Preservative Treatments and Use Classes

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TBC

Introduction

Not all timber is naturally durable. Many lower-durability species can be made suitable for demanding environments through preservative treatment. This allows cost-effective timbers such as Radiata Pine to be used safely in above-ground, in-ground, and even marine applications.

2. The Six Hazard Classes (H1-H6)

AS 1604 classifies biological risk into six hazard classes, each tied to environmental exposure and biological threat. These hazard classes drive the minimum treatment requirement for compliant use.

Hazard Class	Exposure	Biological Threat	Typical Applications
H1	Inside, protected from weather and insects.	Borers only	Furniture, wall linings
H2/H2F	Inside, protected from weather but subject to termite risk.	Borers + termites	Framing in termite areas
H3	Outside, above ground, exposed to periodic wetting and leaching, and allowed to dry after each wetting episode.	Fungal decay, insects	Decking, cladding, fascia
H4	Outside, in ground or continuously damp, non-critical applications such as landscaping and fence posts.	Severe decay, termites	house stumps, piles, poles, and retaining walls.
H5	Outside, in ground or fresh water, critical structural applications such as retaining walls, piles, and stumps.	Extreme decay, termites	Retaining walls, piles
H6	Marine	Marine borers, extreme decay	Jetty piles, sea walls

Note: The distinction between H4 and H5 is important. H4 is not suitable for critical structural elements. H4 is not intended for structural reliability. For any structural element in ground contact, H5 or a Class 1 durable hardwood is required. If in doubt, or if microclimatic conditions increase decay risk, H5 or a naturally durable Class 1 hardwood should be specified.

NCC requires that timber used in high-risk or structural applications must meet these performance requirements - either via natural durability, or treatment to a certified H-class.

Risk-Based Use of Hazard Classes

The correct Hazard Class depends on both exposure and site conditions:

Well-ventilated, above ground – H3 treated softwoods are suitable.
Close-to-ground (<400 mm clearance) – increase to H4 or use Class 1 hardwood.
In-ground structural – H5 or Class 1 hardwood only.
Marine – H6 treatment or durable tropical hardwoods.

Types of Preservative Treatments

Different chemicals are used depending on hazard class, species compatibility, and intended application.

Preservative	Abbrev.	Use Case	Notes
Copper Chrome Arsenate	CCA	Structural timber, decking	Strong, broad-spectrum; some environmental limits. Not allowed for handrails, decking, or playground equipment where frequent skin contact occurs.
Light Organic Solvent Preservative	LOSP	Joinery, appearance-grade cladding.	Low-moisture; sensitive to coatings. Must be primed and finished all-round soon after installation. Does not prevent surface mould growth; finish systems reduce this risk. Not suitable for H3, H4 or H5 applications.
Alkaline Copper Quaternary	ACQ	External structures	Arsenic-free, water-based
Copper Azole	CuAz	Decking, structural timber	Low environmental impact
Boron/Borates	B	Internal framing, furniture	Not for external use - leachable

Figure 2: Copper Chrome Asenate (CCA)

Figure 2: Light Organic Solvent Preservative (LOSP)

Figure 3: Alkaline Copper Quaternary (ACQ)

Figure 4: Copper Azole (CuAz)

Figure 5: Boron/Borates

Detailing and Coating of Treated Timber

Sealing and priming – all treated timber should be sealed or primed all-round to protect against weathering and surface mould.
Cut ends – reseal immediately after cutting with preservative or primer.
Top-edge protection – install flashing strips or damp-proof membranes over exposed beams and joists.
Finishes – apply appropriate stains or coatings to reduce weathering and extend service life.

Sapwood vs Heartwood - Treatability

Preservative treatment is only effective in sapwood, which is porous and can absorb chemicals. In contrast, heartwood - especially in dense hardwoods - is naturally resistant to penetration.

Timber Part	Treatability	Implication
Sapwood	Readily treatable	Can be fully treated to meet H-class requirement
Heartwood	Resistant	Cannot reliably achieve treatment penetration

For species like Blackbutt or Spotted Gum, pressure treatment does not reach H3/H4 levels in heartwood. Rely on natural durability instead, or switch to more treatable timbers.

Figure 6: Decay of sapwood under a film finish.

Application Methods and Effectiveness

Different methods are used to apply preservative chemicals to timber. The effectiveness of each method depends on penetration depth and chemical retention, both defined by AS 1604.

Treatment Method	Depth	Typical Use
Pressure Impregnation	Deep, uniform	Structural and exposed elements
Double-Vacuum (LOSP)	Moderate	Joinery, pre-finished cladding
Dipping / Brushing	Surface only	Temporary protection, site cuts

On-Site Re-Treatment

All cut ends, drill holes, and rebates in treated timber must be re-coated on site using suitable preservatives to maintain integrity.

Verifying Treatment Compliance

Treated timber must be traceable to a certified treatment process under the NCC. This includes:

End tags or face stamps showing:
- Species
- Preservative type
- Hazard class (e.g. H3)
- Standard reference (e.g. AS/NZS 1604.1)
- Treatment plant certification
Certificate of treatment for structural or in-ground applications

Before installation:

Confirm labelling and documentation
Verify that treatment depth and retention meet specification (especially for H4 or H5)
Check cut ends have been sealed

Detailing & Compatibility

Preservative treatments may affect the performance of fasteners, coatings, and adhesives. Correct specification ensures long-term durability.

Component	Compatibility Notes
Fixings	Use hot-dip galvanised or 304/316 stainless steel in CCA/ACQ
Coatings	LOSP may inhibit adhesion of some water-based finishes
Adhesives	Check compatibility with treatment type (esp. LOSP in joints)

Summary

Preservative treatment allows lower-durability timber to be used in high-risk applications
AS 1604 defines six hazard classes based on exposure and biological risk
Only sapwood is treatable - ensure penetration and retention for compliance
All treated timber must be labelled and verifiable under AS 1604
Re-seal site cuts, and ensure compatibility with fixings and coatings
Refer to NCC and local CTIQ guidance for treatment and installation protocols