Q: My neighbor discovered that many of the floor joists on his outdoor deck are rotting. The rot is along the top, where the decking attaches to them. It’s treated lumber and 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?

A: Your questions are spot on. They take me back to 1974, when I saw an advertising placard on the sales desk of an old lumber company. It had a photo of a new, treated lumber and it boasted, “Lifetime guarantee!”

Being young and impressionable, I believed it. But guess what? The ad didn’t specify whose lifetime!

The truth is, treated lumber can — and does — rot. I’ve witnessed it on numerous occasions, and I’ve had countless people send me photos of their own rotten wood with the same questions you’re asking.

Not only can treated lumber rot, but wood-destroying insects can eat it. I had this happen at my own home. I built a play set for my kids using treated lumber that was approved for ground burial.

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


Now, I don’t believe that manufacturers would purposely produce an inferior product. But even with quality controls in place, things can go wrong. For instance, 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 until eight years later.

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.

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 your neighbor’s joists.

This rot can be prevented by applying strips of joist tape on top of the joists before the decking is applied. This should be done for all decks, but it’s mission-critical for any deck that will sport composite decking. You don’t want the undercarriage of the deck to rot when you have expensive decking on top.

The ends of any treated lumber are the most susceptible to wood rot. Water can easily enter the end grain of posts, joists and decking. The end grain is what you look at when you cut down a tree and can see the concentric growth rings. A tree is not much different than having millions of tiny straws next to one another. The sap travels up and down through the straws when the tree is alive. Once cut, the tiny tubes become pathways for water to get deep into the wood.


Use this knowledge to your advantage and routinely apply copper naphthenate solution to any end grain you see. This magic liquid, sold at home-improvement stores and elsewhere, soaks into both the end grain of treated lumber as well as the sides and edges. Be sure to read the instructions and follow them to the letter when applying the solution.

If you have deck-railing posts with cut ends facing the sky, consider covering them with decorative caps. These caps come in wood and metal, and they act like roofs to stop rainwater from entering the exposed ends.

You can’t stop fungi spores from falling on your deck; the wood is food for the fungi. What it needs to start growing and rotting the wood is water. Stop the water and you stop the rot.

Tim Carter has worked as a home-improvement professional for more than 30 years. To submit a question or to learn more, visit AsktheBuilder.com.