Timber Treatment Compliance for Australian Residential Builds: AS 1604 Hazard Levels

What it is

AS/NZS 1604 is the Standards Australia series for preservative-treated wood-based products. It sets six hazard classes, H1 through H6, that define what biological threats a piece of treated timber can resist. Each class specifies the preservative type, retention level and penetration depth required for treatment.

The hazard class is not about strength or grade. It is about exposure. A piece of F17 hardwood and a piece of MGP10 pine can carry the same hazard class if they are treated to the same level. The class tells you whether the timber can survive in the location you want to install it.

Treatment is done in a treatment plant under pressure or by other approved methods. The treated timber is then branded or tagged with the hazard class, treatment type and the treatment plant identifier. That brand is the builder's evidence the timber is fit for the application.

Why it matters for residential builders

Australian homes deal with termites, fungal decay and a wet to dry cycle that punishes anything that is not protected. A timber framing failure caused by using the wrong hazard class can take years to surface and almost always lands during the statutory warranty period. The builder is then back on site cutting open walls, drying subfloors and replacing posts.

State building regulators and home warranty insurers treat the use of correctly treated timber as a baseline compliance item. NCC Volume Two and AS 1684.2 reference the treatment levels required for structural framing in termite risk areas. The NCC also points to AS 3660.1 for termite management of new buildings, which interacts directly with the hazard class chosen for any framing not separated from the ground.

Hazard classes explained

The six classes step up by exposure and biological threat.

H1: borers only

H1 is for timber fully protected from weather and wetting and used inside, where the only realistic threat is wood-boring insects like Lyctus borer. It uses a light surface treatment. H1 is rarely specified in residential framing today.

H2: termites, inside the building envelope

H2 is the workhorse for internal framing in termite-risk areas. It protects against subterranean termites for timber that stays inside the building, fully protected from weather. There are two variants. H2-S is for use south of approximately the Tropic of Capricorn and uses a permethrin-based treatment. H2-F is for use anywhere in Australia and uses a different chemical envelope.

Most pine wall studs, plates and roof framing in southern Australia are H2 treated. The treatment is light and lets the timber be worked with ordinary fasteners.

H3: outside, above ground

H3 is the minimum for timber that can get wet but can also dry out, such as fascia, bargeboards, exposed pergola members, weatherboards and decking joists not in ground contact. It protects against moderate decay and termites.

H4: in ground contact

H4 is required for fence posts, retaining sleepers, garden bed edging and any structural timber in the ground or in continuously damp conditions. It handles severe decay and termite attack.

H5: heavy in-ground

H5 is for structural timber in ground contact in critical applications, such as house stumps, piles and posts holding up structures where failure would be a serious problem.

H6: marine

H6 is for timber in seawater, exposed to marine borers. Most residential builders will never specify H6.

Where each class belongs on a residential job

A working rule for residential framing in southern Australia is H2 inside the envelope, H3 outside above ground and H4 or H5 in or near the ground. The application of this rule depends on the state termite management plan and any local council requirements.

In the Northern Territory, far north Queensland and parts of WA, the termite pressure is higher and authorities may require H3 framing where H2 would suffice further south. Always check the NCC, the state termite management requirements and the engineer's documentation before ordering.

Durability classifications

Hazard class is about treatment. Durability class is about the natural decay resistance of the species. AS 5604 lists species durability ratings for above-ground and in-ground exposure. A naturally durable hardwood like grey ironbark has high in-ground durability with no treatment. A pine has poor natural durability and needs treatment to perform in the same location.

Builders sometimes use durability class as a substitute for treatment when species and section size allow it. The trade-off is cost, availability and weight. A naturally durable hardwood post costs more than a treated softwood post and is harder to fix.

What builders should check

Three checks catch most problems before they hit the job.

Check the brand

Every piece of treated timber should carry a brand or tag with the hazard level, treatment type and plant identifier. If timber arrives without a brand, it should be rejected. Take a photo of the brand on a sample of each parcel and keep it with the job file.

Match the hazard class to the location

A common defect is H2 treated pine used as fascia or bargeboard. That is an H3 application. The pine will start to break down within five years. Order H3 for anything that can get wet.

Document the supply chain

Keep delivery dockets that list the hazard class. If a defect surfaces during the warranty period, you want to be able to show what was specified, what was supplied and what was installed. Without paperwork the dispute is just your word against the homeowner's.

Common defects

Some of the most common timber treatment defects on residential builds are H2 timber used in H3 locations, untreated framing in termite zones with no termite management system, and H4 sleepers used where H5 stumps are required by the engineer. Each of these is easy to spot if someone is looking and almost impossible to find once the wall is sheeted and lined.

A builder who runs a treatment-level check at the framing inspection and keeps the brand photos on file has done what the standard expects.