Do you ever notice roofs that have black staining on them that look something like this one below? I have seen several and more commonly, on houses in Stayner and Wasaga Beach.
They seem to be increasing in number and on a recent inspection of a home with this condition, the home inspector explained that it was an algae that is most common on north facing roofs and, that it is harmless.
Algae is naturally in the air and grows on surfaces where there is food such as lime which is commonly found in roof shingles as well as other materials. It also needs moisture so is more common in humid areas.
While it is commonly called Black Algae, it actually a dark blue-green in most cases. While it can look a bit frightening, in most cases, it is harmless to the roof and building structure. It is possible to clean a roof if it really bothers you and there are various websites with recommendations for doing so but they all agree, don’t use a pressure washer and that could do real damage