H3 timber: what the rating covers and what it leaves out
H3 timber is treated for outdoor use above the ground. That is a narrower promise than most people assume.
What H3 timber is
H3 is a hazard class for treated timber in Australia, set under AS 1604. It marks timber treated for use outside and above the ground, where it gets wet from rain but can dry out again. Think pergola posts clear of the soil, fascias, decking boards and exterior cladding.
Most H3 pine is treated with a preservative that soaks into the sapwood. The label on the end of the board or the tag stapled to the pack tells you the class and the treatment.
Where H3 is the right choice
| Use | H3 suitable? |
|---|---|
| Decking boards, pergola beams, balustrades | Yes, above ground and able to dry |
| Exterior wall framing and cladding | Yes, where the design calls for it |
| Posts set in or touching soil | No. This is H4 territory |
| Dry interior framing | Fine, but H2 is the class designed for it |
If you are comparing the classes side by side, H3 vs H4 timber sets them out in more detail.
Does H3 timber stop termites?
It helps, but it is not a termite barrier. The treatment makes the treated sapwood unattractive or toxic to termites and borers, which slows damage to that board. It does nothing for the untreated timber next to it, and it does not stop a colony building a mud tube over it to reach something else.
That is why a house relies on a termite management system under AS 3660 and regular inspection, with treated timber as a supporting layer. Termite treated timber covers what the label actually promises.
The cut-end problem
Treatment often does not reach the heartwood, and a saw cut exposes it. Every cut end, drilled hole or notched edge is a small patch of untreated timber. Brush the cut surface with a preservative made for the job and follow the instructions on the tin, as the standard expects. Skipping this is the commonest way treated timber fails early.
Mistakes that undo the rating
- Burying H3 in the ground. It stays wet and decays, and decayed timber is easier for termites to get into.
- Piling soil or mulch against it. This hides the timber from view and keeps it damp.
- Treating the class as a guarantee. H3 says where the timber may be used, not that the house is protected.
What to do if you find damage in H3 timber
Treated timber can still be attacked, especially at cut ends and where it meets untreated framing. If you see mud, frass or timber that sounds hollow, do not spray or break it open. Leave it as it is and book a licensed timber pest inspection. Any treatment is for a licensed pest management technician using APVMA registered products. This site does not certify any product or brand.
Frequently asked questions
What is H3 timber used for?
H3 is treated for outdoor use above the ground, such as decking, pergolas, fascias and exterior cladding, where the timber is wet at times but can dry out.
Can I use H3 timber in the ground?
No. Timber in contact with the ground needs a higher class, normally H4 or above. Check the requirement in AS 1604 or ask your builder.
Is H3 timber termite proof?
No. The treatment slows damage in that board, but it does not protect the rest of the house. A termite management system and regular inspections do that job.
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.
- Department of Health, Victoria. Pest control - termites
- Queensland Building and Construction Commission. Protect against termites - termite management systems and inspections
- Standards Australia. Australian Standards (AS 3660 termite management, AS 4349.3 timber pest inspection)