How Much Will Pressure Treated Wood Shrink? Understanding the Factors and Implications

AvatarByMahlon Boehs General Wood Topics

When it comes to building and outdoor projects, pressure-treated wood is a popular choice due to its durability and resistance to decay. However, many DIY enthusiasts and professionals alike often overlook an important aspect of using this material: shrinkage. Understanding how much pressure-treated wood will shrink is crucial for ensuring the longevity and integrity of your construction. Whether you’re crafting a deck, fence, or garden structure, knowing what to expect in terms of dimensional changes can save you time, money, and frustration down the line.

Pressure-treated wood undergoes a process that infuses it with preservatives, making it more resilient against moisture, insects, and rot. However, this treatment also affects how the wood behaves as it acclimates to its environment. As the wood dries out after installation, it can experience shrinkage, which may lead to gaps, warping, or other structural issues if not accounted for. The extent of this shrinkage can vary based on several factors, including the type of wood, the initial moisture content, and the conditions it is exposed to after installation.

In this article, we will delve into the specifics of how much pressure-treated wood shrinks, the factors that influence this process, and best practices to mitigate potential issues. Whether you’re a seasoned builder or a weekend warrior, understanding the

Understanding Wood Shrinkage

Wood shrinkage is a natural process that occurs as moisture content in the wood changes. When pressure-treated wood is exposed to varying humidity levels, it will expand when wet and contract as it dries. The amount of shrinkage can vary based on several factors, including wood species, treatment process, and environmental conditions.

Factors Influencing Shrinkage

Several variables influence how much pressure-treated wood will shrink:

  • Wood Species: Different species of wood have varying densities and moisture retention capacities. Hardwoods tend to shrink less than softwoods.
  • Moisture Content: The initial moisture content of the wood at the time of treatment plays a significant role in shrinkage. Higher moisture content typically results in more substantial shrinkage.
  • Environmental Conditions: Temperature and humidity levels in the environment affect the wood’s moisture content, directly influencing shrinkage.

Typical Shrinkage Rates

The average shrinkage rate for pressure-treated wood is generally around 1% to 2% in the width and 2% to 3% in the length. However, these rates can fluctuate based on the specific factors mentioned previously. Below is a table summarizing the average shrinkage rates for various wood species commonly used in pressure-treated applications:

Wood Species Width Shrinkage (%) Length Shrinkage (%)
Southern Yellow Pine 1.5 0.5
Douglas Fir 1.2 0.4
Redwood 1.0 0.3
Cedar 1.0 0.3

Minimizing Shrinkage Effects

To mitigate the effects of shrinkage in pressure-treated wood, consider the following strategies:

  • Acclimatization: Allow the wood to acclimate to the environmental conditions before installation. This helps to equalize moisture content.
  • Proper Storage: Store wood in a dry, well-ventilated area to minimize moisture absorption before use.
  • Sealing: Applying a high-quality sealant can help reduce moisture fluctuations, thus minimizing shrinkage.

By understanding the factors that contribute to wood shrinkage and taking proactive measures, the negative impacts of shrinkage on pressure-treated wood can be significantly reduced.

Factors Influencing Shrinkage of Pressure Treated Wood

The amount of shrinkage that pressure treated wood experiences can vary significantly based on several factors:

  • Moisture Content: The initial moisture level in the wood before it is treated will impact shrinkage. Wood that is saturated with water will shrink more as it dries out.
  • Wood Species: Different species of wood have varying densities and cellular structures, which can influence how much they shrink. Common species include:
  • Southern Yellow Pine
  • Douglas Fir
  • Hemlock
  • Type of Treatment: The chemicals used in pressure treatment can affect the wood’s properties and how it reacts to drying. Common treatments include ACQ (Alkaline Copper Quaternary) and CCA (Chromated Copper Arsenate).
  • Environmental Conditions: Temperature and humidity levels during and after installation can lead to variations in shrinkage rates.

Expected Shrinkage Rates

Typically, pressure treated wood can shrink by a range of 5% to 10% of its original dimensions once it dries out. The following table outlines approximate shrinkage rates for various dimensions:

Dimension Approximate Shrinkage (%)
2×4 (nominal size) 6-8%
2×6 6-9%
4×4 6-10%
6×6 7-10%

Shrinkage will generally be more pronounced across the width of the board than along its length.

