How to remove algae from shingles and keep it away

October 12, 2012|By Tim Carter, Tribune Media Services | Ask The Builder
  • This roof is badly stained by algae. Note how the roof is clean below the lead chimney flashing.
This roof is badly stained by algae. Note how the roof is clean below the lead chimney flashing. (Tim Carter)

DEAR TIM: 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.

DEAR MOLLY: Hundreds of thousands of roofs on homes 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 help add weight to them.

This limestone never used to be a component of shingles. When the mats used to build shingles switched from traditional felt paper to fiberglass, the manufacturers had a problem getting the shingles up to the minimum weight requirement. Someone decided to use powdered limestone dust -- but they didn't realize that it was a delectable food for the tiny algae. Fortunately, the algae doesn't harm the shingles.

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


A few years ago, I decided to test a pressure washer on asphalt shingles to see what damage it would cause. I did this on my own home on asphalt shingles on my own 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 did damage the shingles, it wouldn't matter.

I used a 25-degree tip on the 2,400 psi pressure washer and held the wand about 9 inches away 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 it was 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 just 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 that surrounds your home. 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 in seconds or 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 only be 4 feet wide. Do NOT stand on a wet part of the roof as you clean. Always be 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 3-inch strip of copper at the top of the roof. Every time it rains, a tiny amount of copper will wash down on the roof. Copper is a natural biocide, and the algae will not grow in its presence. Zinc works too, as does lead, but the copper is more powerful.


You can purchase asphalt shingles that contain copper in the ceramic granules. These shingles are marketed as being algae resistant. I tried these on a shed I built and was disappointed. The north-facing section of the roof developed black algae stains in just three years.

Apparently, the manufacturer hadn't put enough of the copper-containing granules in the shingles, perhaps to save costs. Copper is getting very expensive, and to stay competitive, the manufacturer possibly decided to put less in the shingle.

Look for algae resistant shingles that have the best warranty. These probably have enough copper in them to resist algae growth.

You can watch a video that shows how to clean roof algae at Simply type "removing algae stains video " into the search engine.
