How Long Will A Treated 4×4 Last In The Ground? (Explained)

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If you plan to build a fence or some other structure, then you need to know how long a treated 4×4 can last in the ground.

This will help you determine if you need to reinforce the 4×4 to make it last longer.

Proper reinforcement can ensure the structure lasts for years.

Here, we will look at how long a treated 4×4 will last in the ground and other factors that can influence its lifespan.

How Long Will A Treated 4×4 Last In The Ground? (Explained)

A treated 4×4 will last 20 to 25 years in the ground if the conditions in the soil and climate are favorable.

That number could increase to 40 to 75 years if you install the treated 4×4 in a cement ring rather than the soil.

There are a few factors that influence how long the 4×4 can last in the ground.

What Factors Cause A Treated 4×4 To Deteriorate Faster?

Knowing what factors can cause a treated 4×4 to deteriorate faster can help you avoid them.

In so doing, you can let your 4×4 last even longer.

Here are the factors that can cause treated 4x4s to deteriorate faster.

1. Soil

One of the biggest factors in wood deterioration is the soil.

If the soil already has a lot of moisture in it, then it will take less time for mold and fungus to grow on it.

That’s because the conditions are already ideal for mold and fungus growth.

Even treated lumber will fall victim to fungus growth over time.

If the soil is acidic, then it may also deteriorate the 4×4.

Acidic soil can eat away at the pesticides on the treated lumber’s surface.

Once the pesticides are gone, the insects and fungus are free to start eating the wood.

Before you place your treated 4×4 in the soil, you should first check that soil’s acidity.

It may not be an ideal location for something like wood.

You may want to consider other materials to form your posts instead.

In the case of moist soil, you should check to see if the area floods.

That can present several dangers in itself.

If the soil only happens to be moist because of the climate, then you may want to ensure that your treated 4×4 is the right kind of treated lumber.

It may be a better option to use a treated 4×4 designed for marine conditions instead.

2. Humidity

A factor that may influence the moisture in the soil is humidity.

It can also impact the treated 4×4.

People who live in climates that have high humidity are going to see a faster deterioration of their treated 4x4s than those who live in non-humid climates.

Humidity measures the concentration of water vapor in the air.

If an area has high humidity, then it means there’s a high concentration of water vapor.

That isn’t good for wood.

Even treated wood can succumb to water if it’s present a lot.

In climates with high humidity, treated lumber will be consistently exposed to moisture.

This humidity will wear down the pesticides and sealants that protect the wood from the water.

Once it’s worn down, there’s nothing left to keep the wood protected from the moisture.

If you live in a high-humidity climate, then you may want to consider using another material besides wood.

3. Termites

Most treated 4x4s have pesticides that will ward off termites.

However, not all treatments use pressure.

They only use a topical treatment.

The problem with this is that the topical treatment doesn’t last as long as a pressure treatment.

Once it wears off, termites can start to eat and burrow into the wood.

As soon as that happens, it decreases the lifespan of the treated 4×4.

Knowing what kind of treatment your 4×4 received is important.

It can inform you about whether you need to continue to apply topical treatments or not.

4. Using A Cement Ring

When you bury your treated 4×4 in the ground, you have two options.

You can either bury the wood directly in the soil or place it in a cement ring.

Burying it in a cement ring will give it a longer lifespan.

That’s because treated 4x4s buried in the soil are subject to insect and water deterioration.

The soil, particularly at the surface level, starts to erode the 4×4.

The cement ring protects it.

It keeps bugs from easily accessing it.

Water dries faster on cement than it does in the soil.

This can keep water from pooling around the 4×4 and wearing down the protective layer on it.

If you want your treated 4×4 to last as long as possible, then you need to install it in a cement ring in the ground.

Finally, cement rings add further stability to the 4×4.

Over time, the weight of the 4×4 can cause it to lean in the soil.

This is especially true if the soil is moist.

It weakens around the 4×4 and lets it lean.

Once the post starts to lean, it disrupts the forces keeping the rest of the structure together.

The other 4x4s may start to lean, too.

The cement ring prevents this by holding the 4×4 in place.

It won’t lean and the structure will remain solid.

5. Type Of Wood

One final factor that influences the lifespan of treated 4x4s is the type of wood of the 4×4.

There are two different wood types.

Hardwood is the sturdier type of wood, but it is more expensive and rarer.

Softwood is more affordable and prevalent, but it isn’t as strong as a hardwood.

Sapwood, in particular, is a softwood that’s often used for 4×4 construction.

Heartwood is a hardwood that is a more expensive variant.

If you have the budget, you should try to find treated 4x4s made of Red Cedar or Black Locust.

Both of these wood types are hard and have greater resistance to water and insects.

Using a 4×4 made of softwood automatically starts you off with a shorter lifespan than if you had chosen a hardwood.

If you want the treated 4×4 to last, then you need to invest in quality materials from the start.

What Is Treated Wood?

Treated wood refers to wood that has undergone some sort of treatment with pesticides and other chemicals to make it last longer.

There are two main types of treated wood.

The first is topically treated wood.

This type of wood treatment uses pesticides and other topical agents to spray the surface of the wood.

It’s the more affordable option when it comes to treated wood.

The problem with topically-treated wood is that it wears down fast.

You need to continue to spray the wood to keep it safe from bugs, moisture, and other types of damage.

The second type of treated wood is pressure-treated wood.

In this case, professionals place the wood in a pressurized chamber.

