“My roof is in the shade much of the day and it has significant areas of black discoloration that I suspect are mildew or something similar. Is having my roof cleaned necessary? Is it safe for the roof?” — Dora Garrett

Whether it needs to be cleaned depends on the kind of roof it is, according to Kelly Carter, owner of Window Gang, which offers roof cleaning in San Antonio. “Asphalt shingles can be damaged by [the discoloration] if it’s not washed,” he says.

The discoloration is caused by fungus and moss that feed on the limestone contained in asphalt shingles, says Karen Warren, office manager at AAA-1 Roof Care in Woodinville, Wash. She says the problem is complicated by especially rainy climates, like the Pacific Northwest.

“It rains so much here, so shadowed roofs don’t dry out and these organisms begin to grow,” she says.

Bob Clem, owner of Home Team in Maple Valley, Wash., says the best way to remove the moss from your roof is by applying a solution that kills the organism and its roots. “If it’s done properly, the roof won’t need to be cleaned again for six to nine years,” he says.

Most homeowners choose to clean their roofs for aesthetic reasons, says Joan Crowe, technical services director for the National Roofing Contractors Association, which takes no position on whether roof cleaning is necessary. Crowe recommends using a mild bleach-water solution with a soft brush or sponge. She advises against using pressure washers, which can damage shingles.

"The long-term effect is it will lead to moss, and moss is damaging. So if you get rid of the algae, you're not going to have that moss problem," says Mike Hilborn owner of Roof-to-Deck Restoration in Menomonee Falls, Wis.

Hilborn says cleaning a roof costs $400 on average, depending on roof size and pitch.