AS 1604.1: treated timber and the H1 to H6 hazard classes
AS 1604.1 sets the H1 to H6 hazard classes for preservative-treated timber. Which H-class to order for framing, cladding, decking, and posts, and the treatment types.
Ask Chalkline about this →AS 1604.1 is the Australian Standard that specifies how timber is preservative-treated and defines the H1 to H6 hazard classes that tell you what each piece of treated timber can safely be used for. Every time a framing, cladding, decking, or post order specifies “H2”, “H3”, or “H4”, that letter-and-number is an AS 1604.1 hazard class. Getting it right is the difference between timber that lasts the life of the building and timber that rots or gets eaten in a few years.
What a hazard class means
A hazard class is not a measure of strength. It describes the service hazard the timber will face, exposure to insects, termites, decay fungi, weather, ground contact, or seawater, and the minimum preservative treatment needed to survive it. Higher number, harsher exposure, more preservative.
| Class | Exposure | Main hazard | Typical use |
|---|---|---|---|
| H1 | Inside, above ground, dry, protected | Borers (not termites) | Interior joinery, furniture |
| H2 | Inside, above ground, protected from wetting | Termites and borers | Enclosed framing in termite-prone areas |
| H3 | Outside, above ground, exposed to weather | Decay fungi and insects incl. termites | Cladding, fascia, above-ground decking, pergolas |
| H4 | Outside, in-ground contact or continually damp | Severe decay and insects | Fence and deck posts, retaining sleepers, landscaping |
| H5 | In-ground / fresh water, severe | Very severe decay | Critical in-ground structural, freshwater contact |
| H6 | Marine, immersed in sea water | Marine borers, extreme decay | Marine piles, jetties |
(Verified 2026-05-25, AS 1604.1 hazard class system per Standards Australia and timber-industry sources.)
The jump that catches builders out is H3 to H4: the moment timber goes into the ground or stays continually damp, it needs H4. An H3 post set in the ground will fail early; the H-class has to match the worst exposure that piece sees.
H2 and the termite link
H2 is the framing class. South of the Tropic of Capricorn, enclosed structural framing in termite-prone areas is commonly H2-treated so the timber itself resists termites and borers. This connects directly to termite management: AS 3660.1 recognises appropriately treated timber as one accepted contribution to termite resistance, alongside physical and chemical barriers. Treated framing is not a substitute for a perimeter barrier on its own, but it is part of the picture, and it is why framing orders in termite country call up an H-class.
The treatment types
AS 1604.1 covers several preservative systems, and the one used affects how the timber behaves on site:
- LOSP (light organic solvent preservative): a solvent-borne treatment that does not add water to the timber, so the timber stays dimensionally stable and does not need re-drying. Common for H2 and H3 dressed timber (cladding, framing). For H3, LOSP must include both a fungicide and an insecticide.
- ACQ (alkaline copper quaternary): a water-based copper system (copper plus a quaternary biocide), arsenic-free, used for H3 to H5. The green-ish water-based treatment swells the timber, so it needs re-drying after treatment.
- Copper azole: another water-based copper system (copper plus tebuconazole), arsenic-free, similar uses to ACQ.
- CCA (copper chrome arsenate): the long-standing green-treated system, still used for many H3 to H5 applications, but restricted in Australia from high-contact uses (decking, handrails, garden furniture, playground equipment, picnic tables) following an APVMA review, because of the arsenic content.
The practical upshot: LOSP-treated timber stays the size it was, while water-based (ACQ, copper azole, CCA) timber is wetted in treatment and should be re-dried before it is fixed, or it will shrink in place.
How it is marked
Treated timber is branded with its hazard class (H2, H3, etc.) and treatment information, and the brand is what a certifier or builder checks. If a framing plan specifies H2 and the delivered timber carries no H-stamp or the wrong one, it is not compliant, regardless of how it looks. Treatment certificates and the brand together are the evidence.
How it relates to AS 5604
AS 1604.1 is about added preservative: timber treated with a chemical to reach a hazard class. AS 5604 is about natural durability: the inherent decay and termite resistance of the species itself (a class 1 natural-durability hardwood may not need treatment for some uses). The two work together, you reach the required durability either by choosing a naturally durable species (AS 5604) or by treating timber to the right hazard class (AS 1604.1), or both.
For a builder
- Match the H-class to the worst exposure. Framing inside the envelope is H2; anything outside above ground is H3; anything in the ground or continually wet is H4 or higher. Do not under-spec.
- Mind the LOSP-versus-water-based difference. If you need dimensional stability (dressed cladding, framing going straight up), LOSP-treated stays put; water-based treated timber should be re-dried first.
- Check the brand on delivery. Confirm the H-stamp matches the order before it goes up; an unstamped or wrong-class piece is a delivery to reject.
- Reseal cut ends. Cutting or drilling treated timber exposes untreated core, especially in envelope (shell) treatments; re-coat cut ends and holes with an approved end-sealer so the protection stays continuous.
- Remember it is not a termite barrier on its own. H2 framing helps, but the building still needs its AS 3660 termite management system.
Related
- Treated pine
- AS 5604: timber natural durability
- AS 3660 termite management
- Timber framing basics
- Decks (residential)
See also
Last updated: 2026-05-25. Verified: 2026-05-25. Quarterly review for currency.