Class 1 is the highest natural above-ground durability class for timber. When you see a Class 1 badge on a Sydney Timber Supply hardwood, it means the supplier specification sheet for that species states Class 1 above-ground durability, typically alongside a 40+ year rating. Five species in the STS range carry it: Ironbark, Spotted Gum, Redgum, Gympie Messmate and Blackbutt. This article explains what the class actually tells you, why some popular decking timbers do not carry it, and how to read the 40+ year figure sensibly.
What durability classes measure
Natural durability classes describe how well a timber's heartwood resists decay in service without any preservative treatment, with Class 1 at the top of the scale. The ratings that matter for decking are the above-ground ratings, because a deck's boards live above ground: they get wet, they dry out, and they need to resist rot through that cycle on the strength of the timber itself.
Two points are easy to miss:
- It is a natural property. The class describes the species' own heartwood, not any treatment applied to it.
- Above ground is the context. An above-ground rating tells you about performance in applications like decking boards, not about posts buried in soil.
The STS species with supplier-published Class 1 ratings
STS shows the Class 1 badge only on hardwoods whose supplier specification sheets state it. Across the Australian hardwood range, that is these five species:
| Species | Above-ground durability | Supplier rating | Character notes (supplier-published) |
|---|---|---|---|
| Ironbark | Class 1 | 40+ years | Very hard, dense |
| Spotted Gum | Class 1 | 40+ years | Striking grain |
| Redgum (QLD Reds, Forest Red Gum) | Class 1 | 40+ years | Rich red tones |
| Gympie Messmate (QLD Blondes) | Class 1 | 40+ years | Warm yellow-brown |
| Blackbutt | Class 1 | 40+ years | Straight grained, even texture, creamy blonde to rich honey tones |
Blackbutt (Eucalyptus pilularis) is worth a special mention: its supplier flyer also lists a BAL 29 rating (AS3959-2009), which matters if you are building in a bushfire-prone area. You can read more about each species on our species guide.
Why Merbau and Jambu are not badged Class 1
Merbau is a stable, widely used imported decking hardwood, and Jambu is a hardwood decking timber with durable heartwood that supplier sheets note as resistant to termites. Both are legitimate decking choices. But their supplier-published natural durability is Class 2 above ground, not Class 1, so neither carries the STS Class 1 badge. That is not a criticism of the timbers; it is simply what the specification sheets say, and the badge follows the evidence. If Merbau suits your project on price and appearance, it remains a proven decking option. Jambu is available from STS on a quote-only basis.
Why pine is excluded
Class 1 badges at STS describe natural durability, and structural pine does not earn its outdoor service life that way. Where pine is used outdoors above ground, it relies on preservative treatment such as H3, which manufacturer technical guidance describes as protecting against termites, rot and decay in above-ground outdoor use. Treated pine is a treatment story, not a natural durability story, so it sits outside the Class 1 badge system entirely. You will find our structural pine in the treated pine and untreated pine collections.
How STS badges work: evidence-gated
The rule behind the badge is deliberately strict: STS displays the Class 1 badge only where the supplier specification sheet for that specific species states Class 1 above-ground durability. No supplier sheet, no badge. That is why the badge appears on exactly five hardwoods and never on pine, Merbau or Jambu, even though those products all have their place. The same evidence-first approach runs through our Australian timber and certification page.
What 40+ years means, and what it does not
The 40+ year figure that accompanies Class 1 ratings is a supplier-published durability rating for above-ground use. It is a useful indicator that these species sit in the longest-lasting category of decking timbers. It is not a warranty, and STS does not offer it as a guarantee of service life. How long any deck actually lasts depends on factors the rating cannot see: design, drainage and ventilation, fixing practice, exposure and ongoing maintenance. The supplier installation guidance attached to our product pages (for example, maximum 450 mm joist centres and minimum 400 mm ground clearance for ventilation) exists precisely because good detailing is what lets a Class 1 timber deliver on its rating.
FAQ
Is Class 1 the same above ground and in ground?
No. Durability is rated separately for different exposure. The STS badges and the ratings in this article refer to above-ground durability, which is the relevant rating for decking boards.
Is Class 2 timber a bad choice for decking?
No. Merbau and Jambu are both Class 2 above ground per supplier sheets and both are widely used decking timbers. Class 1 simply denotes the highest natural durability class.
Does the 40+ year rating come with a guarantee?
No. It is a supplier-published rating, not a warranty. Actual service life depends on design, installation and maintenance.
Talk to us
If you want help matching a durability class to your project, get in touch through our request a quote page or drop into the yard at West Gosford for pickup. Bulk, pack and trade orders are quote-first, and we deliver across Sydney Metro, the Central Coast, Newcastle and the Hunter, and Wollongong and the Illawarra.
Sydney Timber Supply team
Technical details drawn from supplier specification sheets; reviewed 19 July 2026.