Many roofs in my neighborhood, mine included, are covered with black streaks. What’s causing this to happen, and can the roofs be cleaned? I’m able to walk on my asphalt shingle roof with ease but am afraid of hurting the shingles by cleaning them. Once the roof is clean, is there a way to prevent the streaking from returning? Could all of this been prevented?

— Molly G., Jacksonville, Fla.

Hundreds of thousands of roofs on houses and other buildings suffer from the stains you’re plagued with. It’s a harmless algae that’s gorging itself on the powdered limestone filler that’s often added to the liquid asphalt in shingles to add weight to them.

Limestone didn’t used to be a component of shingles. When the mats used to build shingles were changed from felt paper to fiberglass, the manufacturers had a problem getting the shingles up to the minimum weight requirement. Someone decided to add powdered limestone dust but didn’t realize that it was food for algae. Fortunately, the algae don’t harm the shingles.

The first thing to do is to read any paperwork that came with your shingles concerning the warranty. You want to know how the manufacturer recommends that you clean its product. If you don’t have the paperwork, you can often get it at the manufacturer’s Web site.

Let me save you some trouble, however. Most manufacturers will void the warranty on shingles if they discover you used a pressure washer on your roof. They’re afraid that if an untrained person gets the cleaning wand too close to the surface, it will blast away the ceramic granules that provide protection to the tender shingle below.

A few years ago, I decided to test a pressure washer on asphalt shingles to see what damage it would cause. I did this to asphalt shingles on my roof that had a 15-year warranty and had been on the roof for 22 years. I was getting ready to install a new roof, so if I damaged the shingles, it wouldn’t matter.

I used a 25-degree tip on the 2,400-psi pressure washer and held the wand about nine inches from the surface, always pointing the wand down the roof. The water spray cleaned off all the lichens, moss and algae, leaving the roof looking like new. I constantly stopped to check to see if I was removing any of the colored ceramic granules. In my case, the shingles were not damaged.

I do not recommend you do this, as you may damage your shingles. I just wanted to share the results of my experiment. You should never do anything that would void a warranty.

Instead, you should clean your roof with regular soap and water and a scrub brush. I’ve gotten stellar results using an oxygen bleach solution. Some people use chlorine bleach and water, but chlorine bleach is highly toxic to any vegetation. Any chlorine solution that drips or that is rinsed off the roof can kill your landscaping. Oxygen bleach does not harm vegetation.

It’s best to clean a roof when it’s cool and the sky is overcast. Working on a sunny day will cause the cleaning solution to evaporate within a few minutes. Spray the oxygen bleach solution on the roof, working in strips from top to bottom of the roof. Each strip you clean should be no more than four feet wide. Do not stand on a wet part of the roof as you clean. Always stand on a dry section, as the cleaning solution is slippery. Consider wearing a fall-protection harness in case something goes wrong.

Once the roof is clean, you can prevent the algae from coming back by installing a three-inch strip of copper at the top of the roof. Every time it rains, a tiny amount of copper will wash down the roof. Copper is a natural biocide, and the algae will not grow in its presence. Zinc works, too, as does lead, but copper is more powerful.

Tim Carter is a columnist for Tribune Media Services. He can be contacted through his Web site at