Q&A / 

Roof Cleaning

Roof Moss
Roof Moss

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 morning  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.


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.

BEGIN AUTHOR UPDATE - November 2015:

This column was written in 2007. At that time all of the asphalt shingle companies and the Asphalt Roofing Manufacturers Association said to NOT use a pressure washer to clean roofs. They still tell you NOT TO USE A PRESSURE WASHER as it can and will VOID the warranty.

The primary reason, in my opinion, is because they knew the shingles currently being made were not as good as the shingles I used on my home when it was built in 1986.

Secondly, I'm convinced they realize that not all homeowners and contractors would do the job correctly. They'd use the wrong tip on the pressure washer, hold the wand too close and or use too much pressure.

There's NO DOUBT that you can damage an asphalt shingle with a pressure washer, but the photos and VIDEO footage on this page is clear proof that you can use the tool to clean asphalt shingles that are MADE CORRECTLY so the ceramic granules are well adhered to the fiberglass mat.

I'm currently collecting data from hundreds / thousands of homeowners who are suffering PREMATURE shingle failure. I was one of them.

opens in a new windowCLICK HERE if your home is suffering from premature shingle failure to take my survey.

Soon I'll be issuing a full report of my findings. You'll be stunned to discover the Asphalt Roofing Manufacturers Association is refusing to cooperate with my investigation.

Tim Carter - Founder - AsktheBuilder.com


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. If I damaged my roof, then no harm no foul as I'd be installing a new one.

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.

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!

Column 672


8 Responses to Roof Cleaning

Leave a Reply

You have to agree to the comment policy.