Does Pressure Treated Wood Shrink?

Pressure treated wood, also known as treated lumber or preserved wood, is a type of lumber infused with chemical preservatives. These chemicals help the wood resist rot, decay, and insect damage, making it a popular choice for outdoor structures like decks, fences, and playground equipment. But, like all wood, pressure treated lumber is subject to natural processes like shrinkage. So, what exactly is wood shrinkage, and does it affect pressure treated wood? Let’s delve into these questions.

Understanding Wood Shrinkage

Wood shrinkage, or wood contraction, is a natural phenomenon that occurs when wood loses moisture. Wood is made up of tiny fibers that absorb and hold water. When the wood is exposed to dry conditions, these fibers release their moisture, causing it to contract and reduce size. This process is influenced by several factors, including the type of wood, the initial moisture content, and the conditions to which the wood is exposed. Now that we understand the basics of wood shrinkage, let’s see how it applies to pressure-treated wood.

Does Pressure Treated Wood Shrink?

The short answer is yes, pressure-treated wood does shrink. Despite its chemical treatment, pressure-treated wood is still susceptible to the laws of nature. When pressure-treated wood is made, it’s often saturated with water to allow the chemical preservatives to penetrate the wood fibers. After treatment, the wood is left to dry naturally, during which it can lose a significant amount of moisture. This moisture loss leads to shrinkage or size reduction.

However, the shrinkage of pressure-treated wood is not necessarily bad. It’s an important consideration in woodworking and construction. Knowing that the wood will shrink allows builders to account for these changes when designing and constructing projects. For example, they might leave gaps between deck boards to allow for shrinkage, preventing buckling or cracking if the wood shrinks without enough space.

Compared to untreated wood, pressure treated wood may shrink more or less, depending on the specific type of wood and the conditions it’s exposed to. But generally, all wood, treated or not, will shrink to some degree when it loses moisture.

How Moisture Affects Pressure-Treated Wood

Moisture plays a significant role in the behavior of pressure treated wood. When the wood is treated, it’s often soaked in water and chemical preservatives. This process saturates the wood fibers with moisture, which is slowly lost as the wood dries after treatment. This drying process can cause the wood to shrink.

The rate and extent of this shrinkage depend on several factors, including the initial moisture content of the wood, the humidity of the environment, and the specific type of wood. In high humidity environments, the wood may absorb moisture from the air, slowing the drying process and reducing shrinkage. Conversely, the wood may lose moisture quickly in dry environments, leading to more significant shrinkage.

It’s also worth noting that moisture can cause other changes in pressure treated wood, aside from shrinkage. For example, it can lead to warping or twisting of the wood, especially if the drying process is uneven. Therefore, properly storing and handling pressure-treated wood is important to minimize these issues.

Comparing Shrinkage: Pressure Treated Wood vs Untreated Wood

When comparing pressure treated wood and untreated wood, one might expect that treated wood would be more resistant to shrinkage due to its chemical treatment. However, this is not necessarily the case. Both types of wood are susceptible to shrinkage, but the rate and extent of this shrinkage can vary.

Pressure treated wood often has a higher initial moisture content due to the treatment process, which can lead to more significant shrinkage as the wood dries. However, the chemical preservatives in treated wood can also help to stabilize the wood and reduce some of the warping or twisting that can occur with shrinkage.

On the other hand, untreated wood may have a lower initial moisture content, leading to less shrinkage overall. However, untreated wood is more susceptible to changes in humidity and can absorb and lose moisture more readily than treated wood. This can lead to more variability in shrinkage, depending on the specific conditions.

Shrinkage Characteristics: Width, Length, and Thickness

When it comes to shrinkage, not all dimensions of pressure treated wood are affected equally. The extent of shrinkage in wood largely depends on the wood’s grain’s orientation and the wood fibers’ direction.

Typically, wood shrinks most in the direction perpendicular to the grain. This means that the width of a piece of wood (the dimension across the grain) usually changes more than the length (the dimension along the grain). The thickness of the wood, which is also perpendicular to the grain, can also experience significant shrinkage.

In pressure treated wood, this pattern of shrinkage is generally the same. However, the exact rates of shrinkage in width, length, and thickness can vary depending on the specific type of wood and the conditions it’s exposed to.

Managing Shrinkage in Pressure Treated Wood

While shrinkage in pressure treated wood is a natural process, there are strategies you can use to minimize and manage it. Here are a few tips:

  1. Proper Storage: Store your pressure treated wood in a dry, well-ventilated area. This can help the wood dry evenly, reducing the risk of warping or twisting.
  2. Acclimatization: Before using pressure treated wood, allow it to acclimate to the environment where it will be used. This can help reduce future shrinkage.
  3. Leave Space: When building with pressure treated wood, leave gaps between boards to allow for shrinkage. This can prevent buckling or cracking.
  4. Seal the Wood: Applying a sealant can help slow down the moisture loss rate, reducing the shrinkage rate.
  5. Choose the Right Wood: Different types of wood have different shrinkage rates. If shrinkage is a concern, choose a type of wood known for its stability.

