Hi guys, I know this thread is old, however, the problem still exists and all information is still very pertinent so I just want to put my 2 cents in. I am also a professional roof cleaner in the Cleveland area. I will agree that the zinc in the flashing DOES, in fact, inhibit or slow the algae growth as evidenced in the prior pics that were posted. I see it all the time, HOWEVER, they DO NOT WORK FOR VERY LONG!!! The Zinc either loses its effectiveness and/or over time mother nature wins. I have also seen over and over where there is no stain immediately under the flashing and even a few feet below it, but depending on the distance to the gutter line, it eventually begins to stain. Also, I have done countless warranty jobs on the "algae-resistant shingles. They just don’t work for very long. The warranty on the Owens Corning Algae Resistant shingles is only 10 years. 1 of their products has a 15 year warranty against the algae but all the rest are only 10 years. It takes about 6-7± years for new (non algae resistant) shingles to start showing staining anyway, so no matter what you buy, if there is staining going on in the area, that roof will eventually fall victim. Light gray shingles are the worst and you will see staining on all sides of the roof. The recommended cleaning method is Chlorine Bleach per ARMA technical bulletin. Common sense will tell you NEVER use a power washer!
As a home inspector, all you need to do is explain what it is, (Gloeocapsa Magma), look up the definition on the web, and recommend that it be cleaned by a professional roof cleaner. The cleaning service is very cheap. I charge anywhere from $250-$675 depending on the size and complexity of the house. (The $675 was a 10,000 sq ft mansion). This is a great thing to recommend/mention on your reports whether it be a seller paid inspection (to help them increase their curb appeal) or a buyers inspection so that they dont think they need a new roof. Even very smart and educated people that I have talked to, assume that the roof needs to be replaced. My experience has proved that the majority have never even heard of roof cleaning.
Feel free to email me any questions or even if you want verbage to put in your report.
Can you recommend a professional roof cleaner in my area (Methuen, MA), so I can personally put a shingle cleaning to a test and see how long it’ll last?