What is hardwood?
Hardwood is timber cut from broadleaved trees. In Australia that mostly means eucalypts and their close relatives: species such as Ironbark, Spotted Gum, Blackbutt and Forest Red Gum. The name has nothing to do with a hardness test. It is a botanical grouping, which is why hardwoods vary widely in density, hardness, colour and durability from species to species. That variation is exactly why durability classes exist: they give builders and homeowners a consistent way to compare how well the heartwood of different species resists decay.
Sydney Timber Supply sells a range of Australian hardwoods alongside imported Merbau and structural pine. This guide explains the durability system those timbers are classified under, and just as importantly, what the system does not tell you.
What natural durability means
Natural durability is the inherent resistance of a species' heartwood to fungal decay and, separately, to termite attack, without any preservative treatment. It is a property of the timber itself, not of a coating or chemical process. In Australia, natural durability is classified under AS 5604, which groups species into four classes for above-ground use and four classes for in-ground use, each with an indicative service life range.
Two points matter here. First, the classification applies to heartwood; sapwood of almost all species is not durable. Second, the ranges are indicative classifications, not guarantees. A Class 1 board installed with poor drainage and no ventilation can fail early, while a well-detailed, well-maintained deck routinely outperforms expectations.
The difference between Class 1 and Class 2
Class 1 is the highest natural durability rating. For above-ground use, AS 5604 gives Class 1 an indicative service life of more than 40 years and Class 2 an indicative range of 15 to 40 years. Both are genuine outdoor-grade classifications. The difference is the margin of decay resistance the heartwood provides, which matters most in harsh, wet or poorly ventilated situations.
In our range, Ironbark and Spotted Gum carry supplier-documented Class 1 above-ground durability and are displayed as QLD Class 1 Hardwood. Blackbutt and Redgum are also supplier-documented as Class 1 above ground. Jambu, Jambu Mixed and Merbau are supplier-documented as Class 2 above ground. For a focused comparison, see our Class 1 vs Class 2 guide.
Above-ground vs in-ground ratings
AS 5604 publishes two separate classifications for each species because the exposure is completely different. Above-ground ratings cover applications such as decking, screening, cladding and battens, where the timber can dry out between wettings. In-ground ratings cover posts and other members in direct soil contact, where moisture is constant and decay pressure is far higher. The same species always has a shorter indicative life in ground than above it.
A decking board only ever needs its above-ground rating. Quoting an in-ground figure for a deck, or an above-ground figure for a fence post, misrepresents the timber. Every durability figure on our site states which rating it is.
Why durability class matters
Decking is a long-term investment. The board you choose is fixed down, oiled and walked on for decades, so the decay resistance of the species sets the ceiling on how long the deck can serve, assuming sound installation and maintenance. Durability class is the single most useful like-for-like comparison between species when longevity is the priority, particularly for exposed decks, coastal sites and areas that stay damp.
What durability class does not tell you
Durability class answers one question only: how well the heartwood resists decay. It does not tell you:
- Strength. Load-bearing capacity is described by strength groups (for example SD ratings for seasoned Australian hardwoods), not by durability class.
- Hardness. Resistance to denting and wear is measured by the Janka test, a separate property again.
- Appearance. Colour, grain and figure vary by species and by board, and have no relationship to durability class.
- Termite resistance. This is assessed separately from decay resistance, even though both fall under natural durability.
- Maintenance needs. No durability class removes the need for cleaning and re-coating. Every hardwood deck needs ongoing care.
Durability vs strength
Strength and durability are independent properties. Within our own range, Grey Ironbark carries a supplier-documented SD1 seasoned strength group (Very High) while Redgum, an equally Class 1 rated species per supplier documentation, is SD4 seasoned (Medium High). Both resist decay to the same classification, but they are not structurally interchangeable. For structural members, always work from the strength group and your project engineering, not the durability class.
Durability vs hardness
Hardness predicts how a deck surface handles furniture, claws and foot traffic, not how it handles decay. Supplier-documented Janka values in our range illustrate the spread: Grey Ironbark at Janka 16 and Blackbutt at Janka 8.9 are both Class 1 above ground per supplier documentation, yet one is nearly twice as hard as the other. A softer Class 1 species will still resist rot; it will simply show dents sooner.
Maintenance and exposure
Durability class assumes the timber gets a fair chance: adequate ventilation gaps between boards, ground clearance so the deck can dry, correct fixings, sealed cut ends and a maintained finish. Exposure conditions such as full sun, salt spray, pooling water and shaded damp corners all shift real-world performance. Cleaning the deck and re-coating with a penetrating decking oil on a regular cycle is part of ownership for every species, Class 1 included. Our supplier's installation guidelines, linked on each product page, set out clearances and fixing practice in detail.
