Zinc, copper kill roof algae
Dear Ms. Builder: We had a new light-colored roof installed about two years ago and it has developed dark mildew stains, but there are no stains by the chimney. What could be causing this and how can we stop it? -- Julia K.
Dear Julia: Unsightly dark stains can really detract from the appearance of your house. Don't feel bad for picking a light roof. It was not the cause of your problem.
You were actually wise to install light-colored shingles. They keep your roof cooler in the summer. Also, by not getting as hot as a dark roof, the lumber in the roof is not subjected to excessive stress and strain.
Now for your roof staining problem. Although I would have guessed that it was an algae problem, not mildew, the fact that it is not near the chimney confirms this. It does not like the metal flashing around the chimney.
First, a little background on the shingle/algae relationship. Many people, who have had light roof shingles for decades without problems, are starting to have staining after they re-roof their homes. It will continually worsen until you clean the roof and kill the algae (gloeocapsa magma to be exact).
Your old roof probably used true asphalt-impregnated felt paper shingles. These were heavy and stable because the asphalt thoroughly saturated the felt. With the source of the felt base material (scrap cotton) dwindling, the shingle manufacturers switched to a durable fiberglass backing. It is an excellent long-lasting product.
To give the fiberglass-based shingle more weight and durability, finely ground limestone is added. This particular variety of algae thrives on limestone. The dark stains that you see are the buildup of the cells and waste over time. They are more apparent on a lighter roof.
Zinc and copper ions, even in very low concentrations, will kill algae. During the Roman Empire era, silver and copper where used to purify water. Today these ions are often used to purify swimming pool and spa water. The metal flashing around your chimney and probably some of the vents has kept the algae in control in those areas.
The best method to solve the problem is to place a thin piece of sheet copper near the roof ridge. It should run the entire length of the roof. Every time it rains, a slight quantity of copper ions flow down over the roof below and kill the algae. It will not harm your flowerbeds or the family pets.
First clean the roof shingles thoroughly. This involves going up on the roof and using a deck cleaner or a sodium percarbonate compound. Common laundry bleach will work, too, but it is hard on the plants below. Always wear a safety harness and be careful, especially on a wet roof.
Most commercial roofing supply outlets have sheet copper flashing material available. Many large home center stores will also have it. Cut it in long strips about seven inches wide. Make them as long as you can easily handle. Lift up the shingles with a stiff paint scraper and slip the copper under it. You can secure it with some roof cement or nails
For people who are going to have their roofs done soon, select anti-algae shingles, which release enough copper during rains to control the algae.
The following manufacturers offer anti-algae shingles -- Atlas Roofing 1-800-933-2721, Celotex Corp 1-800-235-6839), Certainteed Corp. 1-800-782-8777, Malarkey Roofing 1-800-545-1191, U.S. Intec 1-800-624-6832.