Combatting Timber Decay: Exploring Treated Timber Methods

Timber is a fundamental building material because of its strength, workability and appearance. However, despite its many benefits, wood is prone to decay caused by fungi, insects and other environmental factors. For this feature, we investigate the landscape of timber decay, exploring treatments aimed at extending the longevity of timber products. 

What is timber decay?

Timber decay is a complex biological process that can severely compromise the performance of wood-based structures. This degradation is primarily caused by the activity of various wood-destroying fungi and insects. It feeds on the essential components of wood, such as cellulose, hemicellulose and lignin. Fungal decay is one of the most prevalent and damaging forms of timber deterioration. 

Different types of fungi, including brown rot, white rot and soft rot, employ unique mechanisms to break down the wood's cellular structure. For example, brown rot fungi selectively attack the cellulose and hemicellulose, leaving behind a brown residue. However, white rot fungi can degrade all major wood components and often cause significant strength loss. 

Insect infestations also pose a significant threat to timber structures. These pests can tunnel through and consume the wood, weakening the material's structural integrity. The consequences of timber decay can be severe, ranging from aesthetic issues to structural failures. 

As the wood's cellular structure breaks down, its strength and dimensional stability are compromised. This can lead to sagging, cracking and even collapse in extreme cases. It poses significant risks to the safety and longevity of timber-based structures.

What are the different timber decay treatments?

Treatments boost various mechanisms to secure the wood's natural resistance and prevent fungi, insects and environmental factors. The following are the most widely used and timber decay treatment options. 

1. Preservative treatments

Traditional wood preservatives, such as chromated copper arsenate (CCA), copper azole and alkaline copper quaternary (ACQ), are widely used to protect timber from decay and insect infestations. They are typically applied through timber pressure treated methods, allowing them to penetrate deep into the wood's cellular structure. 

The preservative chemicals for pressure treated timber (also known as tanalised timber) create a difficult environment for fungi and insects. This is either through toxic effects or by rendering the wood indigestible.

2. Thermal modification

This innovative treatment involves exposing wood to high temperatures (typically between 160°C and 240°C) in an oxygen-free or low-oxygen environment. The heat causes chemical changes in the wood's cellular structure. It reduces its moisture absorption and improves its dimensional stability. Thermally modified wood exhibits increased resistance to fungal decay and insect attacks, as well as protection against wet weather conditions.

3. Impregnation treatments

These treatments involve impregnating the wood with specialised resins, polymers or nanoparticles to enhance its durability. For example, melamine-based resins can be impregnated into the wood. This creates a polymer-reinforced material with improved decay resistance and dimensional stability. 

Nanoparticles, such as zinc oxide or copper nanoparticles, can be impregnated into the wood to provide harmful properties against fungi and insects.

4. Surface coatings and treatments

Various coatings and surface treatments can be applied to timber to provide a protective barrier against decay and environmental factors. Water-repellent coatings, such as stain solutions, oils, polymers or paint, can help prevent moisture absorption. This is a primary factor of fungal decay. 

5. Biological control agents

Emerging research explores the use of biological control agents, such as fungi or bacteria, to combat wood-decaying fungi and insects. For example, certain strains of the Trichoderma fungus have been found to produce enzymes and metabolites that can stop the growth of decay fungi. Biological control agents offer a potentially eco-friendly and sustainable alternative to chemical treatments.

How to apply timber decay treatments

The timber treating method you use is crucial in ensuring proper penetration and long-term performance. Below is an overview of how to apply these various treatments, including maintenance frequency and suitable wood types.

Preservative treatments

• Pressure treatment: This is the most common and effective method for applying preservatives to timber. The wood is placed in a sealed cylinder, where a vacuum is created to remove air from the wood cells. The preservative is then forced deep into the wood under high pressure.
• Best for: Softwoods like Canadian Lumber Standard (CLS timber) are most commonly pressure-treated because of their permeable nature to create CLS treated timber. Hardwoods can also be pressure-treated. However, the process is more challenging because of their denser cellular structure.
• Maintenance frequency: For ground-contact applications, re-treatment may be needed every 20-40 years. In above-ground applications with minimal exposure, treated timber battens or boards may last 50 years or more with little maintenance.

Thermal modification

• Industrial thermal modification: Large-scale thermal modification is typically carried out in special industrial facilities. 
• Best for: Thermal modification is suitable for both softwood and hardwood treated carcassing timber. However, the process may need to be adjusted based on the wood type and desired end-product properties. 
• Maintenance frequency: The modified timber board or batten should be kept well-ventilated and protected from direct moisture exposure. On top of thermal modification, surface coatings or wood stains may need to be applied every 3-5 years to treat for UV protection.

Impregnation treatments

• Vacuum pressure impregnation: Similar to preservative treatments, this involves placing the wood in a vacuum cylinder and removing air from the cells before introducing the impregnation solution (resins, polymers, nanoparticles) under high pressure.
• Best for: Permeable softwoods like fir and spruce are ideal for impregnation treatments to create types such as C16 treated timber and C24 treated timber. This is because of their open cellular structure. Some hardwoods, like beech and ash, can also be effectively impregnated.
• Maintenance frequency: Impregnated wood products generally require minimal maintenance due to the extensive modification of the wood's cellular structure. In exterior applications, surface coatings or wood stains may need to be applied every 5-10 years for added protection.

Surface coatings and treatments

• Brush, roller or spray application: For applications like a garden fence, surface coatings like water repellents, fire retardants and protective finishes can be applied using various methods. This includes brushing, rolling, or spraying, depending on the product and desired coverage.
• Best for: Surface treatments are suitable for both softwoods and hardwoods, provided the wood surface is properly prepared (cleaned, sanded) to ensure good adhesion and penetration.
• Maintenance frequency: Water-repellent coatings may need to be applied every 2-5 years for optimal protection. Fire-retardant treatments may require re-application every 5-10 years, depending on exposure such as outdoor elements.

Biological control agents

• Dip or spray application: Depending on the specific agent, it can be applied by dipping the wood into a solution or suspension containing the agent or by spraying the solution onto the wood surface.
• Best for: Biological control agents are generally suitable for both softwoods and hardwoods. However, their effectiveness may vary based on the wood species, moisture content and environmental conditions.
• Maintenance frequency: The maintenance requirements for biological control agents are still being researched and may vary depending on the specific agent used. Periodic re-application or supplementation of the biological agents will likely be necessary, as their effectiveness may diminish over time.

Does tanalised timber offer the best protection?

For long-lasting protection against timber decay, pressure treatment stands out as one of the most effective methods available. While each treatment approach has its strengths, pressure-treated timber offers several compelling benefits to protect from decay.

1. Proven results

Preservative treatments have a well-established track record of successfully protecting timber structures against decay. Compounds like CCA and ACQ have demonstrated long-lasting resistance against a wide range of decay agents.

2. Deep penetration

Preservatives are typically applied through pressure treatment processes, allowing the chemicals to penetrate deep into the wood's cellular structure. This ensures that the protection is on the surface and throughout the entire cross-section of the timber.

3. Adaptability

Different preservative formulations can be tailored to specific applications and exposure conditions. This makes it possible to optimise the treatment for various structural components and environments. For example, ground-contact applications may require more robust preservative treatments compared to above-ground structures.

4. Cost-effectiveness

While the initial cost of preservative-treated timber may be higher than untreated wood, the extended service life and reduced maintenance requirements often make preservative treatments a cost-effective solution in the long run.