Prevent roof discoloration with algae-resistant shingles

"My roof is in the shade much of the day, and it has significant areas of discoloration that I suspect are mildew or something similar. Is having my roof cleaned necessary? Is it safe for the roof?" — Angie's List member Dora Garrett

The black marks are caused by algae, which doesn't survive well in direct sunlight, according to Mike Hilborn, owner of highly rated Roof-to-Deck Restoration in Wood Dale, Ill.

"The best way to get rid of it is to kill it with a sodium hypochlorite [bleach] solution, then rinse it off with a fresh water soft wash," he says. He adds that a cleaning should take two or three hours and cost about $400, depending on the size of the roof.

He suggests looking for an algae-resistant shingle such as 3M's Algae Block, which contains copper granules that can help prevent new growth.

"[Discoloration] is usually a problem that affects asphalt shingles," says Joan Crowe, technical services director for the National Roofing Contractors Association, which takes no position on whether roof cleaning is necessary. "You can clean it, but the discoloration will come back again."

Most homeowners choose to clean their roofs for aesthetic reasons, Crowe says, and she recommends using a mild bleach-water solution with a soft brush or sponge. She advises against using pressure washers, which can damage shingles.

Roof-to-Deck used to install zinc or copper strips to prevent algae growth, but doesn't anymore because in Hilborn's experience, they don't help. "The strips only work when it rains, because they're water-activated," he says. "There's not enough exposure, and it'll never fix the problem."