Sometimes, homeowners and woodworkers can regard pressure treated wood as almost magical in its ability to resist stay functional for decades. And it’s true that pressure treated wood lasts far, far longer than untreated lumber.
But pressure treatment doesn’t mean that a given piece of wood will last forever no matter what you do to it. It’s true that pressure treated wood is resistant to rot, bacteria and fungi, and insects. But there’s one thing pressure treatment does nothing about, and that’s moisture.
Moisture Is The Enemy of Any Type of Wood
Moisture and water seeps into wood over time, causing it to warp, expand and contract, crack, and generally lose its strength and integrity. While pressure treatment chemicals prevent many of the things that cause wood to decay, but they don’t keep water out.
Anytime a piece of lumber is buried or underground, it’s almost certain to come into regular contact with moisture.
How To Protect Pressure Treated Wood Underground
If you want that wood to last for more than about a decade tops, you need to protect that wood with a wood sealer. Adding an additional treatment of a liquid wood sealant will greatly extend the life of pressure treated wood if it’s underground.
In addition, there are a couple of other tricks that can extend the life of your underground pressure treated wood.
- How Soon Can You Paint Pressure Treated Lumber?
- Can You Burn Pressure Treated Lumber In a Wood Stove?
How Long Will Pressure Treated Wood Last Underground With No Further Treatment?
Numerous studies have shown that pressure treated wood will remain free of decay and insect damage for 40 to 50 years. That kind of longevity is incredible, especially when compared with untreated wood that can be destroyed after as little as a year or two.
But these studies are only looking at rot, fungal and bacterial damage, and insect damage. Water damage is an entirely different story.
If a piece of pressure treated lumber (not fortified with a wood sealer) is buried underground and in constant contact with moisture or water, the water damage will start to have a noticeable effect on the wood within around 7-10 years.
How to Further Protect Your Pressure Treated Wood Underground
As mentioned above, the best, most reliable way to protect pressure treated wood from water damage is to apply an all-weather wood sealer before burying the wood.
When applying wood sealer – or any other paint or primer – to pressure treated wood, it’s extremely important to understand how pressure treated wood is different then untreated wood. Specifically, you will need to give pressure treated wood time to ‘dry’ before applying wood sealer.
Allow The Wood To Completely Dry
When we say ‘dry’, that’s not talking about wiping any moisture off the surface of the wood. Pressure treated wood is infused with a compound of waterborne chemicals when it’s created. Freshly made pressure treated wood is considered ‘wet’, because the chemical solution hasn’t had a chance to settle or dry within the wood.
This process takes a few weeks, up to a month or a little more. During this time, if you apply paint, primer, or sealers to the wood, the drying process will cause whatever you applied to slough or peel off the wood.
Make sure to let your pressure treated wood sit for a few weeks after purchasing it before applying a wood sealer.
Clean The Surface Before Painting or Staining
Once you’ve let it dry, wash off the surface of the pressure treated wood and let that moisture dry too. Apply the wood sealer, and then apply the finish you’ve chosen. After you’ve waited the specified time period to the let the finish set, you’re ready to use your pressure treated wood.
Separate The Wood From The Soil
Another good method of protecting pressure treated wood from moisture while underground is to separate it from earth or soil. The ground almost always contains a degree of moisture, and if the earth is right up against the wood it will transfer water to the wood over time.
Encase In Concrete
Probably the best way to separate the wood from soil is to encase it in concrete. This is especially useful when your wood is forming a base, foundation, or post, since the concrete can serve two purposes. It anchors the base and makes it more stable and also prevents moisture transfer.
Encase In Gravel
You can also place rocks, stones, pebbles, or gravel into any hole where the wood will go. This fulfills the same function as the concrete, although the seal isn’t nearly as tight. Water will still trickle through rocks or pebbles in many cases. But this still drastically reduces the amount of moisture compared with soil right up against the wood, which will extent the life of the pressure treated wood.
Wrap In Plastic
Finally, another way you can separate the pressure treated post from direct contact with the moisture laden soil is to wrap it in plastic before burying it in the ground. I’ve never done this myself, as I typically burry the post directly into concrete. However, the idea is that it keeps the wood from contacting the soil. The problem I have with this method is what happens if and when moisture does get into the plastic? The moisture will have nowhere to escape, leaving it in long term contact with the pressure treated wood.
In my opinion, your best bet is to burry it in concrete if possible.