Protecting PSE Timber: Boost Outdoor Durability

26th Jun 2024 -

Planed square edge (PSE) timber is a popular choice for outdoor construction projects because of its clean appearance. However, exposing this type of timber to the elements may lead to premature degradation, compromising its structural integrity.

While PSE timber offers many benefits, it still requires proper protection to withstand outdoor environments. This article focuses on the various methods for protecting PSE timber in outdoor applications, offering guidance for maximising performance. 

What is PSE Timber?

PSE timber is milled and planed to create a smooth, uniform surface on all 4 sides. It is typically made from softwood species such as pine, redwood, fir and cedar. The planed timber process removes the rough outer layers of the wood. This exposes the smoother interior and creates a more consistent thickness and width than alternatives like rough sawn timber. 

Because of its refined look and overall dimensional consistency, PSE timber is well-suited for a wide range of outdoor spaces. Some common uses include the following applications.

Decking and patio construction

PSE timber is an excellent choice for building decks and patio surfaces. Its smooth surface provides a comfortable and splinter-free area, while its uniform dimensions allow precise fitting. 

Fencing and railing systems

The smooth surfaces make it ideal for constructing fences and railings that require accurate fitting. Its consistent sizing across length, width and thickness allows for tight joints and clean lines. 

Outdoor furniture and structures

PSE timber is widely used to build sturdy and visually appealing outdoor furniture. For example, you can use it to create benches, tables and chairs, as well as raised garden beds and retaining walls.  

Siding and trim

The uniform dimensions of PSE timber allow for a neat and consistent installation when used as siding or trim on exterior walls, soffits and fascia boards.

Find out more about the properties of PSE timber with our article.

4 Benefits of Using PSE Timber Outdoors

While appearance and general dimensional stability are key, PSE timber offers several other benefits that make it an excellent choice. Below, we explore these 4 benefits in detail.

1. Improved structural performance

PSE timber is typically made from kiln-dried lumber, which reduces its moisture content to a stable level. The drying process enhances the wood's strength and stiffness. It makes it less susceptible to sagging or splitting when used in load-bearing applications like decking or outdoor structures.

2. Easier installation and finishing

Consistent PSE dimensions and smooth surfaces allow quicker installation, limiting the need for extensive cutting, fitting or sanding. This saves time and labour costs during construction. Also, the planed surfaces accept stain, paint and other finishes more evenly, resulting in a long-lasting finish.

3. Increased durability

Its smooth surfaces and lack of rough edges or cracks make it less prone to moisture absorption and fungal growth. While PSE timber still requires treatment for outdoor use, this characteristic reduces the need for frequent repairs or replacements.

4. Versatility

PSE timber is available in various softwood species. This allows for a range of choices based on specific project requirements, such as strength, weight or weather resistance. Its versatility makes it suitable for various outdoor applications, from decking and fencing to furniture and landscaping features.

What Factors Affect PSE Timber Durability? 

The long-term performance of PSE timber is influenced by various factors. Understanding these factors is important for introducing appropriate protective measures. Here are the key factors that impact the durability of PSE timber in outdoor applications. 

Moisture exposure

Prolonged exposure to moisture, whether from precipitation or humidity, can lead to timber decay, warping and loss of structural integrity. The level and duration of moisture exposure are critical elements in determining the rate of deterioration.

Ultraviolet (UV) radiation

Sunlight contains UV radiation that causes discolouration, surface cracking and brittleness in wood over time. The intensity and duration of UV exposure greatly impact the appearance and longevity of PSE timber structures.

Temperature fluctuations

Particularly during freezing and thawing cycles, extreme temperature changes may stress the wood fibres. This potentially causes splitting, cracking or other forms of physical degradation. The severity and frequency of temperature fluctuations accelerate the ageing process of PSE timber.

Biological factors

Wood-destroying fungi, insects and other pests may compromise the structural integrity and appearance if left unchecked. The presence of these biological agents, as well as the susceptibility of the specific wood, can enhance the deterioration rate.

Site conditions

The environment has a significant impact on its durability. Factors such as exposure to direct sunlight, moisture, and vegetation or debris all contribute to accelerated degradation.