AS 5604 indicative durability classes
| Class | General durability level | Above-ground indicative range | In-ground indicative range | Typical suitability | Key limitations |
|---|---|---|---|---|---|
| Class 1 | Highest natural durability | More than 40 years | More than 25 years | Exposed decking, screening and demanding outdoor work where maximum service life is wanted | Still requires correct installation, drainage and maintenance; rating applies to heartwood only |
| Class 2 | High natural durability | 15 to 40 years | 15 to 25 years | Decking, fencing and general outdoor structures | Less margin than Class 1 in severe or persistently damp exposures |
| Class 3 | Moderate natural durability | 7 to 15 years | 5 to 15 years | Sheltered or well-detailed outdoor use; protected applications | Needs careful detailing and protection in exposed positions |
| Class 4 | Low natural durability | Up to 7 years | Up to 5 years | Internal or fully protected use, or treated before outdoor use | Not suited to unprotected exterior exposure without preservative treatment |
Service life ranges are indicative classifications under AS 5604, not product warranties. Actual performance depends on species, installation, exposure, drainage, maintenance and site conditions.
Class 1 vs Class 2 at a glance
| Class 1 | Class 2 | |
|---|---|---|
| Above-ground indicative range (AS 5604) | More than 40 years | 15 to 40 years |
| Outdoor decking suitability | Yes | Yes |
| STS examples (supplier documented) | Ironbark, Spotted Gum, Blackbutt, Redgum | Jambu, Jambu Mixed, Merbau |
| Typical reason to choose | Maximum decay-resistance margin for exposed or long-horizon projects | Sound outdoor performance, often at a more accessible price point |
The Sydney Timber Supply hardwood range
Our decking range covers both classes so you can match the timber to the project and the budget:
- Ironbark: supplier-documented Class 1 above ground, displayed as QLD Class 1 Hardwood. One of the densest decking hardwoods on the market.
- Spotted Gum: supplier-documented Class 1 above ground, displayed as QLD Class 1 Hardwood. Widely available with distinctive gum-vein figure.
- Blackbutt: supplier-documented Class 1 above ground. Straight grained with creamy blonde to honey tones.
- Redgum (Forest Red Gum): supplier-documented Class 1 above ground. Rich cherry to burgundy tones.
- Gympie Messmate: durability classification not currently published on our product pages.
- Jambu and Jambu Mixed: supplier-documented Class 2 above ground, 90 x 19 mm decking by quote.
- Merbau: imported hardwood, supplier-listed Class 2.
All decking is currently supplied by quote. Use the timber calculator to work out your linear metres, then request a quote with your quantities.
Frequently asked questions
Is Class 1 hardwood guaranteed to last 40 years?
No. The 40+ year figure is an indicative classification under AS 5604 for above-ground use, not a warranty. Real service life depends on installation quality, drainage, ventilation, exposure and maintenance as much as on the species.
Is Class 2 hardwood good enough for a deck?
Yes, Class 2 is a genuine outdoor classification with an indicative above-ground range of 15 to 40 years under AS 5604. Merbau, a supplier-listed Class 2 timber, is the most widely stocked decking hardwood in the Sydney retail market. See our Class 1 vs Class 2 comparison.
Does Class 1 mean the timber is harder than Class 2?
Not necessarily. Durability class measures decay resistance; hardness is a separate Janka measurement. Some Class 1 species are softer than some Class 2 species, and vice versa.
Does a Class 1 deck still need oiling?
Yes. Every hardwood deck needs cleaning and re-coating with a suitable decking oil on a cycle matched to its exposure and traffic. Durability class does not remove maintenance.
Why do above-ground and in-ground ratings differ?
Constant soil moisture puts far more decay pressure on timber than above-ground exposure, where boards can dry between wettings. AS 5604 therefore classifies the two situations separately, and the in-ground indicative ranges are always shorter.
What does QLD Class 1 Hardwood mean on your product pages?
It is the display wording we use for Ironbark and Spotted Gum, both supplied as Queensland-sourced hardwoods with supplier-documented Class 1 above-ground durability. The underlying classification is the same AS 5604 above-ground Class 1 described on this page.
Where can I check the durability of a specific product?
Each product page states its supplier-documented durability where published, and our species page summarises the range. If a value is not shown, it is not currently published and we will confirm what the supplier documents at quote stage.
Reviewed by the Sydney Timber Supply team. Last updated 19 July 2026.