Know how to read the labels on treated lumber

Q: I’m a remodeling contractor and have started to see a disturbing trend in the failure of treated lumber. I was hired to rebuild an outdoor stair railing that had a treated 4x4 post wrapped with redwood. A vast majority of the bottom of the post rotted away, and the post was very wet once I removed the redwood. The bolts were very corroded, too. The post was less than 15 years old.

I was also called to raise an outdoor shed that was built on 6x6 treated lumber posts that were partially buried in the soil. It turns out termites had eaten the post. I thought the treatment process was supposed to deter insect attacks. What would you do to create more permanent installations?

— Jonathan S., Milford, Ohio

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A: Whether you're a remodeling contractor or a homeowner, you need to be very careful when it comes to treated lumber. A lot of people think that all treated lumber at the local home center or traditional lumber yard is all the same. That's not true by a long shot.

A visit to the American Wood Protection Association website will clear up this matter. As I write this column, there are more than a dozen categories for treated lumber. Each gets a code: UC1, UC2, all the way to UCFB.

Raw lumber from the forest is treated with different chemical brews at different concentrations to achieve a predetermined level of protection. A good analogy might be winter jackets. You might put on a light puffer jacket if the outdoor temperature is in the mid-40s. But if Old Man Winter is in a foul mood and the wind is howling with a temperature of minus-10 degrees, you might want to have on a Maine mountain parka.

It’s possible the treated lumber that Jonathan is replacing was rated only for interior dry locations. Or, it could have been rated for outdoor use but was rated for rapid water runoff. He said the post was wrapped in redwood, so it stayed damp or was wet most of the time.

When you purchase treated lumber, it's supposed to have a small plastic tag stapled to the end of the lumber. This tag should show the AWPA use category. If I were using treated lumber outdoors and wanted maximum protection from rot and insects, I'd want the tag to say UC4C.

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Much of modern treated lumber has a very high concentration of copper in the wood. If you paid attention in high school chemistry class, you know that when you put copper and steel or iron in a beaker full of water, you start a chemical reaction that starts to corrode the iron or steel.

When you’re working with treated lumber, it’s so important to use bolts, nuts, nails, etc. that have the highest amount of corrosion resistance. The best would be stainless-steel fasteners, but you'll probably have to settle for double-dipped hot galvanized ones. Some high-quality fasteners are sold with a corrosion-resistance scale printed on the box. Match the corrosion resistance to your application.

Deck-collapse autopsies often reveal the cause to be corroded fasteners. If you couple this with poor framing practices and dubious structural construction methods, you can see why I never go onto a high deck before I inspect it first.

You have other options in many cases when building things exposed to the weather. Instead of using treated lumber, you can use metal. The treated 4x4 post that Jonathan removed could have been a galvanized 4x4 metal tube. The homeowner may not have liked the look, but the railing system could have been aluminum.

It pays to talk with a contractor with at least 20 years of building experience in situations like this. I'd also suggest talking with several, not just one, so you get a variety of opinions. If you engage your own critical-thinking skills, you may ratchet back your trust in how long treated lumber can last.

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