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Q.The shady part of a customer’s roof is stained with round, pea-green growths on the asphalt shingles. The round spots are of various sizes, and they are not fuzzy like moss. A bleach solution does not remove these growths. What are they, and how can I remove them?

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. Any more aggressive roof-cleaning measures are likely to damage the asphalt shingles and result in more harm than good. Lichen and algae are primarily an aesthetic problem, and if left alone will not prematurely degrade the shingles.