H3 vs H4 timber: what the hazard class number really tells you
The H number tells you where a piece of timber may be used. It does not tell you the house is safe from termites.
The class is about the use, not the quality
Treated timber in Australia carries a hazard class, written H1 to H6. The class describes the conditions the timber will face and the level of treatment it needs to last there. It comes from Australian Standard AS 1604, which sets the treatment requirements.
So a higher number is not "better timber". It is timber treated for a harsher situation.
H3 versus H4 in plain terms
| Class | Typical use | Main hazard |
|---|---|---|
| H2 | Inside, above ground and dry | Borers and termites |
| H3 | Outside, above ground, such as decking and pergolas | Weather, moisture and decay as well |
| H4 | Outside, in contact with the ground, such as fence posts | Constant damp, severe decay |
The step from H3 to H4 is mostly about ground contact. Timber that sits in soil stays wet, so it needs a heavier treatment than a deck board that dries between showers.
Does a higher class stop termites?
Not on its own. Hazard classes were built mainly around decay and insect attack in the board itself. Treatment slows the damage in that piece, which is useful, but a colony can still reach the rest of the house by another route.
Cut ends, drilled holes and notches also matter. A saw cut exposes untreated heartwood, which is why treated timber is protected again where it is cut. Termite treated timber explains what the label really promises.
Should you pay for the higher class?
Use the class the standard says matches the job, and no more. Buying H4 for a dry interior frame adds nothing you will notice. Using H3 where timber meets soil is the real mistake, because it will decay early and invite trouble.
If you are unsure which class a job needs, ask the supplier to show you the requirement in AS 1604 or ask your builder. This site does not certify any product or brand.
Where termite protection really comes from
A house is protected by a termite management system and by regular inspection, with timber choice as a supporting layer. Do termites eat treated pine? looks at what treatment does and does not do.
If you find signs of termites in any timber, treated or not, do not spray or disturb it. Leave it alone and book a licensed timber pest inspection.
H4 vs H3 treated pine: which one for which job
Match the class to where the timber will sit. If it touches soil, you need H4; if it stays above the ground outdoors, H3 does the job.
- Deck boards and pergolas above the ground: H3.
- Fence posts and garden sleepers in the soil: H4.
- Structural piers and poles in heavy ground contact or wet ground: look at H5 treated pine, and let the design or your engineer decide.
- A dry interior frame: H4 adds nothing you will notice.
The common mistake runs the other way: H3 where the timber meets soil. It decays early and invites trouble. Whatever the class, seal every cut end and drilled hole, because the cut exposes untreated wood. If you are unsure, ask the supplier to show you the requirement in AS 1604, or ask your builder. More detail on the ground-contact class sits in H4 timber.
Frequently asked questions
What is the difference between H3 and H4 timber?
H3 is treated for outdoor use above ground, like decking. H4 is treated for outdoor use in contact with the ground, like fence posts. The class is set by the conditions of use under AS 1604.
Is H4 timber termite-proof?
No. A higher class means a heavier treatment for a harsher setting, which slows damage in that board. It does not make the house safe, which is the job of a termite management system and inspection.
Can I use H3 timber in the ground?
It is not the right class for ground contact. Check the requirement in AS 1604 or ask your builder, because under-treated timber in soil tends to decay early.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- Standards Australia. Australian Standards (AS 3660 termite management, AS 4349.3 timber pest inspection)
- Queensland Building and Construction Commission. Protect against termites — termite management systems and inspections