AS/NZS 1604: timber preservative treatment standard
AS/NZS 1604 is the timber preservative-treatment standard. H1 through H6 hazard classes set chemical type and retention for each exposure: CCA, ACQ, copper azole, LOSP.
Ask Chalkline about this →AS/NZS 1604 is the multi-part joint Australian/New Zealand standard for timber preservative treatment. It defines the chemical types, retention levels, penetration depth, and test methods for treated timber, classified by H1 through H6 hazard classes matching the timber’s exposure environment. Called up by AS 1684 (timber framing), AS 1720 (timber structures), and the NCC Housing Provisions for the structural timber components of every Class 1 build.
The H hazard classes
| Class | Exposure | Typical chemical |
|---|---|---|
| H1 | Inside, dry, fully protected (e.g. internal framing) | Light borer protection |
| H2 | Inside, above ground, protected from weather | Light boron / synthetic pyrethroid |
| H3 | Outside, above ground (cladding, fascia, pergola structure) | LOSP, ACQ, CCA, copper azole |
| H4 | Outside, in-ground (fence posts, garden edging) | ACQ, copper azole, CCA |
| H5 | Outside, in-ground, critical (retaining walls, marine piles) | ACQ, copper azole, CCA at higher retention |
| H6 | Marine, splash zone | Heavy-duty CCA or specialist marine treatments |
The class drives the chemical formulation AND the retention rate. H3 ACQ and H4 ACQ are not the same chemical loading; the treatment plant runs different cycles.
The standard’s parts
- AS/NZS 1604.1: Sawn and round timber (the volume residential standard).
- AS/NZS 1604.2: Round and quarter-split posts (fence posts and similar).
- AS/NZS 1604.3: Plywood.
- AS/NZS 1604.4: Laminated veneer lumber (LVL).
- AS/NZS 1604.5: Glued-laminated timber (Glulam).
Each part sets the test method and the retention table for its product type.
How it shows up on site
- The grade stamp on every piece of compliantly treated timber names the standard (AS/NZS 1604.1), the hazard class (e.g. H3), and the chemical (e.g. CCA, ACQ).
- The mill delivery docket carries the full treatment lot details.
- BASIX and other state planning instruments sometimes specify minimum hazard class for visible external timber (e.g. cladding battens must be H3 minimum).
A piece of timber on site without a treatment stamp = not certified treated. Don’t accept it on the structural-timber delivery.
For a builder
- Match the class to the exposure. H2-treated pine framing inside a wall is fine; H2 used as a fence post will rot inside two years. The class is in the spec for a reason.
- Read the grade stamp on every load. Mill ink-jet stamps fade; ensure the class and standard reference are legible at delivery.
- Don’t cut and “re-treat” on site with brush-on. A site-applied brush coat does not restore the AS 1604 retention; cut ends should be specified to be brush-treated per the manufacturer’s data sheet, and treated timber should be cut to fit at delivery if possible.
- Disposal: CCA-treated timber must not be burned (releases arsenic in smoke); landfill only.
Related
See also
Last updated: 2026-05-29. Verified: 2026-05-29. Quarterly review for currency.