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 that 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 carpenter doing it, as it’s so time consuming. I don’t want you to think it was his fault.
The crack may have been very small at the time the deck was built, but over time it enlarged. This happens when water enters the crack and causes the wood to expand. When the wood dries out, it contracts. This back-and-forth movement can cause the crack to widen. As the crack gets bigger, the water drives deeper into the wood, causing even more stress.
If the preservative chemicals didn’t penetrate deeply into the wood, then it stands to reason that the water entering around the screw can cause the wood to begin to rot.
Treated wood rot has spawned a new category of products to address the three things required to make wood rot: wood, water and the fungi that eats the wood. If you take any of the three things away, you stop the rot.
Water is much easier to stop than trying to keep fungi away. Any number of products are available to stop water from entering the treated wood. Last year, as part of my deck reconstruction project, I applied a special tape on top of my wood joists before I installed the decking. This tape has a butyl rubber adhesive and is designed to seal around the shaft of the screws used to attach the hidden fasteners for the decking.
You can also buy rolls of ultra-thin stainless steel that also has the sticky butyl adhesive. Stainless steel is an excellent product to cover the tops and end grain of wood joists so that water can’t enter. It’s easy to cut the stainless steel with regular scissors, and applying it is no different from using any other tape.
My advice is to replace any rotted pieces of treated lumber and then protect the top edge and hidden end grain of the joists where they may be covered by a band board or butt up against a beam. Do this and you might end up with a deck frame that can last for many decades.
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