How to Protect Pressure Treated Wood Underground

January 29, 2022

Treated wood.

A friend asked me the other day about how long the pressure-treated wood can last. And my answer is that it can last up to forty years without succumbing to rot or deterioration. Besides, if you set it up underground, it might exhibit a high level of rot resistance. But it will never be impervious to the effects of moisture and water. If pressure-treated wood gets constantly exposed to moisture, it will warp, contract, or expand over time. It may also eventually crack and lose its integrity and strength. 

Chemicals, of course, can prevent the wood from decaying. Yet, it can never resist the effects of moisture and water, especially if it is often in contact with water. Hence, if you want to make your pressure-treated wood impervious to the effects of humidity, you must make an extra effort to protect it. 

You can protect pressure-treated wood from water by applying on its surface a wood sealer. This liquid wood sealant will significantly extend the life of pressure-treated wood even if it is set underground. Moreover, you can also employ some other tricks that could stretch out the life of the pressure-treated lumber.

How Long Can Pressure-treated Wood Last Underground w/o Treatment?

Several studies were made about how long pressure-treated wood will last underground without additional treatment. Their findings concur that pressure-treated wood sans further treatment has a lifespan between 40 to 50 years. Well, such longevity is remarkable indeed, especially if you know that the untreated wood will only last up to two years. 

Yet, such studies sometimes might have only factored in the fungal, bacterial, and rot damages as well as those damages caused by insects. These studies fail to factor in the effects of moisture and water on the wood surface. 

For example, if you fail to fortify the fibers of the wood using a wood sealer, you can expect a shortened life for pressure-treated wood buried underground. Within seven to ten years, the constant contact with moisture will manifest its effects on the pressure-treated lumber.

How Can You Make Pressure-treated Wood Underground Impervious to Water and Moisture?

You can apply an all-weather wood sealer onto the wood surface before you bury the wood. But before you apply the wood sealer, it will be best to know that pressure-treated wood is different from untreated wood. Below are the concise steps you can follow when applying wood sealer to pressure-treated wood:

Step 1: Let the Wood Dry Completely

This means your application of wood sealer will be different from how you would apply wood sealer on untreated wood. Pressure-treated wood will need more time to dry. But before you apply the wood sealer on the wood, you need to let the pressure-treated wood dry completely. 

Remember that pressure-treated wood is imbued with a waterborne chemical compound when the wood is pressure-treated. Hence, you should consider a recently made pressure-treated wood as “wet.” It is considered wet because the chemical solution used to treat the wood has not yet settled down or dried completely. 

The drying process can last for several weeks to complete. So, if the wood is wet and you apply wood sealer on it, the wood sealer or paint will only peel off as the wood dries itself. The best thing you can do is to allow your pressure-treated wood to sit for several weeks after buying them before you treat them with a wood sealer.  

Step 2: Clean the Wood Surface Before Staining or Painting

After drying the pressure-treated wood, you can now prep the wood for sealing. Wash off the wood surface and let it dry likewise. Then, apply the wood sealer when the surface has already dried. Let the finish set in before you use the pressure-treated wood.

Step 3: Don’t Let the Wood Touch the Ground!

If you further want to extend the life of the pressure-treated wood, you can separate the wood from the soil or earth. The obvious thing is that the ground will always get saturated with water if it rains or snows. Moreover, the ground will always contain moisture, and if the pressure-treated wood is in contact with the ground, the ground moisture will exert its effect on the wood. Hence, it will be best to separate the wood from the soil.

Step 4: Encase the Wood in Concrete!

You might be thinking of ways to separate the wood from the ground or soil. The best and easiest way, of course, is to encase the pressure-treated wood in concrete. If the wood forms the foundation, base, or post, you can encase the base or lower part in concrete. In this way, you can provide the wood with a stable base and prevent water or moisture from seeping into the wood.

