H3 treated pine is for timber kept off the ground but exposed to weather or periodic wetting. H4 is for timber in contact with the ground or continually damp. H5 goes further, for ground or fresh-water contact. A higher class also suits the conditions of the lower ones. The class is part of the treatment brand on the timber, so you can check it before you build.
The hazard classes
From Business Queensland's summary of AS 1604:
| Class | Conditions | Protects against |
|---|---|---|
| H1 | Off the ground, well ventilated, completely protected from weather and wetting | Insects other than termites |
| H2 | As H1 | Insects including termites |
| H3 | Off the ground but exposed to weather or periodic wetting | Insects including termites, and decay |
| H4 | In contact with the ground or continually damp | Insects including termites, and severe decay |
| H5 | In contact with the ground or fresh water | Insects including termites, and very severe decay |
| H6 | Prolonged contact with sea water | Marine borers and very severe decay |
H4 or H5 in the ground?
WoodSolutions guidance separates them by how critical the element is. H4 is for in-ground, non-critical uses such as landscaping and fence posts; for structural elements in ground contact it points to H5 or a Class 1 durable hardwood (WoodSolutions, preservative treatments and use classes). Your designer or engineer decides which applies to your job.
Reading the brand
Business Queensland's summary says treated wood offered for sale should be preserved to the Standard and branded with a registered brand. The brand must show, in letters and figures at least 4 mm high:
- the treatment plant registration number;
- the preservative identification number;
- the H class.
Pieces thicker than 15 mm are branded with a hammer or burn brand, a stamp or a sticker. For smaller sizes, the brand may be on a label on the piece, bundle or wrapping, or on the delivery docket. Some items, such as battens and palings and pieces of small cross-section, may be exempt from branding.
What we list
We list H3 treated pine framing, plus untreated pine for dry internal work; see choosing an H3 treated pine framing size. We do not list H4 or H5 pine. For ground contact, tell us what your plans call for through request a quote. For deck framing, see deck subframes.
FAQ
Can I use H3 for a fence post?
No. A post in the ground is ground contact, which is H4 or above.
Is H4 fine for structural posts in the ground?
WoodSolutions points to H5 or a Class 1 durable hardwood for structural elements in ground contact. Follow your designer.
Can I use H3 inside?
Yes. A higher class suits the conditions of the lower classes, though untreated or H2 timber is the usual choice for dry internal framing.
Sydney Timber Supply team
Hazard class and branding references from Business Queensland and WoodSolutions. Reviewed October 2026.