Q&A / 

Roof Cleaning

DEAR TIM: Roof cleaning needs to be done at my home. I have a newer asphalt shingle roof, but parts of it are getting covered with moss, lichens and algae. It is really distressing, as it makes my house look like it is being neglected. Is a pressure washer a great tool for cleaning roof shingles? Will it remove all of this growth? A neighbor told me a pressure washer will ruin my shingles. Are there other cleaning alternatives? Once the roof is clean, how do I prevent the stuff from coming back? Tom B., Whittier, NC

DEAR TOM: Welcome to the world of organic chemistry. I'll bet the north, northwest and possibly west-facing sections of your roof are the ones that look bad, while the remaining parts of the roof look like new. What's more, my guess is the parts of your roof that are the worst get shade from some large trees. I know this, because I have the exact same problem as you do. Parts of my roof look like the Amazon rain forest.

Wind broadcasts spores, they can drop from trees and birds can deliver them to a roof surface. The abundant water you get from periodic rainfall fuels the growth of the moss, lichens and algae. Since the north, northwest and west sections of your roof stay in the shade during the early part of each day, the dew that develops on the roof at night does not evaporate quickly. This morningĀ  moisture quenches the thirst of the mini-vegetable garden up on your roof. The other sections of your roof dry too quickly and the moss, lichens and algae die of thirst.

Discover how Tim pressure washed his roof. Discover how Tim pressure washed his roof.


You can use a pressure washer to clean moss, lichens and algae from asphalt shingle roofs. But, you must be careful! PHOTO CREDIT: Brent Walker

You can use a pressure washer to clean moss, lichens and algae from asphalt shingle roofs. But, you must be careful! PHOTO CREDIT: Brent Walker


I have also heard urban legends about pressure washers causing permanent damage to asphalt roof shingles. I know for a fact that pressure washers can damage concrete, so it would seem likely they might harm shingles. But since I know how to quickly and easily replace one or more roof shingles, I decided to do a test of my own. What's more, I felt my roof would be a good one to test since it is now twenty-years old, and at the end of its useful life. I also decided to test some new shingles to see if the pressure washer blasted away many or all of the colored granules.

The roof-shingle-cleaning test started with a gasoline-powered pressure washer that develops 2,500 pounds per square inch of pressure while delivering 2.4 gallons of water per minute. I equipped the pressure washer spray wand with a 25-degree tip. This tip is used for general-purpose cleaning.

The results of the test were astonishing. I was able to remove all of the moss, lichens, algae and twenty years of dirt with the pressure washer. No damage was done to the asphalt shingles whatsoever.

I started with the spray wand tip about twelve inches from the surface of the shingles and aimed the wand down the roof. I didn't get any noticeable cleaning results at this distance. But once I slowly lowered the wand to within 6 inches of the shingles, the organic growth started to disappear. My suggestion to you is to carefully clean just one shingle and stop working. Inspect the shingle for damage.

Walk to another part of your roof where the shingles look great to see if the clean shingle looks just like the freshly-cleaned shingle. Rest assured you will readily spot damage. If you see small or large patches of solid black or fiberglass mesh, you are ruining your roof.

You can use a regular garden hose and a scrub brush to clean your roof. It will be an enormous amount of work to say the least. Always point the garden hose or pressure washer wand down the roof. Never point it up the roof, as water can be driven up under the shingles creating massive leaks inside your home.

Tim Carter shares an easy way to keep your roof shingles free from moss, algae and mildew. Click to watch the video. Tim Carter shares an easy way to keep your roof shingles free from moss, algae and mildew. Click to watch the video.

You can keep the roof looking like new once cleaned, by installing strips of copper at the top of the roof. You need about two to three inches of copper exposed along the ridge line. If your roofer had installed shingles that contained invisible copper-coated granules, you would not have to do any of this work. The slow release of copper on the roof surface prevents the growth of moss, lichens and algae.

Be extremely careful working on a roof with water. The water makes the moss, lichens and algae as slippery as if you had spread oil on the roof. Try to always work on a dry section of the roof. Work in ribbons that are maybe two-feet wide going from the ridge line down to the gutter line. The hoses also are dangerous slip hazards. Use fall-protection harnesses to ensure you will not get hurt if you slip.

When you get to within ten feet of the edge of the roof, stop working and let the freshly-cleaned roof dry. Once the roof is dry, then reverse the direction of cleaning so you are walking forward towards the end of the roof, not backwards and falling off!

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2 Responses to Roof Cleaning

  1. The assumption that Pressure-washing damages concrete is completely false! A pressure-washer does not have enough power to tear apart concrete. I can run a pr. washer tip across my hand while it is running... I can remove dried paint or gum from concrete without damaging it. Concrete has many variations in quality of formula, and has improved a lot over 50 years. The thing that most damages concrete is Mother Nature, meaning age, moss, freezing & thawing cycles, de-icer, traffic, etc.
    Concrete can be made to last longer by applying a sealer to it every few years. Or, cleaning it annually.
    The breakdown of concrete is not due to washing it, but rather to many other factors, and washing it is necessary and can prolong its lifespan.

  2. I hate to say it but you're completely wrong. All you have to do to see the power of water flowing across GRANITE at just 14.2 pounds per square inch (PSI) is hike to the bottom of the Grand Canyon on the Kaibab Trail. You'll then instantly understand that water squirting out of a pressure washer at 2400 PSI can and will erode the cement paste at the surface of concrete. If you want further proof, go to Chicago. Look at the sidewalk in front of the Chicago Tribune building. See all that exposed aggregate? That's because it gets PRESSURE WASHED at least once a week.

    Industrial pressure washers are used all the time to cut through concrete and metal. Do us all a favor and STOP posting false information here at my website. Your comment about your pressure washer not hurting your hand is ludicrous. Just talk to any ER doctor and she'll tell you all about the people that come in with severe injuries from pressure washers.

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