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H1 to H6 timber hazard classes explained
What each hazard class means, where treated timber can safely go, and how the Australian system compares with Europe and the US.
Sterling Trading Ventures · Updated October 2026
Every piece of treated timber sold in Australia carries a hazard class, from H1 to H6. The number tells you where the timber can safely go: inside a dry house, out on a deck, in the ground, or in sea water.
Pick a class too low and the timber can rot or be eaten by termites within a few years. Pick one higher than you need and you pay more for treatment you don’t use. This guide explains each class, how to choose, and how the Australian system compares with Europe and the US.
In short
Hazard classes run from H1 (indoors, dry, no termite risk) to H6 (in sea water). The higher the number, the harsher the exposure and the more preservative the timber carries.
- H1 and H2 are for inside, protected from the weather.
- H3 is for outside, above the ground.
- H4 and H5 are for timber in the ground, with H5 for critical or heavy-duty uses.
- H6 is for marine use, in salt water.
In Australia, hazard classes are set by AS 1604.1, the standard for preservative-treated sawn and round timber. New Zealand uses its own standard, NZS 3640, with similar but not identical class names.
The six hazard classes at a glance
| Class | Where it goes | Main threat | Typical uses |
|---|---|---|---|
| H1 | Inside, above ground, dry and well ventilated, no termite risk | Borers (insects other than termites) | Interior joinery, furniture, linings |
| H2 | Inside, above ground, protected from the weather | Borers and termites | House framing, roof trusses, floor joists |
| H3 | Outside, above ground, rained on but able to dry out | Decay and termites | Decking, pergolas, fascias, cladding, fence palings |
| H4 | Outside, in the ground or constantly damp | Severe decay and termites | Fence posts, garden sleepers, landscaping |
| H5 | In the ground or fresh water, heavy-duty or critical | Very severe decay and termites | Retaining walls, house stumps, building poles, piles |
| H6 | In sea water | Marine borers and decay | Jetty and wharf piles, sea walls, marine bracing |
Each class explained
H1: inside and dry
H1 timber is used indoors, above the ground, where it stays dry and well ventilated and there is no termite risk. The only threat is borers, small beetles that attack the sapwood of some hardwoods. H1 is mainly used for joinery, mouldings and furniture.
H2: inside, with termites about
H2 is for the same dry, indoor conditions as H1, but where termites may be present. Most treated pine house framing sold in Australia is H2.
You may also see H2F and H2S. These are lighter treatments, often a surface or glue-line treatment, approved for framing only in areas south of the Tropic of Capricorn.
H3: outside, off the ground
H3 is the most common outdoor class. It suits timber that gets rained on but sits clear of the ground and can dry out between wettings. Decks, pergolas, handrails, cladding and fence palings are typical H3 uses.
H4: in the ground
H4 is for timber in contact with the soil, or so close to it that it stays damp. Fence posts, garden edging and landscape sleepers are H4. It suits uses where a failure would be an inconvenience rather than a safety risk.
H5: in the ground, where failure matters
H5 carries more preservative than H4. It is used where timber sits in wet ground or fresh water, or where the structure must not fail. Retaining walls, house stumps, building poles and piles are typically H5.
H6: in sea water
H6 is the highest class. It is for timber permanently or regularly in salt water, where marine borers can destroy untreated timber within a year or two. Marine piles, jetty timbers and sea walls are H6.
How do you choose the right hazard class?
Start with where the timber will sit, then ask how bad a failure would be. These rules cover most jobs:
- Inside and dry: H1 if there is no termite risk, H2 if there is.
- Outside, above ground, able to dry: H3.
- Within about 400 mm of the ground, or somewhere that stays damp: go up to H4. A deck frame low to the ground is a common example.
- In the ground: H4 for fences and landscaping, H5 for anything structural.
- In sea water: H6.
When in doubt, go one class higher. The extra cost is small compared with replacing a deck frame or retaining wall that rotted early.
What about cut ends and drilled holes?
