The short answer
Class 1 and Class 2 are the top two natural durability ratings under AS 5604, describing how well a species' heartwood resists decay without treatment. For above-ground use such as decking, Class 1 carries an indicative service range of more than 40 years and Class 2 a range of 15 to 40 years. Both are genuine outdoor classifications; Class 1 simply provides a larger decay-resistance margin, which matters most in harsh, damp or long-horizon applications.
Class 1 vs Class 2 compared
| Class 1 | Class 2 | |
|---|---|---|
| What it measures | Natural decay resistance of untreated heartwood | Natural decay resistance of untreated heartwood |
| Above-ground indicative range (AS 5604) | More than 40 years | 15 to 40 years |
| In-ground indicative range (AS 5604) | More than 25 years | 15 to 25 years |
| Suitable for decking | Yes | Yes |
| STS species (supplier documented, above ground) | Ironbark, Spotted Gum, Blackbutt, Redgum, Gympie Messmate | Jambu, Jambu Mixed, Merbau (140 x 19 and 90 x 19), Peninsula Blonde (140 x 19) |
| Says anything about strength or hardness? | No | No |
| Removes maintenance? | No | No |
Service life ranges are indicative classifications under AS 5604, not product warranties. Actual performance depends on species, installation, exposure, drainage, maintenance and site conditions.
When Class 2 is a sound choice
Class 2 hardwood is not a compromise timber; it is the workhorse of Australian decking. Merbau, supplier-listed as Class 2, is the most widely stocked decking hardwood in the Sydney retail market precisely because it balances outdoor durability, stability and price. A Class 2 species is a sound choice when:
- The deck is well detailed, with good ventilation, drainage and ground clearance, so the timber gets every chance to dry.
- Budget matters and the indicative 15 to 40 year above-ground range comfortably covers the life you plan for the structure.
- You want a stable, easy-working board for a straightforward residential deck, steps or fencing.
Class 1 earns its premium where exposure is severe, where the deck must outlast everything around it, or where replacement would be disruptive and costly.
Species examples from our range
On the Class 1 side, Ironbark and Spotted Gum carry supplier-documented Class 1 above-ground durability and are displayed as QLD Class 1 Hardwood on our product pages. Blackbutt and Redgum are also supplier-documented as Class 1 above ground.
On the Class 2 side, Jambu and Jambu Mixed are supplier-documented as Class 2 above ground and supplied as 90 x 19 mm decking, Merbau is an imported hardwood with supplier-listed Class 2 durability in 90 x 19 and 140 x 19 mm profiles, and Peninsula Blonde is an imported 140 x 19 mm board with supplier-documented Class 2 above-ground durability.
Remember that class only describes decay resistance. Strength groups and Janka hardness are separate properties; our hardwood durability guide walks through the distinctions, and our species comparison table puts the supplier-documented numbers side by side.
Choosing for your project
Work out your quantities first with the timber calculator. Linear metre prices are shown on every product page, so you can compare a Class 1 and a Class 2 option on real numbers for your deck and add your metres straight to the cart, or request a quote for pack quantities and trade volumes.
Reviewed by the Sydney Timber Supply team. Last updated 25 August 2026.