Lichens, which require no soil, are often the first culprit on a roof. It seems that as the lichens get established, they provide the "soil" for other things that can grow on roofs, such as moss. I have to tell you, lichens are tough critters. Anyone who can grow on an asphalt roof in 140 degrees of summertime heat is one tough customer.
Concerns of Roof Lichen and Mosses
Well, we all want our roofs to last as long as possible, and many folks are concerned that their roof could be impacted by the growths. The lichens themselves, having no (or virtually no) root system, are unlikely to cause damage themselves to a roof, but since they can invite other organisms that do, over a long period those other organisms can interfere with the granuals that protect your asphalt roof, and perhaps with the roof base itself. (Note: as Asphalt is far and away the most common roof type, we are specifically talking about that type.) If you're concerned that the moss is getting out of hand on your roof, it may make sense to call in an expert, and see if it's serious enough to shorten your roof life.
Treatment of Roof Lichen and Mosses
There are chemical treatments for the problem, some are quite pricey, but I've also seen a simple mix of bleach and water to be recommended. Again, much would likely depend on the amount, and the types, of growths you need to battle, and in any case I would have discuss with an expert any serious problem, to make sure you don't do more damage than is already there! For example, I've also seen (on the great wide internet) recommend that you sweep or brush it off, but I think most people would tell you that sweeping your roof is probably both hazardous and NOT good for your roof - you could be sweeping loads of good granuals off your roof, and that is not good either!
Prevention of Roof Lichen
I think the simplest, most cost effective way is probably to add zinc strips to your roof. Again, probably not something to do yourself, but this is probably pretty cost effective for a contractor. The zinc mixes with rain water and creates an environment unsuitable for the lichens that started the problem, and if you get rid of those, or just keep them at bay, you can probably deter the rest of the organisms that come your way. In practice, I haven't seen very many homeowners, or home inspectors, issue much concern over these growths, but we can always file this under the best practices section!
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