Merbau Decking — Pros, Cons, and Care in Adelaide

Merbau is the most popular hardwood decking timber in Adelaide. There are good reasons (price, durability, availability) and a few caveats (sustainability, tannin bleed, oiling commitment). Here’s an honest take.

What merbau is

Merbau (Intsia bijuga, also sold as kwila) is a tropical hardwood from South-East Asia and parts of the Pacific. Naturally durable Class 1 — meaning it lasts 25+ years in-ground without preservative. Deep reddish-brown when fresh, weathers to silver-grey if left untreated.

Density: 800–900 kg/m³. Janka hardness rating: 8.5 — harder than most flooring timbers, plenty hard for decks.

Why it dominates Adelaide decks

• Cost. Cheaper than spotted gum, blackbutt, or jarrah. A few dollars per linear metre cheaper than the equivalent Australian hardwood.
• Stability. Cups, twists, and warps less than most cheaper hardwoods.
• Availability. Stocked at every major Adelaide timber yard.
• Durability. Naturally rot-resistant and termite-resistant.
• Look. The fresh reddish-brown is striking.

What you should know before specifying merbau

Tannin bleed

Merbau leaches a deep red tannin from the timber for the first 3-6 months of weathering. The tannin runs onto whatever is below the deck — pavers, concrete, lawn — and can permanently stain light-coloured surfaces. Pre-soak the boards before laying, or pre-oil before installation, to reduce the bleed. Cover delicate surfaces below for the first wet season.

Oiling

To keep the rich reddish-brown look, merbau needs annual oil. The right oil for Adelaide conditions: a tung-oil-based decking oil (Cabot’s Aquadeck, Cutek, Intergrain UltraDeck or similar). Apply two coats, dry between, sand lightly, brush off, oil. Plan on 4-6 hours for a 30sqm deck.

If you skip oiling, the deck weathers to silver-grey within 18 months. It’s still structurally sound — silver-grey merbau is often a deliberate aesthetic. But once weathered, restoring the original colour requires sanding back to fresh wood.

Sustainability

Merbau is on the IUCN vulnerable list. Look for FSC-certified merbau or PEFC-certified stock. Most Adelaide timber yards now stock certified merbau by default; ask explicitly if you care.

Spotted gum and blackbutt — Australian-grown — are sustainable alternatives at slightly higher prices.

Splinters when un-maintained

Weathered merbau (un-oiled) can develop surface splinters. Light sand annually if you want bare-feet-friendly even on weathered timber.

What it costs in Adelaide

Supply only: $7–$12 per linear metre for 90×19mm boards.

Supplied and installed: $230–$320 per square metre. Includes:

• Subframe build (bearers, joists, posts)
• Boards laid with stainless 316 fixings (or hot-dipped galvanised inland)
• 3-6mm gap spec for drainage
• Edge trim

A 30sqm merbau deck in Adelaide: $7,000–$9,500 typical install.

Maintenance schedule

When	Job	Time
First 3 months	Cover surfaces below from tannin bleed	passive
1 month after install	Light wash to remove construction dust	30 min
6 months	First oil application	4-6 hours
Annual	Re-oil (Cabot’s Aquadeck or equivalent)	4-6 hours
Every 2-3 years	Light sand and oil	6-8 hours
Every 8-10 years	Heavy sand back to fresh wood and oil	full day

When merbau is the right call

• You want a hardwood deck and you’re cost-sensitive
• You’ll commit to annual oiling
• The deck is in a location where occasional silvering is acceptable
• You don’t have light-coloured paving or surfaces directly below the deck

When something else is better

• Don’t want to maintain it: composite decking
• Want top-tier Australian sustainability: spotted gum or blackbutt
• Pool deck with chlorine exposure: composite or jarrah (more chemical-resistant)
• Heritage home where merbau looks wrong: Australian native hardwood

Get a free decking quote

Request a free decking quote — mention merbau in the brief if it’s your preference. Adelaide deck builders will quote merbau against alternatives so you can compare.