For a Northern Beaches deck, Spotted Gum is the best all-rounder — Class 1 durability, Janka 11.0, locally grown, available everywhere. Blackbutt is the lighter-coloured close second. Ironbark is the hardest and longest-lasting but 25–35% more. Merbau is cheap but imported with sustainability issues — we don’t use it on new builds.
Four hardwoods come up on every Northern Beaches deck conversation: Merbau, Spotted Gum, Blackbutt, Ironbark. They look different, cost different, and age differently. Here’s how they compare on the specs that actually matter.
Side-by-side specs
| Timber | Janka hardness | Durability class | Cost (per LM 86×19) | Origin |
|---|---|---|---|---|
| Merbau | 8.6 | Class 1–2 | $10–$13 | Indonesia / Malaysia |
| Blackbutt | 9.1 | Class 2 | $13–$17 | NSW / SE QLD |
| Spotted Gum | 11.0 | Class 1 | $14–$18 | NSW / QLD |
| Ironbark | 14.0 | Class 1 | $18–$24 | NSW / QLD |
Janka hardness measures resistance to denting and wear. Durability class measures resistance to rot and termites in-ground (Class 1 is highest). Both matter for a coastal deck.
Merbau
The default budget hardwood for two decades. Reddish-brown, fine grain, easy to work, takes oil well. The downsides have caught up with it:
- Sustainability. Most Merbau is logged from Indonesian rainforests. FSC-certified stock exists but is rare and not always honestly labelled.
- Tannin leaching. Merbau bleeds dark red tannin in the first 6–12 months. If your deck is over white render or pavers, plan to seal underneath or accept the staining.
- Imported supply chain. Lead times can be unpredictable, and quality varies between shipments.
Cheap, available, fine if you’re not fussy. We don’t spec it for new client decks anymore — the local hardwoods are now competitive on price.
Blackbutt
Pale, golden-blonde to light-brown timber, grown in NSW. Janka 9.1 (softer than Spotted Gum but harder than Merbau). Class 2 durability above ground. The look:
- Pale and modern. Suits coastal homes, white-rendered facades, and contemporary architecture. Doesn’t shout.
- Ages light. Greys to a soft silver if left unoiled, much lighter than Spotted Gum or Ironbark.
- BAL-29 rated. One of the seven species accepted for bushfire-prone areas without further treatment.
Best for: clients who want hardwood look without the dark, heavy tone of traditional eucalypt decks. Common across Avalon and Newport new builds.
Spotted Gum
The Northern Beaches workhorse. Janka 11.0, Class 1 durability, grown locally, widely stocked. Colour ranges from chocolate brown to pale grey within a single board — that variation is what makes a Spotted Gum deck look alive.
- Strength. Hard enough that dragged outdoor furniture doesn’t leave permanent marks.
- Durability. Lasts 25–30 years on the Northern Beaches with annual oiling and stainless fixings.
- Look. Rich, varied, distinctly Australian. Greys beautifully if left unoiled.
- BAL-29 rated. Accepted in bushfire zones.
If a client doesn’t have a strong preference, this is what we recommend. It’s the default in our deck builds.
Ironbark
The premium option. Janka 14.0 (hardest of the four), Class 1 durability, dense and heavy. Deep red-brown to almost-black colour. The trade-offs:
- Cost. 25–35% more than Spotted Gum.
- Weight and density. Pre-drilling required for every screw. Slower to build, costs more in labour.
- Movement. The denser hardwoods can cup or twist if not properly seasoned and acclimatised. Buy from a yard that air-dries stock for at least 12 months.
- Lifespan. 35–40 years with maintenance. Genuinely long-term.
Best for: heritage homes wanting deep, traditional colour; clients building a once-in-a-lifetime deck and willing to pay for it.
What we spec on a typical job
For most Northern Beaches clients we recommend Spotted Gum 86 × 19mm or 86 × 21mm boards, fixed with 65mm stainless 316 decking screws, on H3 or H4 hardwood joists at 450mm centres. That spec lasts and looks good without being silly money.
If the client is matching a pale modern facade we’ll go Blackbutt. If they’re prepared to spend up, Ironbark. Merbau only if they specifically request it and understand the trade-offs.
Composite as the alternative
If you’ve read this and are wondering whether you want to deal with hardwood at all, fair question. Composite skips the oiling and the splinters but costs 30–40% more upfront. We’ve broken down the brands in our composite decking brand comparison.
Frequently Asked Questions
What is the best decking timber for the Northern Beaches?
Spotted Gum is the best all-rounder — Class 1 durability, Janka 11.0, locally grown, looks great oiled. Blackbutt is a close second with a lighter colour. Ironbark is harder and longer-lasting but 25–35% more.
Is Merbau still a good choice in 2026?
It’s still cheap and widely stocked, but Australian Spotted Gum and Blackbutt are now competitive on price, look better, and don’t have the sustainability issues of imported Indonesian Merbau.
How long does a Spotted Gum deck last?
A properly built Spotted Gum deck with stainless fixings and annual oiling lasts 25–30 years on the Northern Beaches. Without oiling, boards still last 15–20 years but go silver-grey.
What is the hardest decking timber in Australia?
Ironbark has a Janka hardness of 14.0 — the hardest commonly available decking timber. Spotted Gum is 11.0, Blackbutt 9.1, Merbau 8.6.
Want a Hardwood Deck Built Right?
We build Spotted Gum, Blackbutt and Ironbark decks across the Northern Beaches with stainless 316 fixings and joists at proper spacing. Get in touch for a quote that has the spec in writing.
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