The Impact of Treatment Depth and Moisture Content on Shrinkage

The depth of treatment and the moisture content of the wood are two key factors that can significantly impact the shrinkage of pressure treated wood.

The treatment depth refers to how deeply the preservative chemicals have penetrated into the wood. Deeper treatment can lead to more moisture being introduced into the wood, leading to more significant shrinkage as the wood dries. However, a deeper treatment also means better protection against rot and insects, which can enhance the longevity of the wood.

The moisture content of the wood at the time of treatment can also influence shrinkage. Higher initial moisture content can lead to more shrinkage as the wood dries. However, it’s important to note that the moisture content can vary widely depending on the specific type of wood and the conditions it’s exposed to.

These two factors—treatment depth and moisture content—can interact in complex ways. For example, a deeper treatment might increase the initial moisture content, leading to more shrinkage. But it could also slow down the rate of moisture loss, potentially reducing shrinkage over time. Understanding these interactions can help you make more informed decisions when working with pressure treated wood.

Practical Tips to Minimize Shrinkage in Pressure Treated Wood

While it’s impossible to eliminate shrinkage in pressure-treated wood, there are practical steps you can take to minimize it:

  1. Choose the Right Wood: Some types of wood are more stable and less prone to shrinkage than others. Research the characteristics of different types of wood to choose the best one for your project.
  2. Control the Drying Process: Slow, controlled drying can help minimize shrinkage and prevent warping or cracking. Consider using a kiln or other controlled drying method if possible.
  3. Seal the Wood: Applying a sealant can help slow down the rate of moisture loss, reducing shrinkage.
  4. Use Proper Construction Techniques: When building with pressure-treated wood, use techniques that allow for shrinkage, such as leaving gaps between boards.
  5. Maintain the Wood: Regular maintenance, including sealing and staining, can help maintain the moisture content of the wood and reduce shrinkage over time.

By understanding the factors that influence shrinkage and taking steps to manage it, you can ensure that your pressure-treated wood projects are durable and long-lasting.

The Impact of Shrinkage on the Use of Pressure-Treated Wood

Shrinkage can significantly impact the use and applications of pressure-treated wood. For instance, in construction projects, shrinkage can affect the fit and finish of the structure. If not properly accounted for, shrinkage can lead to gaps, cracks, or buckling in the finished project.

In the case of outdoor structures like decks or fences, shrinkage can affect the durability and lifespan of the structure. Over time, shrinkage can lead to loosening of fasteners, creating instability. It can also open up gaps that allow water to penetrate the structure, potentially leading to rot or decay.

In furniture making, shrinkage can affect the fit of joints and the smoothness of surfaces. For example, a tabletop made from pressure-treated wood might warp or develop a rough surface as the wood shrinks and expands with changes in humidity.

Therefore, understanding and managing shrinkage is crucial when using pressure-treated wood. It can help ensure the performance and longevity of the finished product, whether it’s a deck, a fence, or a piece of furniture.

Real-World Examples of Shrinkage in Pressure-Treated Wood

To illustrate the impact of shrinkage, let’s consider a few real-world examples.

Imagine you’re building a deck with pressure-treated wood. You install the deck boards tightly together without leaving any gaps for shrinkage. After a few months, you notice that the boards have shrunk, leaving noticeable gaps between them. Not only does this affect the appearance of your deck, but it also creates spaces where water can collect, potentially leading to rot or decay.

Or consider a wooden fence made from pressure-treated posts. Over time, the posts shrink, causing the fence to become loose and unstable. This could lead to the fence leaning or falling over, requiring costly repairs.

These examples highlight the importance of accounting for shrinkage when working with pressure-treated wood. By understanding the nature of wood shrinkage and taking steps to manage it, you can avoid these issues and ensure the success of your woodworking projects.

Expert Insights on Shrinkage in Pressure-Treated Wood

Experts in the field of woodworking and construction have a wealth of insights to offer on the topic of shrinkage in pressure-treated wood. The consensus among professionals is that shrinkage is a natural and expected characteristic of wood, including pressure-treated varieties.

Experts advise that the key to managing shrinkage is understanding and anticipating it. This means recognizing that different wood and woodcuts will shrink differently. It also means understanding that factors like the moisture content of the wood, the humidity of the environment, and the specifics of the wood treatment process can all influence the extent of shrinkage.

Professionals also emphasize the importance of proper construction techniques when working with pressure-treated wood. This includes leaving space between boards to allow for shrinkage, using appropriate fasteners that can accommodate some movement in the wood, and applying sealants or finishes that can help control the rate of moisture loss.


In conclusion, shrinkage is a natural characteristic of pressure-treated wood that can significantly affect its use and performance. Shrinkage occurs as the wood loses moisture, which can be influenced by factors like the type of wood, the depth of treatment, and the conditions the wood is exposed to.

While shrinkage can present challenges, it can also be managed through proper storage, handling, and construction techniques. By understanding and anticipating shrinkage, you can make more informed decisions in your woodworking projects and ensure the longevity and success of your creations.