture and eats the organic material in your shingles. By holding in moisture, Gloeocapsa Magma can cause premature rotting of your shingles.