Dear James: Our three-year-old light-colored roof already has dark stains over most of it, except for near the chimney. Why is that, and how can we stop the staining from getting worse?—Phil R.
Dear Phil: Your roof-staining problem is not uncommon, especially with a light-colored roof where it is most noticeable. You made the correct choice in installing light-colored shingles, and the color is not causing the staining problem.
Actually, it was an energy-efficient decision to select light-colored shingles. They reflect heat and 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 to address 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 reroof 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 were 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 flows down over the roof below and kills the algae. It will not harm your flower beds 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 into 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. These shingles have copper formulated into the colored granules on the shingles. These release enough copper during rains to control the algae.
The following manufacturers offer anti-algae shingles: Atlas Roofing, www.atlasroofing.com; Certainteed Corp., www.certainteed.com; GAF, www.gaf.com; and Malarkey Roofing, www.malarkeyroofing.com.