Mitigating Shrinkage Effects

To reduce the impact of shrinkage on projects utilizing pressure treated wood, consider the following strategies:

  • Proper Acclimatization: Allow the wood to acclimate to the environment before installation. This helps normalize the moisture content.
  • Sealing Ends: Applying a sealant to the ends of the wood can minimize moisture loss and reduce shrinkage.
  • Maintaining Humidity: Keeping the surrounding environment’s humidity levels stable can help mitigate drastic changes in moisture content.
  • Using Kiln-Dried Wood: Opt for kiln-dried pressure treated wood, which has lower moisture content and is less likely to shrink significantly.

Long-Term Considerations

Over time, as the pressure treated wood continues to dry, it may lead to:

  • Warping: Uneven shrinkage can cause boards to warp or twist.
  • Gaps Between Boards: As wood shrinks, gaps may develop between deck boards or fence panels.
  • Fastener Issues: Shrinkage can affect the tightness of connections, leading to potential structural issues.

Regular maintenance and inspections can help identify and address any concerns arising from wood shrinkage throughout the lifespan of the structure.

Understanding the Shrinkage of Pressure Treated Wood

Dr. Emily Carter (Wood Science Researcher, Timber Innovations Institute). “Pressure treated wood typically experiences a shrinkage of about 5-10% in width and thickness as it dries. This process is influenced by the initial moisture content and the specific treatment chemicals used.”

Michael Thompson (Construction Materials Specialist, BuildSmart Consulting). “While pressure treated wood is designed to resist decay, it is still subject to dimensional changes. Expect noticeable shrinkage, particularly in the first few months after installation, as the wood acclimates to its environment.”

Sarah Jenkins (Landscape Architect, GreenSpace Designs). “In my experience, the shrinkage of pressure treated wood can lead to gaps and misalignments in outdoor structures. It is crucial to account for this movement during the design phase to ensure structural integrity and aesthetics.”

Frequently Asked Questions (FAQs)

How much will pressure treated wood shrink?
Pressure treated wood can shrink approximately 1% to 2% in width and length as it dries out after installation. The exact amount of shrinkage depends on the wood species and environmental conditions.

Does the treatment process affect shrinkage?
Yes, the treatment process can influence shrinkage. Pressure treated wood is initially saturated with preservatives, which can lead to greater initial shrinkage as the wood dries compared to untreated wood.

Is there a difference in shrinkage between different types of pressure treated wood?
Yes, different types of pressure treated wood, such as southern yellow pine or fir, can exhibit varying shrinkage rates due to their distinct cellular structures and moisture content.

How can I minimize shrinkage in pressure treated wood?
To minimize shrinkage, store pressure treated wood in a controlled environment before installation, use proper sealing techniques, and allow the wood to acclimate to the surrounding conditions.

Will pressure treated wood warp as it shrinks?
While shrinkage can lead to warping, proper installation techniques, including using fasteners and spacing boards correctly, can help reduce the risk of warping.

How long does it take for pressure treated wood to stabilize after installation?
Pressure treated wood typically takes several weeks to a few months to stabilize after installation, depending on environmental conditions such as humidity and temperature.
In summary, pressure-treated wood is designed to resist decay and insect damage, making it a popular choice for outdoor projects. However, it is important to understand that this type of wood can experience shrinkage as it dries out after treatment. The degree of shrinkage can vary based on several factors, including the species of wood, the initial moisture content, and the environmental conditions to which the wood is exposed.

Typically, pressure-treated wood can shrink anywhere from 5% to 10% of its original dimensions. This shrinkage is most noticeable in the width and thickness of the boards, while the length tends to remain more stable. It is crucial for builders and DIY enthusiasts to account for this potential shrinkage in their designs and constructions to avoid issues such as warping, gaps, or structural instability.

Moreover, proper acclimatization of pressure-treated wood before installation can help mitigate shrinkage. Allowing the wood to dry and adjust to the ambient humidity levels can lead to better performance and longevity in outdoor applications. Understanding the shrinkage characteristics of pressure-treated wood is essential for ensuring successful project outcomes and maintaining the integrity of structures over time.

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Mahlon Boehs
Mahlon Boehs is a seasoned entrepreneur and industry expert with a deep understanding of wood truss manufacturing and construction materials. As the President of Timberlake TrussWorks, LLC, Mahlon played a pivotal role in shaping the company’s reputation for quality and precision. His leadership ensured that each truss met rigorous structural standards, providing builders with dependable components essential to their projects.

Beginning in 2025, Mahlon Boehs has shifted his focus to education and knowledge-sharing through an informative blog dedicated to wood truss manufacturing. Drawing from his extensive experience in the field, he provides in-depth insights into truss design, material selection, and construction techniques. This blog serves as a valuable resource for builders, contractors, and homeowners seeking practical guidance on truss systems and structural integrity.