[USA] Here in Oregon, when you combine the common blue-green algae that grows on your roof shingles with moss, you have a bad combination that can damage your roof, even promote rot.
What happens is the algae grows and becomes food for the moss. That moss then collects rainwater, like a sponge. If that top layer of moss gets heavy enough, the moisture seeps under the roof shingles and into the understructure. This brings mold build-up and eventually decay.
That means if you see algae or moss on your roof shingles, it’s time to remove it.
The only way to take care of it is to use a high-base chemical solution, like zinc sulfate.
Zinc sulfate most often comes in powder form. To start cleaning, apply it directly and generously to moss areas – it’s recommended to use three pounds of powder for every 600 square feet. Spread it along the ridge of the roof, then let the rain do its job of dissolving it.
Generally, this takes care of moss for 1-3 years. The zinc needs to be applied annually though to keep the algae from regrowing.
The downside to zinc sulfate is that can be damaging to plants if it hits them directly. That’s why, before using it, most people will drape their plants to protect them. We also recommend applying it on a calm day.
View original article at: Concentrated CO2 enables growth spurt in algae