Redwood deck unleashes black soot attack

Q: My redwood deck, which has never had any type of waterproofing or other protective coating put on it, always turns black after I power wash it. Whatever this black stuff is, it comes off on shoes, socks, skin, material–I can’t even lean on the railing for fear of getting black soot on my clothes.

I have had several redwood decks in the past and have never had this happen before. What is it? What causes it? What can I do to prevent it? –John, San Francisco

A: Without seeing the deck, we believe your redwood deck is leeching tannin or you have a mildew problem.

Tannin, or tannic acid, is the substance in the redwood that makes it resistant to fungus and termites. It’s black. If the deck is in a protected area that gets little sun, a fungus is the more likely culprit.

Kevin had a similar problem in Alameda, Calif., with a deck that was on the north side of his house. It got some sun, but in winter the black slime came out. At one point he even started to grow a nice crop of moss.

If leeching tannin is the problem, a good sealer is the answer. It will inhibit water from penetrating the wood and thus the tannin from leeching out from the treated surface. The sealer also will help the redwood maintain its natural oils and keep the moisture content of the wood somewhat stable.

If it’s a mildew issue, you need to kill the mildew with a mildewcide. Check out a paint store or hardware store for recommended products. A simple solution is household bleach.

After scrubbing a small portion of an out-of-the way part of the deck, mix a 50/50 solution of bleach and water and try it to see if it affects the deck’s color. If the result is acceptable, pressure-wash the deck to clean it, then apply the bleach. Let the deck dry a day or two, then apply a couple of coats of sealer.

At any rate, we think that if one of us were faced with this situation we would pressure wash, apply mildewcide, then seal. The only variable is possible color variation.

If we didn’t like the result, a transparent stain could correct that flaw.

Bill and Kevin Burnett will attempt to answer your questions although the volume of e-mail sometimes makes this impossible. Contact them at sweat-equity@comcast.net.