Treated pine: hazard classes H1 to H6, chemicals, and where each lands

TL;DR

Treated pine is Radiata Pine (and some Slash Pine in QLD) impregnated with a chemical preservative under pressure to make it resist termites, decay, and weathering. The Australian standard is AS 1604.1:2021, which sets six hazard classes (H1 through H6) by exposure condition: H1 (indoor, weather-protected only), H2 (indoor framing in termite-risk areas), H3 (outdoor above-ground, weather-exposed), H4 (in-ground, retaining walls and posts), H5 (severe in-ground and freshwater contact), and H6 (saltwater marine). The four common chemical systems are CCA (copper-chromium-arsenic, restricted from some applications since 2006), ACQ (copper + DDAC quaternary ammonium), MCA (micronised copper azole, the modern volume default), and LOSP (light organic solvent preservative, for H2 and H3 only). The two job-killers: using H2-treated pine in an external above-ground location where it sees rain (rots within 5 years), and using H3 in direct ground contact (rots within 3 to 5 years). The hazard-class-to-application mapping is non-negotiable: AS 1684 specifies the minimum class for each location, and the certifier checks the timber stamp (verified 2026-05-13, Business Queensland Treated Timber).

What it is

Treated pine is structural softwood, almost universally Radiata Pine in Australian residential use (occasionally Slash Pine in tropical Queensland), with a preservative chemical impregnated into the timber via pressure treatment. The chemical changes the wood’s resistance to:

Termites: chemical toxic to or repellent to subterranean termites
Wood-decay fungi: chemical inhibits the fungi that cause rot
Wood-boring insects: chemical kills or repels Lyctus, Anobium, and other timber-boring beetles
Marine borers (H6 only): teredo and similar marine organisms

The treatment is mandatory under AS 1684 for any pine framing or structural timber in residential construction; untreated Radiata Pine is rarely used in Australia outside specific protected internal applications.

The hazard class determines:

What chemical is used and how much penetrates the timber (the retention specification)
How deep the chemical penetrates (typically full sapwood penetration, or heartwood penetration for higher classes)
Which exposures the treated timber can survive in service

The six hazard classes

Class	Exposure	Where used in residential
H1	Inside, weather-protected, dry; insect protection only	Furniture, cabinets, internal joinery (rarely structural)
H2	Inside, weather-protected, framing; termite protection in termite-risk areas	Wall studs, top plates, ceiling joists, roof rafters in termite-protected sites
H3	Outside above-ground; weather and decay exposed	Decking joists (sub-frame), pergola beams, cladding battens, fascia, fence palings
H4	In-ground or in-fresh-water contact	Garden retaining wall sleepers, in-ground posts, sleeper retaining walls
H5	Severe in-ground exposure, freshwater	Heavy retaining walls, bridge piles, foundation pads in soil contact
H6	Marine, saltwater immersion	Marine piles, jetty timbers (rare in residential)

The hazard class is stamped or branded on every piece of treated timber. Without the stamp, the timber cannot be used as treated; it’s been mishandled or is undocumented stock and must be rejected.

Chemical systems

Different chemicals deliver different hazard classes, and not every chemical is approved for every class.

Chemical	Hazard classes	Notes
CCA (copper-chromium-arsenic)	H3, H4, H5	The legacy default. Restricted from H1 and H2 (residential indoor) since 2006 due to arsenic concerns; still approved for H3 + outdoor applications and for some H4 and H5. Greenish-tint colour.
ACQ (alkaline copper quaternary)	H2, H3, H4	Copper + DDAC quaternary ammonium. Greenish-tint, similar to CCA visually. Common in residential framing
MCA (micronised copper azole)	H2, H3, H4	The modern residential default. Lighter green-brown colour; less chemical migration to surface; less chemical staining of fasteners
LOSP (light organic solvent preservative)	H2 and H3 only	Solvent-based, kerosene-carrier. Cures faster than waterborne treatments; produces stable, low-shrinkage finished timber. Common in southern Australia for H3 decking sub-frame. Not approved for H4
Boric	H1 and H2	Glassboard treatment; used for internal framing in non-termite areas (rare residential)
Creosote (coal-tar-based)	H4, H5, H6	Sleepers, rail ties, marine work. Strong smell; staining; restricted from residential aesthetic use

The chemical is the supplier’s choice within the hazard class. For most residential orders the spec is just “H3-treated Radiata Pine” and the supplier delivers what they stock (typically MCA or ACQ in 2026).