Step 5: Encase the Wood base in Gravel

You can also encase the wood base in rocks, pebbles, stones, or gravel when you set the wood in place. Although the seal is not that tight and encompassing, this gravel will provide the pressure-treated wood with enough buffer zone from the soil, preventing the wood from directly touching the ground. 

Such a buffer zone will radically reduce the amount of moisture that will get in touch with the wood. Hence, this method can prolong the life of the pressure-treated wood.

Step 6: Wrap the Wood in Plastic

You can also prevent the wood from directly contacting the soil by wrapping it in plastic before burying it in the ground. Not many would employ this method of preserving the pressure-treated wood. Yet, it might prove effective if your only desire is to keep the wood from directly touching the ground or soil. 

However, one problem might ensue if you opt for this method. If ever the moisture or water manages to enter the plastic barrier, it will have nowhere to go but to remain inside the plastic, leaving the wood base constantly in contact with water.

Wood Types Best for Use as Fence Post

Aside from knowing how to protect your pressure-treated wood and prolong its usability, it will likewise help if you know the following most recommended wood types for fence post:

Yellow Pint

The yellow pine isn’t hardwood. Yet, it responds positively to pressure treatment and absorbs the chemical treatment well. Yet, among pines, the southern yellow pine is considered the most receptive to chemical treatment. The surface of the yellow pine not only absorbs the chemical treatment. It also lets its wood grains and cells absorb the chemical substances.

Hence, the yellow pine, when treated chemically, will have extended longevity. The yellow pine is quite accessible for woodworkers, and you can buy it from any local hardware

White Cedar or Black Locust

Some regions in the world got a serious problem with fungi. Fungi, of course, thrive in swampy and marshy areas, and they are the primary causes of wood rotting in fence posts. So, if you want to sidestep this issue, you can select wood types that resist fungi. Go for eastern white cedar or black locust, for these two wood types are known for their natural resistance to fungi. Cedar also features extraordinary beauty and durability.

Cypress or Redwood

If you happen to inhabit a place where moisture or humidity is high, it will be best to look for a fence post suited for such a condition. Of course, when it comes to wood types that resist moisture, the redwood and cypress will come to mind quickly. These wood types are highly resistant to water and soaking. 

On the one hand, cypress is perfect for resisting moisture and offers remarkable color consistency. It is characterized by a high grain density and is very tough. It also lacks knots. Thus, it is perfect as a fence post for highly humid conditions.

On the other hand, redwood is also like the cypress in its ability to withstand rotting due to moisture. Nevertheless, even if the abovementioned wood types are highly resistant to moisture and fungi, you still need to provide them with additional protection and treatment. 

So, it will be best to use pressure-treated redwood, cedar, cypress, black locust, or yellow pine. With the proper pressure treatment, you can expect them to add years to their longevity and usefulness.

Chemicals Used for Pressure-treating Wood

Chemicals commonly used for pressure-treating wood include Chromated Copper Arsenate (CCA) and Alkaline Copper Quaternary (ACQ). The element responsible for resistance to decay is copper. On the other hand, the arsenic compound is quite toxic and works to keep away marine borers, termites, and other wood-boring insects. The chromium, however, serves as the binding element. 

ACQ is less poisonous than CCA because it lacks arsenic and chromium. In this compound, the copper still serves its purpose of repelling decay. However, the Quaternary Ammonium acts as a shield against wood-boring insects. 

The abovementioned two compounds are the most well-known compounds used for pressure-treating wood. Yet, you will still find other compounds used for pressure-treating wood. 


One of the issues in using wood for your fence post is that wood is a natural material susceptible to rot and decay. If you set them underground, the wood might succumb to decay and rot. Many factors could speed up the rotting and decay of fence posts, such as moisture, fungi, insects, and water. 

If you would use wood for a fence post, you might as well go for pressure-treated wood. Pressure-treated wood can last for several decades. Yet, as mentioned above, they aren’t impervious to decay and deterioration. However, you can prolong their life by applying wood seals on their surface before using them as fence posts.

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