Treated wood can rot, and here’s why – Orange County Register

Q. I’m starting to have some rot issues with my 10-year-old deck. Not only are some of the treated wood decking boards rotting, but I’m also noticing that the tops of some of the joists are showing severe rot where the decking screws pass into the tops of the joists. The rot is 2 inches deep in a few of the joists! Is the lumber defective? Are the chemicals in the wood so strong as to cause the rot? What’s going on and is there anything that can be done to prevent it?

A. I’ve experienced similar rot with treated lumber I’ve owned. Years ago I built a treated wood play set for my kids. After 15 years, I got rid of it to build a garden shed for my wife.

I was shocked when I pulled the buried posts from the ground. Even though the lumber was rated for burial and direct contact with the soil, termites had consumed quite a bit of several posts.

Last year I visited the home of a childhood friend, and her treated lumber deck posts had severe rot where the end grain at the top of the posts was exposed to the weather and rain. A month after seeing this rot, I found some on my own treated lumber deck that I was rebuilding.

About 20 years ago, I was involved in a massive lawsuit over windows made by a national brand that were treated with a defective clear wood preservative. While the windows were not made with the same species of wood as your deck, I have first-hand knowledge that treatment chemicals can be defective. Suffice it to say, treated wood does rot.

There are many reasons why it can happen. A defective pressure gauge at a treatment plant might be the culprit. The manufacturer of the chemical brew might have made a mistake in its testing procedures, and the chemical may not perform as expected. Scientists in labs can and do make mistakes, even though they try everything to prevent them. The list of possibilities is endless.

There’s a secondary issue that could be in play. When your deck was built, the carpenter could have unintentionally helped accelerate the rot. Based on the photo you sent to me, it’s obvious the top of the joist has a crack in it that extends back to the corroding screw.

This crack no doubt originated when the carpenter drove the screw through the decking into the top of the joist. Without drilling a pilot hole, the twisting screw produces enormous amounts of tension in the lumber as the mass of the screw pushes aside wood fibers.

I’ve never taken the time myself to drill pilot holes, and I’ve never heard of a…

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