Construction details

Proper installation, including adequate ventilation, drainage and avoidance of direct ground contact, greatly enhances the longevity of PSE timber. Poor construction practices or design flaws may lead to premature failure or increased maintenance requirements.

Wood species and quality

Different wood species have varying natural durability and resistance to decay, insects and weathering. Also, the quality of the PSE timber like density and knot content influences its overall performance in outdoor environments.

How to Boost PSE Timber Durability

To maximise its longevity and performance in exterior settings, it is essential to implement appropriate protection measures. The following are some effective techniques to consider.

1. Wood preservatives

Treating PSE timber with wood preservatives improves its resistance to fungal decay, insect damage and moisture-related issues. They are applied using pressure treatment, dip diffusion or brush/spray application methods. Common preservatives include the below.

• Copper-based preservatives: Examples like alkaline copper quaternary (ACQ) and copper azole (CA) offer excellent protection against fungal decay and insects. They are suitable for ground contact applications like decking and fencing. 
• Boron-based preservatives: Particularly effective against wood-destroying insects and fungi. Boron preservatives like zinc borate are ideal for above-ground applications. 

2. Water repellents

Applying water-repellent coatings or sealants prevents moisture absorption and subsequent swelling, cracking or warping. These coatings form a barrier that repels liquid water while allowing the wood to breathe and release moisture vapour.

3. UV-resistant finishes

Exposure to ultraviolet radiation from sunlight may cause PSE timber to become brittle over time. Applying UV-resistant finishes, such as exterior-grade paints, stains or clear sealants, protects the wood from these damaging effects.

4. Proper installation and design

Certain installation practices can also enhance the durability of PSE timber in outdoor applications. These include the following.

• Allowing for proper ventilation and drainage to prevent moisture accumulation. 
• Avoid direct ground contact by using appropriate spacers or supports. 
• Incorporating overhangs or other protective features to shield the wood from direct exposure to rain and sunlight. 
• Ensuring proper spacing and fastening techniques to minimise the risk of splitting or cracking.

5. Regular maintenance

Even with proper treatment and installation, regular maintenance boosts the long-term durability of PSE timber in outdoor applications. This may involve the below.

• Periodic cleaning (annually) to remove dirt, debris, surface mould and mildew buildup. 
• Re-application of protective coatings or finishes every 2-3 years for optimal UV and moisture protection. 
• Prompt repair or replacement of any damaged or deteriorated components. 
• For preservative-treated PSE timber, re-treatment is required every 5-10 years for ground-contact applications and every 10-15 years for above-ground use.

How Does Planed Timber Differ From Rough Sawn? 

When evaluating PSE and rough sawn timber performance, both options offer distinct benefits. Understanding these characteristics ensures the structural integrity of outdoor projects. Below is a comparison table between the 2 timber types.

Performance aspect	PSE timber	Rough sawn timber
Surface finish	Smooth, uniform surfaces on all 4 sides resulting in a clean, finished appearance.	Rough, unplaned surfaces with a natural texture from the sawing process.
Dimensional accuracy	PSE timber is made with consistent dimensions across length, width and thickness.	Varying degrees of dimensional accuracy, prone to warping or twisting.
Moisture resistance	Can be treated with water-repellent coatings or preservatives for enhanced moisture resistance.	Rough surfaces and irregularities can trap moisture, increasing susceptibility to decay.
UV protection	Smooth surfaces facilitate better adhesion and uniform application of UV-resistant finishes.	The rough sawn timber surfaces may not accept UV-resistant finishes as evenly.
Aesthetic appeal	Clean, refined look suitable for visible outdoor applications like decking, fencing and furniture.	Offers a natural, rustic appearance that may be preferred for certain outdoor projects.
Installation	Smooth PSE wood surfaces reduce the risk of splinters and improve ease of handling and installation.	Rough surfaces may pose safety risks and installation challenges despite treatment.
Preservative treatment	It can be treated with copper or boron-based wood preservatives for durability.	Also treated with preservatives, but may prevent even distribution and penetration.
Maintenance	The smooth surfaces of PSE timber allow easier cleaning and maintenance techniques.	Rough surfaces can trap dirt, debris and moisture, requiring frequent maintenance.
Versatility	It is suitable for a wide range of outdoor applications, both structural and decorative.	Primarily used for structural outdoor projects or where a natural, rustic appearance is desired.