Timber Treatments Explained: CCA, ACQ & LOSP

Timber Treatments – when and why to use them

Timber is one of the most versatile building materials – strong, natural and sustainable. When timber is used outdoors or in exposed environments, it needs extra protection against decay, insects and weathering. That’s where timber treatments come in.

Below is a breakdown of the three most common treatments, CCA, ACQ and LOSP – explained in simple terms so you know which is right for your project.

CCA (Copper Chrome Arsenate)

What it is: CCA is one of the oldest and most widely used timber treatments in Australia. It combines copper (for resistance against fungi and decay), chromium (to bind the treatment into the timber), and arsenic (to protect against insect attack such as termites and borers).

When to use CCA: CCA-treated timber is designed for outdoor and structural applications where long-term durability is essential.

• Retaining Walls & Sleepers – excellent for garden beds, landscaping and soil retention
• Fencing – posts, rails, and palings that need to withstand years of weather exposure
• Outdoor Structural Framing – pergolas, carports and similar projects (above-ground or in-ground contact)

Important Notes: CCA-treated timber is not recommended for use on playgrounds, furniture or food-related projects due to the arsenic content. Best for structural and industrial use as its highly effective where strength, durability and cost-efficiency are the main priorities.

ACQ (Alkaline Copper Quaternary)

What it is: ACQ is a modern, arsenic-free timber treatment that uses copper (to fight fungi and rot) and quaternary ammonium compounds (to repel insects such as termites). It’s considered a safer alternative to CCA, especially where children will come into contact with the timber.

When to use ACQ: ACQ-treated timber is designed for residential, landscaping and outdoor living projects where both safety and durability matter.

• Children’s Play Equipment – safe option for cubby houses, swings or climbing frames
• Outdoor Furniture – benches, tables, and features where people have direct skin contact.

Important Notes: The copper in ACQ can be corrosive to some metals – always use hot-dip galvanised or stainless steel fixings.

LOSP (Light Organic Solvent Preservative)

What it is: LOSP is a solvent-based preservative (rather than water-based) often used for appearance-grade timbers. It doesn’t cause timber to swell or distort during treatment, making it perfect for detailed joinery and decorative applications. LOSP typically protects against rot, decay, and insect attack and it’s often available in clear or tinted finishes.

When to use LOSP: LOSP is best for above-ground applications where aesthetics and dimensional stability are important.

• Cladding and Weatherboards – holds shape well and can be pre-coated for smoother finishes
• Fascia Boards – lightweight and easy to install
• Pergolas and Decorative Timberwork – keeps its form for joinery or architectural features
• Windows and Doors – doesn’t warp or swell, perfect for precision joinery
• Balustrades and Verandahs – ideal for decorative or exposed but above-ground applications

Important Notes: LOSP timber must be painted or stained after installation for weather protection. Not suitable for in-ground or high-moisture environments.

Quick Comparison

Choosing the right timber treatment comes down to where the timber will be used and who will be around it. By understanding the differences, you can select the treatment that delivers both performance and peace of mind for your next project.

Contact the team at 21 Timbers for further information or advice on 07 3801 1122 or admin@21timbers.com.au.

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