How to Maintain Dimensional Stability in PSE Timber

25th Jun 2024 -

Dimensional stability is key when working with planed square edge (PSE) timber to achieve consistent performance. This article explores methods for maintaining dimensional stability, from proper drying and acclimatisation to handling and storage practices. 

What is PSE timber dimensional stability?

Dimensional stability in PSE timber refers to the wood’s ability to maintain its precise dimensions and square edges over time. Achieving dimensional stability is crucial for ensuring structural integrity, aesthetic appeal and overall performance. Several factors influence PSE timber dimensional stability. 

1. Moisture content

Moisture content is perhaps the most significant factor. Wood is a hygroscopic material, meaning it naturally absorbs or releases moisture in response to changes in humidity and temperature. As the moisture content fluctuates, the wood fibres swell or shrink. 

Dimensional changes lead to warping, twisting and shrinkage or expansion, compromising the precision of the square edges and overall dimensions. Maintaining a consistent moisture content level is essential to minimise these effects.

2. Wood density and species

The density and species of wood used for PSE timber also influence its dimensional stability. Denser hardwoods like oak or maple hold greater dimensional stability compared to softer woods like pine, redwood or cedar. They have a tighter cellular structure and fewer air pockets, making them less prone to swelling and shrinkage.

3. Sawing pattern and grain orientation

The way the lumber is sawn and the orientation of the wood grain can impact dimensional stability. Flat-sawn or plain-sawn boards tend to display more tangential shrinkage (across the growth rings). Quarter-sawn or rift-sawn boards are more dimensionally stable because of their radial grain orientation (parallel to the growth rings).

4. Drying process

The drying process is critical in achieving dimensional stability in PSE timber. Improper or uneven drying may lead to internal stresses and moisture gradients within the wood. This causes warping, checking and uneven shrinkage or expansion.

Specifying controlled temperature, humidity and airflow settings for kiln-drying allows moisture removal and minimises dimensional instability. Also, air-drying or conditioning the timber after kiln drying further boosts dimensional stability. It ensures the wood acclimatises to the environment.

5. Preservative treatment

Many PSE timber materials are treated with preservatives to enhance their resistance to decay, insects and other environmental factors. However, the preservative treatment process itself may influence dimensional stability.

Certain treatments like those involving heat or pressure may cause additional stresses and moisture gradients within the wood. This potentially leads to warping, twisting or uneven dimensional changes.

6. Surface coatings and finishes

Applying surface coatings or finishes like paint, stain or sealant can impact PSE timber dimensional stability. These coatings may act as a barrier, preventing or slowing down the moisture exchange between the wood and the environment. If applied unevenly, they can lead to differential moisture movement. 

7. Weathering exposure

Prolonged exposure to UV radiation, moisture and temperature fluctuations can degrade the wood's surface and compromise its dimensional stability. Proper maintenance such as periodic reapplication of coatings or finishes is necessary to maintain PSE timber in these conditions.

Learn more about the characteristics of planed timber with our comprehensive guide.

PSE vs Rough Sawn Timber: Which is Best for Stability?

The manufacturing process has a crucial role when it comes to dimensional stability. PSE and rough sawn timber represent 2 distinct product types, each with its own considerations. The following table compares the dimensional stability characteristics of these timber options.

Aspect	PSE timber	Rough sawn timber
Manufacturing	Precise planing ensures uniform dimensions and square edges.	Minimally processed, retaining natural unplaned surfaces.
Moisture control	Typically kiln-dried to precise moisture content levels.	Rough sawn timber has a wider range of moisture content levels.
Species/density	Often dense hardwoods with greater dimensional stability.	Commonly softwoods with higher tendency for moisture movement.
Grain orientation	Quarter-sawn or rift-sawn for minimised tangential shrinkage.	Grain orientation is not optimised for dimensional stability.
Dimensional consistency	A high degree of consistency in dimensions and square edges.	Potential for irregularities and dimensional inconsistencies.

How to Maintain PSE Dimensional Stability 

To address the factors that impact PSE timber dimensional stability, proper material selection, storage, post-treatment handling and maintenance are necessary. Here are some effective strategies to consider.

Careful material selection

• Choose PSE timber made from wood species known for their dimensional stability like oak or maple. 
• Ensure the PSE timber is properly seasoned and dried to an appropriate moisture content level before installation. 
• Opt for quarter-sawn or rift-sawn PSE timber. The radial grain orientation minimises tangential shrinkage and boosts dimensional stability.

