Black roof stains are caused by algae and lichen infestation.Not even zinc roofing strips eliminate this compulsive nuisance.
Algae also feeds on limestone, so it's no wonder your sills, caps, retaining walls and other hardscapes are inundated with these naturally occurring spores.
Painted and stained surfaces also suffer as black algae masquerades as dirt or worn paint. Think about the benefits of not exposing your siding and trim to the hazards of pressure washing. And, enjoy the savings in time and money by avoiding painting ... over algae and mildew.