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Pressure Washing Damage on Cedar Roof

Thoughts from an Honest Roofer on Washing Cedar Shake Roofs, Part One.

Why Equipment Matters:

If your roof is in good enough condition to wash, congratulations. You have the opportunity to add years to its remaining life through occasional maintenance. But it is important to understand that a cedar roof should only be washed using specialized equipment. Standard pressure washers, like the one pictured to the right, will do a good deal of damage to a cedar roof regardless of the user. That damage will most likely not be visible from the ground because it is a matter of hundreds of an inch. In fact, consumer-grade pressure washers will remove between five and eight times the amount of wood from your cedar roof than professional equipment. If you have ever seen a cedar deck that looks furry and splintery after washing you know what we are talking about.

Case Study 7

Kuhl only uses high volume, low-pressure equipment
to wash cedar roofs. Our trailer mounted washing
unit is custom built for washing cedar roofs and cost us $25,000, not including Steve’s free labor to weld it all together. There are only a couple of wood restoration firms in the country that use this specialty equipment. Where the standard consumer-grade machine will push 3-4 gallons per minute through the wand, ours pushes 12! This is why we often refer to our process as power rinsing, not power washing.

Why should you care?

Because regular pressure washers–the kind used by painters, handymen and some of our competitors–are an entirely different machine. Such units should never be used on a cedar roof as they operate at inappropriate water volume to pressure ratios, resulting in significant wood loss. (See the photo in the header of this Case Study. Right half shows major wood loss.)

Case Study 7.2

If something like this shows up in your driveway the day your roof is to be cleaned, you’ve hired a pro with the commitment to invest in the right tools for the job. If someone in a minivan shows up, you may want to reconsider.

Companies that don’t invest in industrial equipment do so for
one reason; it is much cheaper to use a cheap machine than an industrial unit. Furthermore from the ground the results look about the same to you. The discoloration on your roof is gone, the roof looks fresh and clean again. But upon close inspection you will see that those machines have removed a lot of wood. Not good. Regardless of the appearance from the ground, using a typical pressure washer on a cedar roof will take years off its existing lifespan even if wood cleaners and/or brighteners are used. Ultimately, wood loss in the keyways is one of the primary reasons your roof will fail in the future.

Case Study 7.3

High volume machinery cleans cedar without damaging it.

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