Black stain on a roof is algae called Gloeocapsa Magma. It spreads from roof to roof through airborne spores, birds and squirrels. These spores begin landing on the roof the day of installation but can take many years to become visible. The spores do not choose where to land. They land everywhere but will grow faster on the shady sides of the roof. Shady sides of the roof are cooler and hold more moisture. Heavy algae stained areas will hold even more moisture promoting the growth of other organisms like moss and lichen. Moss and lichen develop root systems into the shingle. These root systems slowly tear through the shingle leading to premature failure. The root system s what prevents them from blowing away in the wind or washing away with rains.
It is thought that the algae feed on the limestone in the roofing shingle. Roof manufacturers started adding limestone filler to add weight and strength to the shingle. Since limestone is organic it provides abundant food source for the algae to grow and thrive. Roof shingle were designed to reflect the suns harmful UV. A darkened stained roof absorbs UV and can raise attic temperatures causing your air conditioning to run harder in the summer.
Maintaining your roof can add years of life to your roof. Roof eating organisms allows the roof to retain moisture. This leads to premature granular loss. Most people do not know what the stains are on the roofing shingle. Many mistake stained roofs as damaged shingles. When selling a home you can eliminate one the biggest bargaining chips, the roof.