H4 timber: what the rating covers and what it leaves out
H4 timber is treated for ground contact. That does not make it a termite barrier for the house next to it.
What H4 timber is
H4 is a hazard class for treated timber in Australia, set under AS 1604. It marks timber for use in contact with the ground or in other places where it stays damp for long periods. Think fence posts, retaining wall sleepers and garden edging.
It carries a heavier treatment than H3 because ground contact exposes timber to rot and termites together. The label on the pack or the tag on the end of the board tells you the class.
Where H4 is the right choice
| Use | H4 suitable? |
|---|---|
| Fence posts set in soil | Yes |
| Retaining walls and garden sleepers | Yes, for light residential use |
| Pergola posts on footings clear of soil | H3 is usually enough, check the design |
| Structural posts carrying a building | Ask your engineer, the class may need to be higher |
For the class below it, see H3 timber, and for the side by side, H3 vs H4 timber.
Does H4 timber stop termites?
It slows them in that post, but it is not a termite barrier. Untreated timber nearby is still food, and a fence or retaining wall touching the house can become a bridge to it.
Keep ground-contact timber away from the walls and subfloor vents. A termite management system under AS 3660 and regular inspection protect the house. Termite treated timber covers what the label promises.
The cut-end problem
Treatment often does not reach the heartwood, and a saw cut exposes it. Brush every cut end and drilled hole with a preservative made for the job, as the instructions on the tin direct. A post cut to height and set in the ground is a common early failure.
Mistakes that undo the rating
- Using H4 where a higher class is specified. Some in-ground structural uses need more than H4.
- Letting H4 timber touch the house. A garden bed or deck built hard against the wall hides the join and gives termites a path.
- Treating the class as a guarantee. H4 says where the timber may be used, not that the house is protected.
What to do if you find damage in H4 timber
If you see mud tubes, frass or a post 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 H4 timber used for?
H4 is treated for ground contact and long damp exposure, such as fence posts, garden sleepers and light retaining walls.
Is H4 timber the same as H3?
No. H4 carries a heavier treatment for use in the ground. H3 is for outdoor use above the ground.
Is H4 timber termite proof?
No. The treatment slows damage in that post, 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)