Stains on asphalt roofing shingles make a house look shabby, which detracts from its value. In some cases, stains are merely a cosmetic issue. But sometimes they’re symptomatic of a problem that, if left unchecked, can lead to more serious damage and, eventually, roof failure. It’s not always hard to distinguish the causes of stains, nor, in most cases, to get rid of them and prevent the stains from recurring.
Dark stains on an asphalt roof could be caused by a number of conditions, including:
Both algae and moss can be easily removed from asphalt shingles with a 50/50 solution of chlorine bleach and water. Laundry-strength bleach is sufficient, or you can opt for any of a number of proprietary roof cleaners, some of which don’t contain bleach, lye, or other potentially harmful chemicals.
Since bleach and some cleaners can be harmful to plants and humans, it’s a good idea to take some precautions when working with them, including the following:
Before climbing up to clean stains from your roof, be aware that about 30,000 people fall off ladders and roofs each year. Consider using a safety harness, just as the pros are required to do, and follow the common-sense rules for properly positioning and using a ladder. Also, be sure to notify someone that you’ll be on your roof. In case of an accident that incapacitates you, you’ll want someone to know where to look for you.
Apply the bleach solution with a garden sprayer. Let it stand on the surface for about 20 minutes, then rinse it off with spray from a garden hose. Don’t let the bleach solution stand on the roof for more than 30 minutes or so without rinsing. And don’t use a pressure washer, which can damage the shingles by removing their protective layer of asphalt granules.
If accumulations of algae or moss are heavy, at least some of it should wash off the roof surface right away with the stream from the hose. You can try brushing off algae and moss with a brush or broom with medium-stiff bristles, but don’t scrub too hard. You don’t want to separate the mineral granules from the shingles.
If chunks of algae or moss or heavy stains remain on the surface after rinsing, let the roof dry, then spray on the bleach solution again. Wait 30 minutes and rinse. Don’t worry if some staining remains after the second rinse. It should wash off over time with exposure to rain and sunlight.
Algae and moss tend to grow roof surfaces that are shaded and retain moisture. So, it’s a good idea to cut away tree branches that overhang the roof and block sunlight. Keep the roof surface clean by blowing off leaves and fallen branches during seasonal maintenance.
For long-term stain prevention, have zinc or copper strips installed under the cap shingles, leaving an inch or two of the surface exposed at roof peaks, along hips, and under the first course of shingles at the base of dormers. Copper and zinc are sacrificial metals that shed tiny bits of their surface with each rainfall. The metals coat the roof and inhibit organic growth for many years.
Following these maintenance tips can help homeowners enjoy an attractive roof. They can also help extend the roof’s service life, which is important whether you plan to stay in your home or sell it in the future.
This article originally appeared on nachi.org and is authored by Michael Chotiner of The Home Depot. It is used with permission.