In Australia, timber treatment is governed by the Hazard (H) Class system, which ensures that the chemicals used are appropriate for the environment where the timber will be installed.
Whether you are building an indoor frame or an in-ground retaining wall, understanding these treatments is the key to structural longevity.
1. The Hazard Class (H) Scale
The “H” rating tells you what the wood is protected against and where it can be used.
-
H1: Interior, above ground. Protects against borers. Used for furniture and joinery.
-
H2 / H2-S: Interior, above ground. Protects against termites. Used for framing and trusses.
-
H3: Exterior, above ground. Protects against rot and termites. Used for decking and cladding.
-
H4: Exterior, in-ground. Protects against severe decay. Used for fence posts and sleepers.
-
H5: Exterior, in-ground/freshwater. Protects against extreme decay. Used for retaining walls and house stumps.
2. Common Treatment Chemicals
CCA (Copper Chrome Arsenate)
CCA is the most common and cost-effective treatment in Australia.
-
How it works: Copper prevents fungal decay, Arsenic prevents termite attack, and Chromium “fixes” the chemicals into the wood fibers.
-
Appearance: Leaves a distinct green tint on the timber.
-
Restrictions: Due to the arsenic content, CCA is not permitted for use in structures where there is frequent skin contact, such as children’s playground equipment, garden furniture, or residential decking boards. It is still widely used for retaining walls, fence posts, and utility poles.
ACQ (Alkaline Copper Quaternary) & TanE (Copper Azole)
These are modern, arsenic-free alternatives to CCA.
-
Use Cases: Ideal for decking, outdoor furniture, and play equipment where safety is the priority.
-
Performance: They offer similar protection to CCA but are generally more expensive and can be more corrosive to standard fasteners (always use 304 or 316 stainless steel or high-quality galvanized screws).
LOSP (Light Organic Solvent Preservative)
LOSP uses a white spirit-based solvent to carry the fungicides and insecticides into the wood.
-
Benefits: Unlike water-based treatments, LOSP does not cause the timber to swell or warp during the process. This makes it perfect for high-precision items like windows, doors, and decorative cladding.
-
Limitation: It is only suitable for H3 (Above Ground) use and is not designed for contact with the soil.
3. Structural Innovation: The H2-S “Glue-Line” Treatment
In modern engineered wood products like meySPAN LVL, the treatment process is integrated directly into the manufacturing.
-
The Process: A termiticide is added to the glue between the layers of wood veneer.
-
The Advantage: Because the protection is inside every layer, you don’t lose the treatment’s effectiveness when you cut or drill the beam on-site. Traditional “envelope” treatments only sit on the outer surface, requiring you to re-seal every cut end with a brush-on preservative.
4. Safety and Handling Tips
Regardless of the treatment type, always follow these Australian safety standards:
-
Wear PPE: Always use a P2 dust mask and safety glasses when cutting treated timber.
-
Never Burn: Do not burn treated timber (especially CCA), as the smoke and ash contain concentrated toxic chemicals.
-
Reseal Cuts: For H3 and H4 timber, always apply a “Can-Seal” or “Enseal” preservative to any ends you cut to maintain the protective barrier.
Would you like me to help you identify which treatment is required for a specific part of your build, such as your sub-floor or garden edging?
When selecting timber for a project in Australia, matching the Hazard (H) Class to the environment is the most important step for durability.
Here is a breakdown of the specific requirements for your sub-floor and landscaping projects based on Australian Standards (AS 1604).
1. Sub-Floor Framing (Bearers & Joists)
Sub-floor requirements vary depending on how high the floor is off the ground and how much ventilation is available.
-
Internal Sub-Floors (Above Ground): For standard house framing that is protected from the weather, H2 or H2-S (like meySPAN) is the standard. It is treated to resist termites and borers but is not designed to get wet.
-
External Sub-Floors (Decks & Verandahs): For a deck that is “above ground” (usually defined as having at least 400mm clearance and good airflow), H3 is the minimum requirement. It is treated for “periodic wetting.”
-
Low-Level Decks (Near Ground): If your sub-floor is very close to the soil (less than 400mm) or has poor ventilation, it can create a “micro-climate” of high humidity. In these cases, builders often upgrade to H4 to prevent premature rot.
2. Garden Edging & Landscaping
Landscaping timber is in constant contact with moist soil, making it a “High Hazard” environment.
-
Garden Edging (Non-Structural): H4 is the industry standard. It is designed for “severe decay” and direct ground contact.
-
Note: If you are building a vegetable garden, look for ACQ or TanE (Copper Azole) treatments. These are “CCA-free” and safer for contact with edible plants.
-
-
Retaining Walls (Structural): If the wall is over 1 metre high or supporting a load (like a driveway), H5 is often required. H5 has a higher concentration of preservatives to handle the extreme moisture and pressure of being buried deep in the earth.
3. Quick Reference: Which “H” do I need?
4. Pro Tip: The “End-Seal” Rule
Most H3 and H4 timber is “envelope treated,” meaning the chemicals penetrate about 5-10mm into the wood. Whenever you cut a piece of treated timber to length, you expose the untreated heartwood in the center.
You must apply a brush-on preservative (like Enseal or Can-Seal) to every cut end. If you don’t, the timber will rot from the inside out, regardless of its H-rating.