A. Jeff Jacobs, a specialist working in product development at 3M, responds: From the description, it is impossible to identify the discoloration with certainty. The only way to know is to look at a sample under a microscope. If the spots vary in size from a dime to a dollar and the edges are irregular and flat, it could be a lichen. Otherwise, it could be green algae growing in clumps — although algae is usually removable with a bleach solution.
If the discolored areas can’t be cleaned easily, it’s probably best to leave them alone. Since it sounds like this discoloration is occurring only in the shaded area, it may help to trim back any overhanging tree limbs, allowing more light to reach the roof. More light could help retard the growth of any organisms partial to damp, shady areas....
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