Preservative does not always reach the core of a thick section. When you cut, notch or drill treated timber, seal the exposed wood with a brush-on preservative.
Avoid cutting the end of a post that goes into the ground, as that end needs the most protection.
Do you need treated timber at all?
Not always. Some hardwoods are naturally durable without treatment. Australia rates them by durability class under AS 5604, where Class 1 is the most durable.
Class 1 heartwood, such as ironbark, spotted gum, azobé, okan and tali, can often be used outdoors and even in the ground without treatment. The sapwood of these species is not durable, though, so it needs treating or cutting away.
See our Australian hardwood and African hardwoods pages for durability ratings by species.
Which preservatives are used for each class?
The hazard class sets how much protection the timber needs. The preservative is how that protection gets in. The three most common for outdoor and in-ground timber are:
- CCA (copper chrome arsenate): the long-standing choice for poles, piles and marine timber, used up to H6. In Australia it is not permitted for garden furniture, picnic tables, children’s play equipment, domestic decking boards or handrails.
- ACQ (alkaline copper quaternary): arsenic free, commonly used for H3 and H4 decking, handrails and landscaping.
- Copper azole, including Tanalith E: also arsenic free, widely used for H3 and H4 decking, framing and fencing.
Which preservative can reach which class depends on the registered product and its retention. The treatment certificate for each consignment should state both. Our treated timber H3 to H6 page covers the treatments we supply.
How do H classes compare with Europe and the US?
Other countries use different systems. Europe uses use classes under EN 335, and the US uses use categories set by the AWPA. The nearest matches by exposure are:
| Australia (AS 1604.1) | Europe (EN 335) | United States (AWPA) | Exposure |
|---|---|---|---|
| H1 | Use Class 1 | UC1 | Inside, dry |
| H2 | Use Class 1 to 2 | UC2 | Inside, termite risk |
| H3 | Use Class 3.2 | UC3B | Outside, above ground |
| H4 | Use Class 4 | UC4A | In ground, general |
| H5 | Use Class 4 (severe) | UC4B to UC4C | In ground, heavy duty or critical |
| H6 | Use Class 5 | UC5A to UC5C | Marine, sea water |
These are approximate matches by exposure, not certified equivalents. Each system sets its own preservative retentions and test methods. If your specification uses EN or AWPA classes, send it to us with your quote request and we will match the treatment to it.
Tell us what you need. We reply within two working days.
Common questions
What does H3 mean on treated pine?
H3 means the timber is treated for outdoor use above the ground, where it gets wet but can dry out. It is the standard class for decking, pergolas and fascias. It should not be used in the ground.
Can I use H3 timber in the ground?
No. Timber in contact with soil needs at least H4, and H5 if it is structural, such as a retaining wall or a house stump.
What is the difference between H4 and H5?
Both are for in-ground use. H5 carries more preservative and is used where the timber is in wet ground or fresh water, or where a failure would be dangerous or expensive. Fence posts are usually H4. Retaining walls and building poles are usually H5.
How do I know what class my timber is?
Treated timber in Australia should be branded or tagged with its hazard class, the preservative and the treatment plant code. For imported or exported timber, the treatment certificate for the consignment should state the class and the retention.
Is treated timber safe to use?
Yes, when it is used for the purpose it was treated for. Wear a dust mask and gloves when cutting, do not burn offcuts, and check the restrictions on CCA-treated timber for decks, furniture and play equipment.
Need treated timber or poles?
Tell us how the timber will be used and where it is going. We will confirm the right hazard class and treatment before we quote.
We supply treated pine posts, poles, sleepers and marine timber from Australia and New Zealand, and treated eucalyptus poles from South Africa, shipped by the container to any port.
Related guide: Choosing a decking timber: seven species compared
Products in this guide: Treated timber H3 to H6 · Sleepers and poles · Eucalyptus poles · Radiata pine · All guides
Hazard class descriptions follow AS 1604.1 as summarised by WoodSolutions and the Queensland timber database. Check the current standard and your local building authority for project-specific requirements.