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Ask the Builder: Rot in treated lumber — yes, it happens

Tim Carter, Tribune Content Agency on

Q: Tim, my neighbor discovered that many of his outdoor deck floor joists are rotting. The rot is along the top where the decking attaches to them. It’s treated lumber rated for outdoor exposure. How can this be possible? I thought treated lumber was rot-proof and would last for a lifetime. What’s going on and are there ways to prevent treated lumber from rotting in the event something’s wrong? —Andy D., Lexington, Ky.

A: These questions are spot on. They take me back to 1974 when I first saw an advertising placard on the sales desk of an old lumber company I patronized in Cincinnati. Yes, that was a long time ago!

The full-color ad with a photo of this new treated lumber said, “Lifetime Guarantee!” Being a young, impressionable lad, I believed it. Well, guess what? The ad didn’t specify whose lifetime!

The truth is treated lumber can and does rot. I’ve witnessed it at numerous locations and I’ve had countless people send me photos using the Ask Tim page at AsktheBuilder.com. The stories and the photos are often heart-wrenching.

Not only can treated lumber rot, but wood-destroying insects can also eat it as well. I had this happen at my own home. I built a playset for my kids made from treated lumber. I used lumber approved for ground burial.

I had to remove the playset after the kids grew up to make room for a deluxe shed. Lo and behold, two of the buried 4x4s were eaten by termites! The manufacturer said the wood would be immune from termite destruction.


I’ve often thought about the process of treating lumber. I have a hard time believing that manufacturers would purposely produce an inferior product. Even with quality controls in place, things can go wrong. The liquid that’s injected into the lumber might not be the correct formulation. This happened to a major window manufacturer back in the 1980s. What they thought was a fantastic wood preservative turned out to be defective, but the issue didn’t show up for about eight years.

The gauges on the pressurized vessel where the lumber is treated could be out of calibration. The computer program that runs the operation could have a bug. The things that can go wrong in the treating process are many.

Let’s talk now about the rotting deck floor joists. When you drive nails or screws through the decking into the joists, you can cause cracks to develop on top of the joists. These cracks allow water to enter the joist with ease.

If the treatment process was inferior for whatever reason, then the water enters the wood and fuels the growth of wood-destroying fungi. This is most likely what’s happening to the neighbor’s joists.


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