As a professional home inspector in South East Michigan, a day doesn't go by that I don't see black stains on shingles. The black stains on a roof are actually an airborne fungus. This fungus grows and is attracted to certain areas of roofs that have any of the following characteristics:
This black fungus is a pesky little thing flying around in the wind and looking for roof prospects to grow on. Normally when the airborne fungus shows up on one house in a neighborhood, it will eventually spread to some of the other houses in the same neighborhood (as long as the proper growing conditions exist on a particular roof).
Do not despair, this fungus can be removed with the proper cleaner and with the use of a power washer (the concentrate comes in a 1 gallon container and usually costs around $25.). This roof cleaner can be purchased at Lowe’s, Home Depot, or any roofing supply house. If you would rather hire someone to do this, there are companies in town who specialize in power washing roofs.
One can also have a roofer to install zinc shingle shields at the top of the roof. These are relatively inexpensive metal strips (2" wide) that are nailed along the entire tops of each roof line. When it rains, the rain water reacts with the zinc strips creating zinc oxide that then runs down the roof and kills any fungus or algae that might be growing. Further information on the Shingle Shield can be seen at www.shingleshield.com.
For those homeowners who are getting close to replacing their shingles, shingle manufacturers are now offering FR (fungus resistant) shingles. The FR shingles come in the full range of colors/styles and they only cost a small fraction higher than normal shingles. The roofing manufacturers have been doing research on this roof fungus problem and they have changed the granule formulation (on the top of the shingle) to include more zinc particles which inhibits fungus growth. I have heard that within the next several years, all shingles made will automatically be the FR shingles.
The bottom line…the roof fungus problem is just another thing to add to one’s home maintenance list. If you don’t have roof fungus, consider yourself lucky. If your roof does have it, it’s not the end of the world. Just clean it yourself or get someone else to clean it for you. Your house will be happy that you did, plus you’ll be happy coming home to a “cleaner” looking house.