Where each hazard class goes

Location on a residential build	Hazard class required
Wall studs, top plate, internal framing (termite-risk areas)	H2
Wall studs, top plate, internal framing (non-termite areas, e.g. parts of Tasmania)	H1 sometimes acceptable; usually H2 specified anyway
Roof rafters, ceiling joists	H2
Decking joists, bearers (sub-frame to deck)	H3
Decking boards (face-fixing exposed)	Hardwood preferred (see decking-timbers); H3 treated pine acceptable for low-budget decks
Pergola rafters and beams (above-ground exterior)	H3
Fascia and barge boards	H3
Cladding battens	H3
Posts in concrete-encased footing, above ground	H3 (where the post sits on a galvanised post base above ground); H4 if any part is below ground
Retaining wall sleepers, garden walls, in-ground timber	H4
Heavy retaining walls, long-term in-ground structural	H5

The general principle: the timber must be treated to a class at or above the most severe exposure it will see in service. Under-class treatment is a defect; over-class is wasted cost.

Treatment retention and stamp inspection

AS 1604.1 specifies the minimum retention of preservative chemical (in kg/m3) for each class. The treatment process is:

Timber is dried to below 25% moisture content
Loaded into a pressure vessel
Vacuum to remove air from wood cells
Pressure treatment with the chemical solution
Vacuum to remove excess
Air-dried or kiln-dried to remove the carrier (water or LOSP solvent)

Each treated piece carries a brand showing:

Treatment hazard class (H2, H3, H4, etc.)
Chemical type (ACQ, MCA, CCA, LOSP)
Treater identification
Date or batch code

A piece without a brand is not treated for compliance purposes, regardless of any green tint or staining present.

Common defects and on-site issues

Wrong hazard class for application: H2 framing used externally; rots within 3 to 5 years. Inspect timber stamps before site delivery is accepted.
Cut ends not treated: pressure treatment penetrates the sapwood and partially the heartwood. Cut ends expose untreated heart. Use end-grain preservative (CCA paste, ACQ field treatment) on any cut end exposed to moisture or in-ground contact.
Holes drilled through treated timber for service penetrations: similar issue, the hole exposes untreated timber. Apply field-treatment chemical to the cut.
Galvanic corrosion with fasteners: copper-based treatments (CCA, ACQ, MCA) react with mild steel fasteners. Use hot-dip galvanised or stainless steel fasteners on any treated timber. Hot-dip galv minimum 600 g/m2 for most residential applications; stainless 304 or 316 for coastal.
Wet weather during treatment cure: timber rained on within hours of treatment may have surface chemical washed away. Reputable suppliers cover stacks during cure; receive supply from sheltered yard.
CCA in food and child contact areas: CCA-treated timber should not be in direct food or skin contact (children’s play sets, garden vegetable beds). Use ACQ or MCA for these applications since the early 2000s regulatory change.
LOSP-treated timber smell on internal applications: LOSP carrier (kerosene) takes weeks to fully off-gas. New LOSP-treated internal framing has a noticeable solvent smell at first; this is normal and dissipates.

MGP grades vs treatment

A confusion point: MGP (Machine Graded Pine, MGP10 / MGP12 / MGP15) is a stress grade, not a treatment. MGP grades describe the timber’s mechanical properties (bending stress capacity). The same MGP10 timber can be supplied as H2-treated, H3-treated, or untreated. A typical residential framing order is “MGP10 H2-treated 90 × 45”: MGP10 for the structural grade, H2 for the treatment level.

Pricing (2026 indicative, ex-GST, ex-Sydney metro merchant)

Section / treatment	Per linear metre
90 × 45 MGP10 H2 (wall stud, internal)	$5-8
90 × 45 MGP10 H3 (external use)	$7-10
140 × 45 MGP10 H2 (top plate, joist)	$9-13
200 × 50 H3 LOSP (decking joist)	$18-26
100 × 100 H4 ACQ (in-ground post)	$14-22
200 × 75 H4 (retaining wall sleeper)	$22-32
150 × 38 H5 (heavy in-ground structural)	$24-36
Field treatment paste (end-grain treatment)	$25-45 per 1 kg pack

H4 and H5 prices vary materially by region; QLD and NSW are typically cheapest, WA and SA carry freight premiums.

Standards and references

Standards Australia, AS 1604.1:2021 Specification for preservative treatment Part 1: Products and treatments. https://store.standards.org.au (verified 2026-05-13).
Business Queensland, Using treated timber. https://www.business.qld.gov.au/industries/building-property-development/building-construction/laws-codes-standards/using-timber/treated (verified 2026-05-13).
Queensland Department of Agriculture and Fisheries (QTimber), H codes: Hazard treatments H1 to H5. https://qtimber.daf.qld.gov.au/guides/h-codes-hazard-treatments-h1-h5 (verified 2026-05-13).
Standards Australia, AS 1684.4:2010 Residential timber-framed construction. https://store.standards.org.au (verified 2026-05-13).
Standards Australia, AS 3660.1:2014 Termite management Part 1: New building work. https://store.standards.org.au (verified 2026-05-13).
Australian Building Codes Board, NCC 2022 ABCB Housing Provisions (termite and timber durability references). https://ncc.abcb.gov.au/editions/ncc-2022/adopted/housing-provisions (verified 2026-05-13).