The roof on my split-level house is "stained," says e-mailer "Bob from the Suburbs." Is it mold? My roof looks discolored and it looks like I'm ready for a new one. In my development, roofs facing the sun are in excellent condition. Roofs facing away from the sun also have that black stain, so it can't be just me. I'm afraid to put detergent on it after the Home Depot roofing manager said the store doesn't sell a product for this problem.

Answer: I talked with Berwyn roofer Dennis Dunbar, owner of Dunbar Roofing & Siding, about the issue. The "stain" is, of course, black algae. The problem began surfacing several years ago when asphalt-shingle manufacturers switched from rags to ground limestone as a filler material, he says.

"Black algae really loves the stuff, with all those nutrients, and shady spots on the roof supply constant moisture, keeping the limestone damp and helping the algae grow," Dunbar said.

The mildew, that green stuff, feeds off the black algae, he said.

Black algae typically takes about 10 years to begin appearing, about the time the 10-year manufacturer's warranty on algae expires, Dunbar said. Some shingle manufacturers have been adding waste from copper processing to the limestone filler because copper has been proven to deter algae growth.

"Roofing has become almost a chemistry class," Dunbar said. He and other roofers say copper tends to be the best deterrent to the start and growth of black algae, although zinc, too, is sometimes used.

Roofers nail copper or zinc strips at the peak of the roof above the portion affected by the black algae. When it rains, water reacts with the copper strip, and what is produced - this isn't a chemistry class - continues down the roof to kill the algae, and the stain disappears.

While Dunbar thinks copper is better, zinc is what most suppliers sell, he said. Other metals also appear to work to some degree, though not consistently, at least from his observations.

The obvious question, of course: Is the black algae, or the green mildew, damaging your shingles?

"The mildew simply makes your roof slippery to walk on," Dunbar said, just as it does bricks and wood decks.

Black algae? Dunbar has broached the question with his shingle suppliers, and the consensus is no.

Joe Wertz, sales rep for asphalt-shingle manufacturer CertainTeed of Valley Forge, said his company's official policy is that the black algae is an aesthetic problem only, and he agrees with that assessment.

DaVinci Roofscapes in Kansas City, Kan., manufactures synthetic shingles from resins. Technical-services director Tim Gentry said black algae might affect long-term performance, but minimally.

While some reports suggest that algae affects asphalt-shingle life by eating away and loosening the granules, the process of cleaning the algae is probably more detrimental than leaving it on, Gentry said.

Black algae is ugly, however, and that's why homeowners want to get rid of it. Dunbar and his crew use a product called Shingle Shield,, which the manufacturer says contains no bleach or chlorine. The active ingredient is sodium hydroxide, also known as lye or caustic soda, which requires the user to don protective clothing - mask and goggles, too.

"It does the job, but when it heads down the downspouts, it can damage all the plants," Dunbar warned.

Power washing with the stuff at the lowest, gentlest setting possible is the way to go, he said.

"You don't scrub, because it disturbs the granules that protect the shingles," and that will surely reduce the life of your roof, Dunbar said.

Humid and healthy: New research suggests humidifiers may play an important role in reducing the survival of the flu virus on surfaces and in the air. It suggests that houses kept at a 40 to 60 percent relative humidity are likely to have fewer flu viruses lingering in the air and on faucets, door handles, and countertops. The report, and cold and flu season tips, are available at