Spotted Gum vs Merbau Decking: How They Compare

Spotted gum decking and merbau decking are two of the most requested hardwoods for Adelaide decks, and blackbutt is often the third option on the table. All three are durable, hard and good-looking. The differences are in colour, density, tannin bleed, where the timber comes from and how it suits bushfire-prone sites.

Here is how they compare, using published data from WoodSolutions and the Queensland Government’s timber property sheets.

Spotted gum vs merbau vs blackbutt at a glance

Spotted gum is the hardest and densest of the three, merbau has the most even colour, and blackbutt is the lightest in tone. All three are rated as durable above ground and resistant to termites.

Property	Spotted gum	Merbau	Blackbutt
Origin	Australian east coast hardwood	Imported tropical hardwood	Australian east coast hardwood
Colour	Creamy brown to dark brown, varied	Even orange-brown ageing to deep red-brown	Pale straw to light brown
Natural durability above ground	Class 1	Class 1	Class 2
Seasoned density	About 990 kg/m³	About 830–860 kg/m³	About 900 kg/m³
Janka hardness (seasoned)	10.1 kN	8.5 kN	8.9 kN
Termite resistance	Resistant	Resistant	Resistant
Tannin bleed	Some when new	Pronounced when new	Some when new
Bushfire-prone areas	Listed for some BAL levels at 18 mm or thicker	Check the product and your site’s BAL	Listed for some BAL levels at 18 mm or thicker

Durability classes run from Class 1 (very durable) to Class 4 (non-durable), under the Australian rating system in AS 5604. The ratings apply to the heartwood of the species, and real-world life depends heavily on how the deck is built, ventilated and maintained.

Spotted gum decking

Spotted gum is a dense, hard Australian hardwood that is very durable above ground and suits some bushfire-prone sites. It is the go-to choice for many people who want a long-lasting, natural-looking deck.

What it looks like. The colour ranges from light creamy brown through to deep chocolate brown, often within the same pack of boards. The grain can be wavy, and some boards show an attractive “fiddleback” figure. If you like a natural, varied deck, spotted gum delivers that.

How it performs. WoodSolutions rates spotted gum as Durability Class 1 above ground and resistant to termites. It has a high Janka hardness, so it stands up well to foot traffic, outdoor furniture and dog claws. WoodSolutions also notes that it shrinks very little as it dries, which helps it stay stable.

Things to know.

• It is hard, so boards are pre-drilled for fixings.
• Colour variation is part of the timber. If you want a uniform look, it may not suit.
• Like all new hardwood, it releases some tannins, so surface preparation before oiling matters.

Merbau decking

Merbau is a durable imported hardwood prized for its even, rich red-brown colour. It is a reliable decking timber, but it needs care in its first months because of tannin bleed.

What it looks like. New merbau is orange-brown and deepens to a rich red-brown as it ages. The colour is more consistent from board to board than spotted gum, which gives a tidy, uniform finish. Some boards show small yellow flecks in the grain.

How it performs. The Queensland Government rates merbau (also sold as kwila) as Class 1 above ground and notes its strong resistance to termites. It is hard and holds fixings well once pre-drilled.

Tannin bleed. This is the key thing to plan for. Merbau contains water-soluble tannins that leach out when the boards get wet, especially in the first months. The brown-red run-off can stain concrete, paving, render, pool coping and even nearby fences. The bleed eases as the boards weather and are cleaned, but it is worth:

• Keeping run-off away from light-coloured paving or concrete where possible
• Hosing down and cleaning the boards before the first coat of oil
• Using a tannin and oil remover or allowing the deck to weather, as coating makers recommend

Both the Queensland Government and WoodSolutions sheets also note that gum bleed or oily patches can affect finishes on merbau, which is another reason good preparation matters.

Sourcing. Merbau is imported from Southeast Asia and the Pacific. If responsible sourcing matters to you, ask about the supply chain and certification of the timber you are buying.

Blackbutt decking

Blackbutt is an Australian hardwood with a pale, light brown colour that suits a modern or coastal look. It is a good middle ground between the two.

WoodSolutions rates blackbutt as Durability Class 2 above ground and resistant to termites. It is a little less durable than spotted gum or merbau on the rating scale, but it is still a solid decking timber when detailed and maintained well. Its pale colour is the main reason people choose it, and it pairs well with lighter oils that keep the deck looking fresh.

Which timbers suit bushfire-prone areas?

Some hardwood species are suited to bushfire-prone areas, but it depends on the species, the board thickness and your site’s Bushfire Attack Level (BAL). Parts of the Adelaide Hills and the fringe suburbs are bushfire-prone, so this question comes up often.

WoodSolutions lists both spotted gum and blackbutt, in components 18 mm or thicker, as suitable for BAL-12.5, BAL-19 and BAL-29 applications under AS 3959. We have not seen merbau listed in the same way, so if your site is bushfire-prone and you like merbau, check the specific product and your BAL first.

Rules also apply to the deck frame, the gap under the deck and the materials nearby, not just the boards. Check your BAL assessment and the current requirements with your council or a building surveyor, and read our guide to council approval for decks and pergolas in SA for the general picture.

Merbau vs spotted gum: which suits your home?

Pick spotted gum if you want the hardest, most durable option with natural variation, or if your site is bushfire-prone. Pick merbau if you want an even, rich red-brown deck and can manage the tannin bleed in the first months.

If you want…	Consider
Natural, varied tones	Spotted gum
A uniform red-brown finish	Merbau
A pale, contemporary look	Blackbutt
A deck in a bushfire-prone area	Spotted gum or blackbutt, subject to your BAL
A deck next to light paving or a pool	Spotted gum or blackbutt, or merbau with careful tannin management
The hardest surface for heavy use	Spotted gum

Budget is also a factor. Prices move with supply, board width and grade, so we price each species for your deck rather than relying on general rules. Our guide to decking costs in Adelaide explains what drives the final figure.

Looking after a hardwood deck

All three species are durable without a coating, but oiling keeps their colour and reduces surface checking. Left uncoated, they weather to a silver-grey.

A simple routine works for each:

1. Sweep leaves and debris often, especially from the gaps between boards.
2. Clean the deck with a timber deck cleaner and a stiff broom a few times a year.
3. Recoat when water stops beading on the surface. Many owners recoat about once a year, more often on decks in full sun. Check the product’s guidance.

Our step-by-step guide on how to oil a deck walks through the full process, including how to prepare new merbau.

If you are also weighing hardwood against manufactured boards, see composite vs timber decking.

Your next step

The best way to choose between spotted gum decking, merbau and blackbutt is to see the boards in person and talk through your site, sun and nearby surfaces. Have a look at our timber decking page and our overview of decking in Adelaide, then call us on 0485 008 292 or use the quote form. We will get back to you within 24–48 hours to arrange a measure and bring samples.