Ask the Builder

Blasting the Roof Clean

By Tim Carter
Saturday, May 12, 2007

Q: DEAR TIM: My roof needs cleaning. It's a newer asphalt shingle roof, but parts of it are covered with moss, lichens and algae. Is a pressure washer a good tool for cleaning roof shingles? Will it remove all of this growth? A neighbor told me a pressure washer would ruin my shingles. Are there alternatives? And how do I prevent the stuff from coming back? -- Tom B.

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

Spores are broadcast by the wind, dropped from trees and delivered by birds to a roof. Rainwater fuels the growth of the moss, lichen and algae. Because 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 miniature vegetable garden on your roof. The other sections of your roof dry quickly, and the moss, lichens and algae die.

Like you, I have heard about pressure washers causing permanent damage to asphalt roof shingles. I know pressure washers can damage concrete, so it would seem plausible they would harm shingles. But I wanted to try it.

I know how to replace one or more roof shingles quickly and easily, so I decided to test my roof. I figured it would be an especially good subject because it is 20 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 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 were astonishing. I was able to remove all the growth and 20 years of dirt with the pressure washer. There was no damage to the asphalt shingles.

I started with the spray wand tip about 12 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 six inches of the shingles, the organic growth started to disappear. I recommend that you clean just one shingle, then stop working and inspect for damage.

Walk to another part of your roof, where the shingles look great, to see if the clean shingle looks like the freshly washed shingle. Be 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 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 leaks inside your home.

Once the roof is clean, you can keep it looking new by installing strips of copper at the top. You need about two to three inches of copper exposed along the ridgeline. If your roofer used 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 prevents organic growth.

Tim Carter can be contacted via his Web site,

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