Ask the Expert: The Power of Powerwashing

“If your friend trades stocks or practices law, what do they know about power-washing?” That’s not some kind of riddle, but rather a good question posed by Benjamin Smith of Mildew Busters. When you have a question about the stock market, go to your friend. For your question about how to clean a cedar roof, you go to the expert.

Mildew Buster
Ben the Mildew Buster!

The Question: My cedar roof is covered with black and green mold and I’d like to remove it, but a friend told me that you can’t wash cedar roofs because they will get damaged. Is this true?

The Answer from Mildew Busters: Unfortunately this is one of the biggest misconceptions that we run into each season. The answer is that you can wash a cedar roof. If done properly—meaning using the correct pressure machines, tip size and mildicide solutionwashing roofs is no different than washing the sidewall shingles on a house and the results are just as good. Your roof will be free of the mold that rots the shingles, causes costly repairs and when done will look brand new.

The method we recommend in using a nontoxic, “green” citrus-based solution sprayed over the roof with a special applicator gun. This kills the mold and mildew and allows it to then be removed easily. Then the roof gets washed off with a low-pressure power-washing machine with a wide degree tip.

After washing the roof off, there is always a huge amount of organic matter that is essentially the mold and mildew that was on your shingles. The entire roof should then be rinsed with fresh water, and the gutters and down spouts cleaned out so they drain freely. When the roof is clean, the application of a nontoxic “brightener” will result in a roof that essentially looks as if it were just put on.

RELATED: Mildew Busters’ tips for taking care of your deck

Once the moisture content of the roof is below .2 (verified with moisture testers), a clear sealer or a mixture of clear oil, weathering stain and bleaching oil will waterproof the roof, while at the same time giving it a very silver gray look—just like a piece of driftwood that you might find on the beach.

With proper care a cedar roof should last many years before having to be replaced. In fact, our summer house on Shelter Island had a cedar roof that was treated and waterproofed every few years and it lasted over 50 years before having to be replaced.

Next time you have a question about whether or not a job like cleaning a roof or house is possible, you can contact Benjamin Smith and Mildew Busters at 631-495-6826 or online at  

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