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Spokane, Washington  Est. May 19, 1883

Treated lumber can rot, so build accordingly

This is a treated lumber deck post on a deck that is rotting. Can you imagine the safety issues this poses? (Tim Carter)
By Tim Carter Tribune Content Agency

Dear Tim: In just a few weeks a builder will show up to completely rebuild my deck. The house is only 20 years old and the treated lumber used to build it is rotting away. The centers of the deck railing posts are crumbling. I thought treated lumber was supposed to last forever. What is the problem? I religiously applied sealer to the wood, thinking that would help, but alas it was just cosmetic. Can you trust treated lumber? Is it possible my deck could have collapsed? – Cathy B., Cincinnati

Dear Cathy: Your dilemma reminds me of past email exchanges I’ve had with builders and companies that swear treated lumber foundations are so much better than poured concrete. They go on about how the wood is so much more energy efficient. My reply is that the great manmade wonders of the world are all built with rock, and things can go wrong in the treatment process that cause the wood to rot prematurely.

I’ll also add that I was part of a giant class-action lawsuit a few decades ago involving a major window and door manufacturer and a paint company. The paint company was supplying the window company with a clear wood preservative. The preservative was used to prevent the window and door frames and sashes from rotting.

It didn’t work and it took about eight years to discover the preservative formulation was defective. The dream house I built had these windows with the defective clear wood preservative. Fortunately I was able to negotiate a fair settlement. The bottom line is that wood rots, and many things can go wrong in the treatment process causing premature failure.

About 25 years ago I used treated lumber to construct a play set for my kids. The chemical treatment guarantee mentioned that it was formulated to resist termites and other wood-destroying insects.

After about 10 years, my kids no longer used the play set and I needed a place for a garden shed. When I pulled the 4-by-4 posts out of the ground, one-third of the wood was gone, and the termites were feasting away. I could see the little white insects gorging themselves on the wood.

I’m telling you these stories to make a point. Treated lumber is a good product, but you need to realize there’s an element of risk when using it. As with a chain, treated lumber is only as rot proof as its weakest link. Here’s just a partial list of things that can go wrong in the treatment process:

The actual chemicals used in the process could be defective, or the wrong concentration could be made at the chemical factory. A lab technician doing quality control can make a mistake. If the chemicals are diluted at some point in the process, a human error could cause the solution to be too weak. If the process involves pressure to drive the chemicals into the wood, the pressure gauge on the vessel could be defective. If the pressure treatment is to last for a specific amount of time, it’s possible the wood could be removed too early from the treatment chamber. I could go on and on with other scenarios.

There’s no doubt that rotting treated lumber can lead to structural defects that cause decks to collapse. Sometimes these stories make it to the news, but my guess is you’d have to dig deep into insurance industry records to see the extent of treated lumber structural collapse incidents that are the result of rotting wood.

I’m rebuilding a large deck on my own home that’s made from treated lumber. Fortunately, I’ve not encountered any severe rot in the treated lumber, although I have come across three minor areas where the wood has been compromised. What’s more, I just added a new treated lumber deck to my existing one, so I continue to use the product.

I have faith in treated lumber and have taken steps using new products to minimize water infiltration into both the new deck and the existing treated lumber. You can buy tape that is applied to the top of deck floor joists. I prefer the tape that comes with the butyl adhesive rather than asphalt-based adhesive. This tape helps to keep the treated lumber dry where the decking lays on top of the joists.

You can also install membrane systems over the entire deck structure before you install the decking. Not only does this system stop all water from touching the treated lumber joists and beams, but it also creates a dry space under the deck, which is especially handy if your deck is high enough to walk and sit under.

I’m just finishing up now installing this membrane system on my large existing deck. The treated lumber will never get wet again, and this deck should last for a very long time. I think the only danger at this point is collapse from extreme snow load. But knowing this is a possibility, I added lots of special timber screws to help make the deck stronger.

You can also help prevent water from entering the end grain of deck posts and support columns. The open end of timber facing the sky invites water to penetrate deeply into the wood. Install decorative post caps that create a small roof over the tops of exposed deck railing posts. Use the special deck joist tape to cover the vertical exposed tops of any deck posts before you rest a beam on top of them.

Finally, do periodic inspections of any treated lumber using an ice pick. Poke beams and columns to ensure they’re not getting soft.

Need an answer? All of Tim’s past columns are archived for free at You can also watch hundreds of videos, download Quick Start Guides and more, all for free.