Ask the Builder: Rot in treated lumber — yes, it happens
This rot can be prevented by applying strips of deck 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 out when you have expensive decking on top.
The ends of any treated lumber are the most susceptible to wood rot. Water readily enters 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.
You need to understand that a tree is not much different than having millions of tiny straws next to one another. The sap travels up and down through these straws when the tree is alive. Once cut, the tiny tubes become an excellent pathway 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 is easy to obtain and readily soaks into both the end grain of treated lumber as well as the sides and edges of the lumber. Be sure to read the use instructions and follow them to the letter.
If you have deck railing posts with cut ends facing the sky, think about covering them with decorative caps that act like roofs. These caps come in both wood and metal and stop water from entering the end grain of the posts.
Keep in mind that water is your enemy. Water is what fuels the growth of fungi. You can’t stop fungi spores from falling on your deck. The wood itself is the food for the fungi. All it needs to start growing and rotting the wood is water. Stop water and you stop the rot. Period
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