How to Protect Pressure Treated Wood Underground

Fence post, you should know How to Protect Pressure Treated Wood UndergroundThere is so much confidence in pressure treated wood amongst the woodworkers and homeowners. Well, this is understandable, thanks to the reliability that comes with this wood. Even so, treated wood remains vulnerable to different environmental elements, such as water and UV light. So, if you have part of your treated wood underground, you need to know how to protect pressure treated wood underground.

The best way of protecting your treated wood underground is by properly coating the wood using a wood sealer. If you have a pump garden sprayer, spray one layer onto your dried treated timber. Give the sealer enough time to soak thoroughly into the lumber grains and give your structure enough time to dry- usually two hours between coats. Once the first layer dries, apply the second one using a paintbrush or a roller.

Will Pressure Treated Wood Rot In the Ground?Image of ready seal, it's one way of How to Protect Pressure Treated Wood Underground

According to the Forest Products Laboratory research and other like-minded groups, treated wood stakes that are in the ground can stay up to 40 years and remain rot free. One clear thing is that the pressure treatment of wood makes the wood rot resistant. However, the treatment does not make the wood water-resistant. We all know that constant exposure to water can cause the wood rot in the long run; therefore, the first protection you can give your treated wood underground is to ensure that it is sealed with a waterproofing stain and sealer such as the Ready Seal.

How Long Will Treated 4×4 Post Last In The Ground?

Well, the duration that a treated 4×4 lasts in the ground is dependent on your region, the weather, and the climatic condition of your location. In most cases, do not expect these woods to last more than one decade.

So if you have your posts treated and set in concrete, it can last up to 25 years. However, if the bottom of the post is wet and the wood is continuously in contact with water, it might not last more than 7 years. With the stone at the base of your post, expect it to exceed 20 years.

For the untreated ones, expect them to last a maximum of one year.

If you happen to be setting posts and want to save some money by ensuring they last longer, you need to use ‘concrete anchor posts’ or ‘deck blocks.’ Ensure that you have undisturbed soil beneath, do brutal leveling, and ensure that your posts are pressure treated.

How to Protect Pressure Treated Wood Underground (Making it Weatherproof)

  • Cement the Submerged Part 

The very first thing that you should do is to surround your submerged treated wood with cement. A move to add the cement to your submerged wood structure makes it stable and seals it from destructive elements such as water and insects. With cement in the wood’s submerged part, you can only worry about treating the upper parts from time to time, as you deem necessary.

  • Seal the Wood Before You Submerge It. 

Pressure-treated lumber is reliable and repels insects such as termites that “eat away” wood and rot. However, for extra security and improved durability of your wooden structure, you need to protect your wood by sealing it using an all-weather wood sealer.

How To Seal Pressure Treated Wood

The very first step is to clean the wood. Remove all the staining, grease, mold/mildew using a commercial deck cleaning solution. If you are working with older structures, you can choose to clean the surface using a pressure washer. If you go the pressure washer way, ensure that you set it so that you don’t use excess pressure as it might damage the structure. Usually, a pressure range of 1500-2500 PSI is sufficient to clean your pressure-treated lumber. In case you are working with wood types such as cedar and redwood, ensure that you set your pressure to a range of 1200-1500 PSI.

Once you have cleaned your surface, allow it enough time to dry. As you shall see from the labels on most coatings, it is recommended that you don’t seal your structure when it is still wet. One unique deck sealant that you can apply even on a damp wood is the Thompson’s Water Seal Advanced Wood Protectors; this wood sealer allows you to clean and seal your structure the same day. Even as you apply this formula to your structure, ensure that you are working within the application’s requirements to get the best results possible; some of the things you must consider are the temperature and humidity.

Choose your finish. When it comes to choosing the finish, it trickles down to individual preference; you can decide to go with an oil-based or water-based deck sealer or stain. Other options are solid or semi-transparent. You can apply these formulas by using a paintbrush or a paint pad with a long handle; these applicators are available in most local hardware stores; additionally, you can order them online. Some of the finishes can also be applied by spraying. If you are working on large structures such as decks, spraying will help you save a lot of time though you will need to brush the materials into the wood to achieve great consistency and uniformity with a perfectly looking finish.

  • Add Rocks and Pebbles to The Hole

As you would expect, the wood’s submerged section usually rests on the top of the moist ground. Considering that you might not have cement in that section of the earth, I advise that you add thick layers of stone pebbles (approximately 6 to 8 inches). This practice ensures that the moisture will get a passage to drain away from the wood; this helps protect it from cumulative moisture that might contribute to the wood’s softening and rotting.

The Types of Wood That Are Suitable For Use as Fence Post

  • Yellow Pine-The Easy-to-Treat Option.

Pine is not a hardwood; however, it comes with a great response, especially to treatment thanks to the ease at which it absorbs the industrial treating chemicals well. The southern yellow pine is on record as the most receptive to chemicals. 

What’s unique about the yellow pine is that unlike other woods that only absorb the treatment chemicals into their surfaces, this pine absorbs the substances into the cells and wood grains, which offers enhanced and total protection against insects over an extended duration.

If you wonder where you can get the yellow pines post, try from your local hardware store. Alternatively, you can buy it here:

  • Go for White Cedar or Black Locust if Fungi is Your Worry

There are regions whose biggest problem is fungi. If, for instance, you live in swampy and marshy areas, fungi will likely be the primary cause of rot in your fence posts. So if you are looking to counter such issues, you should go for the black locust or eastern white cedar to give a natural resistance to the fungi.

So if you hail from regions with similar conditions, you know what you need for your fence posts. Another reason you need to select cedar is that it offers so much when it comes to the beauty around your how and durability.

  • Cypress or Redwood for Naturally Moisture-Resistant Options. 

If you live in areas that experience high humidity levels and are looking to get fence posts that can stand such conditions, then go for Cypress or Redwood as an alternative.

These wood types offer a natural resistance to the soaking of moisture and will make the right choice for your posts considering your environment. Cypress, for instance, gives you so much more than resistance to moisture as it offers excellent levels of color consistency, it has a high density, tough and lacks knots. The stated factors make the wood perfect for fence posts.

Note: Much as these woods have a reputation for resisting moisture, they still need to be protected through treatment. I’d advise that you buy pressure-treated redwood or cedar if you use it for fences. A treated post stands a chance of surviving rot over an extended duration.

Watch the video below on how to waterproof fence posts:


You can find pressure treated wood in almost every part of the world though treated with different chemicals. However, most people have the notion that this wood type is perfect and can resist virtually every destructive element of the environment from water, UV light, and infestation by insects. Well, treated wood indeed resists insects, yet the same cannot be said for water and Uv light. Therefore, it means that you still need to offer your treated wood protection, especially if you are using it on underground structures. So…

Here’s How to Protect Pressure Treated Wood Underground

By now, you must already be aware of some of the protective actions that you need to undertake to ensure the safety of your pressure-treated wood underground. If I can briefly highlight some of those measures, they include:

  • Cementing around the submerged section of the wood
  • Using wood sealers to protect the wood against moisture
  • Addition of stones and pebbles to allow the draining of excess water away from the wood.

I want to believe that you know what you need to do to guarantee the safety of your treated wood underground until this point. If you have a question, opinion, or suggestion, please share it with me in the comment section below.

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Tyron Otieno

Tyron is an avid woodworker and writer. He founded this website to help other woodworkers, whether hobbyists or professionals by sharing his knowledge and experiencie after a decade of woodworking.