The chamber vacuums out any moisture inside of the wood.

Then it applies pesticides and other chemicals to the wood through force.

The sheer amount of force and pressure allows the chemicals to leach into the wood.

While it won’t reach the wood’s core, it does extend deep enough to provide lasting protection.

The best kind of treated wood is one that uses both of these treatments.

Manufacturers treat wood to protect them from certain threats.

These threats include:

  • Marine organisms
  • Fire
  • Decay fungi
  • Termites
  • Structural degradation

Here’s how each threat can damage wood and the treatment process used to protect it.

1. Marine Organisms

Marine organisms present a threat to wood because they eat away at the wood.

Algae and other marine organisms cling to the wood and start to eat it.

They’re similar to decay fungi, but they’re based in the water.

There’s also the water itself, which is a threat to the wood if it’s submerged in it.

Even wood that is near water, like a bridge, will undergo treatment to ensure it’s safe from water erosion.

To treat wood for water applications, the wood first undergoes vacuuming to remove moisture from it.

Then it’s placed in a pressurized chamber.

Pesticides designed to kill and repel water organisms are forced into the wood.

Certain chemicals that help prevent water erosion are also forced into the wood.

After the treatment is over, the wood goes to a drying area.

The treatment drips from it until it’s dry.

The wood is then ready for installation.

When it’s inserted into the water, it will ward off or kill marine organisms around it.

The type of wooden structures that use this particular type of treatment process are docks, bridges, and other water-based structures.

2. Fire

With wildfires on the rise, many homeowners are looking for 4x4s and other types of lumber that are fire-resistant.

Manufacturers can treat wood to make them more resilient against fire.

To do this, the wood undergoes a similar treatment as the water-protected wood.

It’s placed in a pressure chamber where certain chemicals blast and seep into it.

These chemicals make the wood fire-retardant.

It works by releasing water vapor and a non-combustible gas when fire touches the wood.

The result is char.

The char forms on the surface of the wood and keeps it safe from allowing the fire to burn deeper inside of the wood.

It prevents the wood from combusting with fire.

This method is useful in the construction of homes and buildings.

It can be particularly life-changing for areas that see dangerous bushfires or wildfires.

The treated wood will last far longer than untreated wood.

3. Decay Fungi

There are two main threats to wood when it comes to decay fungi.

The first is wet rot.

Wet rot requires there to be moisture levels of 50% or above.

It also needs oxygen.

If those two factors exist, then fungus can start to eat away at the wood.

A good aspect of wet rot is that it’s localized.

When the rot occurs, it happens slowly and only in one area.

It gives the individual time to stop the rot and prevent it from spreading.

That isn’t the case with the second threat.

Dry rot is the second threat that comes from decay fungi.

It can occur when the moisture level is only 20–30%.

It’s more dangerous because it can spread fast.

Stopping its spread is more difficult than wet rot, especially since it doesn’t require much moisture to spread in the first place.

Treated wood protects the wood from both types of rot.

The wood undergoes treatment in a pressurized chamber.

Pesticides designed to kill fungus leach into the wood.

The wood also goes through a vacuuming process to remove any moisture from within it.

This can make it difficult for the fungus to find the right amount of moisture it needs to spread.

In the case of decay fungi, a topical treatment is also often used along with the pressure treatment.

It further protects the wood from decay fungi.

If you plan on using your 4×4 for a fence or other outdoor structure, then you’ll want to use one treated for decay fungi.

It’s also a good idea to spray topical treatments on the 4x4s every year.

4. Termites

Another serious threat to wood is insects.

Termites and carpenter ants eat wood.

They leave the wood hollow and weak.

Using treated wood can maintain the structural integrity of your buildings.

To treat wood against insects, pesticides are the primary concern.

A pressurized chamber blasts pesticides into the wood.

A topical agent is then sprayed over the wood to give it lasting protection.

You can also increase the lifespan of your treated 4×4 against termites by installing it in a cement ring.

The ring will prevent the termites from being able to tunnel to the wood.

Spray the wood with a topical pesticide once a year to keep it safe in the future.

While the pressure treatment will help keep the wood safe, you can reinforce its ability to kill and ward off termites with an annual topical spray.

5. Structural Degradation

Structural degradation means weathering.

Wood weathers when it becomes wet and dry in rapid succession.

It also occurs slowly over time.

Weathered wood is weak.

Its surface can splinter and if the water seeps to its core, then it can also weaken the wood from the inside.

Treatments can help prevent such weathering from happening.

One step in this process is to remove moisture from the wood.

Without moisture, the wood has a harder time reaching the degree of moisture that it needs to in order to weather.

It also dries faster.

Certain topical agents and chemicals are also used to make it water-resistant.

If the treatment can prevent water from soaking into the wood, then the surface and its core will be safe from weathering.

You’ll need to continue to seal the wood with water-resistant chemicals over the course of its life.

Doing so will prevent your treated 4×4 from succumbing to structural degradation.


Treated 4x4s can last for several decades in the ground.

However, certain factors can make even treated 4x4s deteriorate faster.

Understanding the particular threats to your 4×4 can help you choose the right treated lumber for the job.

Using topical pesticides or other sealers every year on your treated 4×4 can also extend its life.

You can also extend its life by several decades by installing the 4×4 in a cement ring instead of directly in the soil.

Consider these tips above when buying and installing treated 4x4s to ensure whatever you’re building will last as long as possible.

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