- 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.