If you build a ground level deck with the joists close to the ground or with a drop beam trenched into the ground to get the right height. You are going to need to figure out how to protect that wood. In fact, any wood in contact with dirt, you will need to take steps to prevent rot.
Some of the most common and possibly best ways to prevent the wood from rotting in contact with the ground are
- Ground contact wood
- Water Sealing the wood
- Wrapping the Wood with Flashing Membrane
- Surround with Gravel
Even pressure-treated wood decks can become susceptible to rot. This is especially true if one or more parts of the structure are in the ground, where moisture and fungi will collect most of the time. In this article, I’ll walk through the different steps to take to protect pressure-treated wood from the elements, even when the wood is partially buried in the ground
I’ll cover which level of pressure-treated wood is the most resistant to water and fungi, as well as ways to protect the buried wood. Keep reading to learn more about protecting pressure-treated wood in the backyard deck.
Why Does Underground Pressure-Treated Wood Need Protection?
Yes, pressure-treated wood is treated to be used outside. Giving it protection against the elements.
But when you bury wood or it’s touching the dirt, it will need more protection.
The level of moisture and fungi is more than most treated wood is treated for. There is a big difference between a little rain running off the deck and sitting in wet dirt for days.
A twenty-minute shower will not wrinkle your skin the same as soaking in the pool will. The same is true with outdoor wood. It will be exposed to more water on the ground having a greater impact than rain falling on the deck.
Beyond the moisture and fungi, there are many more wood-eating insects in the ground. Who doesn’t know the difference between a twig and your deck beam. It’s all food to them. Unless you take steps to protect it, they will just eat it.
Making buried wood vulnerable to rot and decay. If you don’t want to replace it every few years, you will need to take steps to stop.
Wood In Contact with Dirt Should Be Treated for Ground Contact
This is a common DIYers mistake. I have made it a few times myself.
“All treated wood is the same.”
Not a chance. There are wood that is treated to be outdoors, and there is wood treated to be in contact with the ground. Wood treated that it can be buried and still not rot.
Pressure-Treated wood is not all the same. In fact, American Wood Protection Association classified treated woods into 12 levels of use. The first 4 are not for ground contact. Regardless of what extra steps you take, using anything less than ground contact, you are stepping up to bat with 2 strikes against you. The chance of the buried wood-rotting is high.
But if you use wood treated for ground contact, which has twice the level of treatment of standard outdoor wood, Above ground pressure-treated wood, you start to have a chance of survival.
The bare minimum, use wood stamp with UC4, if it will be buried. Then you know it is treated for ground contact.
Now you could do even better and use wood treated for Marine application, UC5. Which are treated for full submersion in water, even saltwater. Which is pretty harsh, and it is treated to withstand that.
But most times, you will only need to use ground contact wood. UC4C treated for extreme duty will be sufficient in most situations.
But if the potential of the buried wood-rotting is great. Here are a few ways you can decrease the chance of the wood rotting. Decreasing the possibility that you will need to replace it.
Additional Ways to Protect Buried Wood from Rotting
Most of the time, using ground-treated wood will be enough, but you can take it to another level in the following ways.
It really is the water that is the biggest threat to buried wood. Therefore, almost all of them are ways to reduce water contact with the wood.
Seal the Wood Against Moisture
This is the easiest one. Pressure-treated wood is treated for fungi not water. But wet wood will rot. With increased levels of water comes increased rot.
This is why wood is dried to 19% moisture content at the mill. Dried wood lasts longer.
But buried wood, acting like a sponge, will soak up all the water in the surrounding ground. Increasing the moisture content, speeding up rot.
You will reduce how quickly water is absorbed by sealing the wood, thus how moist the wood becomes.
The biggest challenge is using a sealant that will last.
Wolman is one of the top-rated below-ground preservatives. You can order it from amazon here.
As a bonus tip, all cuts of pressure-treated wood should be brush-treated. This is especially true if the wood will be in contact with dirt.
Liquid Rubber is going beyond sealing with a membrane sealing of the wood. This is taking it to the level that a wood foundation is treated. A little messier to use and work with, but it will seriously protect the wood.
Brush on the liquid rubber, let it fully dry before installing, but you cannot seal the wood much better than this.
Wrap with Joists Membrane
If you are looking for protecting membrane but don’t want the mess of liquid rubber joist membrane is a cleaner option. I regularly use joists membrane to prevent deck joist from rotting, and is my number pick for protection.
But here we are using the wider rolls, say 9″, which we can fully wrap the wood. The self-adhesion will make this easy. Using a brand like G-tape, which allows for adjustment while installing, will make this even easier. Cover the edges, folding over the excess tape, then cover the sides. This will ensure the beam is fully enclosed.
Once fully wrapped, it should be watertight and protected against both water and fungi even when in contact with dirt.
The biggest drawback is cost. The speed and ease of application may make up for this but budget accordingly.
Surround Wood with Gravel
This is the most full proof method and one that I regularly do when building a floating deck. By surrounding the wood with drain rock water and fungi, exposure is limited. Which will reduce rotting.
Gravel also limits deck movement with less expansion and contraction of the material around the wood.
But most importantly, the wood will be able to dry, extending its deck life.