Proper storage and handling 

• Store PSE timber in controlled environments with appropriate temperature and humidity levels. This minimises moisture fluctuations before installation. 
• Avoid direct contact with concrete, soil or other moisture sources during storage and handling. 
• Use handling practices such as proper lifting techniques and avoid damage to the square edges.

Protective coatings and finishes

• Apply high-quality, moisture resistant coatings or finishes to protect the PSE timber from environmental exposure and minimise moisture exchange. 
• Ensure coatings are applied evenly and consistently, without creating barriers that could lead to differential moisture movement. 
• Regularly maintain and reapply coatings or finishes as needed, especially in outdoor or high-moisture environments.

Regular maintenance

• Conduct inspections every 6-12 months to identify any signs of warping, twisting or dimensional instability in PSE timber. 
• Address any issues promptly by adjusting environmental conditions, reapplying coatings or conducting necessary repairs or replacements. 
• Clean and reapply protective coatings or finishes as needed.
• Adjust environmental conditions (humidity, temperature) if necessary.
• Address any moisture issues or sources of moisture exposure.

6 Installation Tips for PSE Dimensional Stability

Implementing the right installation techniques is crucial for ensuring the dimensional stability of PSE timber products throughout their service life. Below are some key installation practices and their benefits. 

1. Proper spacing and ventilation

Leave enough spacing (10-15mm) between PSE timber members. Ensure sufficient airflow and ventilation around the installed PSE timber. Spacing and ventilation prevent moisture accumulation, which may lead to dimensional changes, warping or twisting.

2. Using appropriate fasteners

Choose corrosion-resistant fasteners (screws, nails) that are compatible with the PSE timber and the intended application. Avoid over-tightening fasteners which may create localised stresses and potentially lead to dimensional instability. 

Consider using specialised fastening systems or techniques, such as concealed fasteners or slotted holes. These systems allow for natural expansion and contraction and proper fastening methods minimise restraints on the wood.

3. Effective moisture management

Install flashing, drip edges and other moisture-shedding details to divert water away from the PSE timber surfaces. Introduce drainage systems or channels to remove any accumulated moisture quickly. 

Use moisture-resistant barriers or vapour retarders in high-moisture areas or exterior applications. Effective moisture management techniques prevent excessive moisture exposure, which impacts dimensional stability.

4. Protecting exposed end grains

Seal or coat the end grains of PSE timber members with a moisture resistant sealant or end-grain sealer. End grains are more susceptible to moisture absorption and can contribute to localised dimensional instability if left unprotected. Sealing the end grains helps maintain a consistent moisture content throughout the timber member.

5. Acclimatise PSE timber before installation

Allow the PSE timber to acclimate to the environment's temperature and humidity conditions before installation. Acclimatisation helps minimise dimensional changes caused by sudden environmental shifts after installation. Generally, you should allow the acclimatisation period to last between 48-72 hours.

6. Expansion joints and trim details

Add expansion joints or trim details that allow for natural expansion and contraction of the PSE timber during dimensional changes. These details prevent buckling and warping caused by restraining the wood's natural movement. Ensure expansion joints are properly sized and located based on the expected dimensional changes. 

Final Checklist: PSE Timber Dimensional Stability

To ensure the long-term performance and precise dimensions of PSE timber, it is essential to follow a thorough checklist. This checklist serves as a guide, covering all the critical aspects you should consider to achieve optimal dimensional stability. 

• Choose dense hardwood species and quarter-sawn/rift-sawn PSE timber. 
• Ensure proper kiln drying to an appropriate moisture content level.
• Store in a controlled environment, avoiding moisture exposure. 
• Follow proper handling techniques to prevent damage.
• Allow PSE timber to acclimate to the service environment before installation.
• Leave adequate spacing and ensure proper ventilation.
• Use appropriate fasteners and allow for expansion/contraction.
• Introduce effective moisture management strategies (flashing, drainage).
• Seal end grains and incorporate expansion joints.
• Apply high-quality, moisture resistant coatings evenly.
• Conduct regular inspections (e.g. annually). 
• Address any dimensional stability issues promptly. 
• Reapply protective coatings and adjust environmental conditions as needed.