Merbau decking: tannin bleed, upkeep and lifespan vs composite

Merbau decking is a dense imported hardwood that lasts. Its heartwood is rated Class 1 for durability above ground, which the Queensland Government puts at a life expectancy of more than 40 years¹. The catch is tannin. New boards leach a stain when they get wet, and it can be hard to shift from unsealed paving and brickwork nearby³. Composite avoids the yearly oil and most of the bleed, but it can cost more up front and run hotter in full sun.

What is merbau decking?

Merbau is a tropical hardwood (Intsia bijuga, and a close relative) that grows across South-East Asia and the western Pacific, and naturally in two areas of north Queensland¹. What is sold here is imported from South-East Asia and the Pacific islands².

Fresh-cut heartwood is yellowish or orange-brown and darkens with age to brown or a deep reddish-brown. The grain is usually interlocked or wavy, with a coarse but even texture. The timber is heavy, at 830 kg/m³ at 12% moisture content, and rated hard, 2 on a 6-class scale¹. It seasons with very little shrinkage or movement². Merbau is one of the hardwoods in the deck designer, so you can lay it out on your own deck shape and see the boards and frame it needs.

Is kwila decking the same as merbau?

Yes. Kwila and merbau are the same timber. The Queensland Government's species page is filed under kwila and gives merbau as its Malaysian name, along with scrub mahogany¹. If a quote says kwila, read everything below as applying to it.

Merbau tannin bleed: why it stains paving and how to limit it

Most hardwoods carry water-soluble compounds that give the wood its colour and some of its decay resistance. They wash out to the surface as the timber dries after wetting. The stain can be hard to shift from unsealed concrete or paving under the deck³. One coating maker groups merbau with spotted gum and jarrah as tannin-rich, oily timbers that take longer than the usual 4–6 weeks of wetting and drying to weather⁵.

Owners say it goes on longer than the first few wet days. In an October 2019 Whirlpool thread about laying merbau onto an exposed-aggregate driveway, Donnie Darko wrote that "Merbau leaches for ages" and will stain concrete if nothing stops it⁸. Skitza replied that theirs was still leaching after a year. Another member, subyroo, had tannin run off merbau gates onto a concrete path and tried bleach and then a laundry stain remover, both with a pressure washer, "all to no avail".

Here is how I'd keep it off the paving:

• Keep the packs dry, off the ground and under cover until they go down. In the same Whirlpool thread, 2 Fast advised laying dry boards and letting them weather together, since a stack left out in the rain leaches unevenly and leaves "Random red patches"⁸.
• Cover the paving and pool coping under and beside the deck with builder's plastic for the whole build.
• Seal pale paving before the boards go down, because unsealed paving is where WoodSolutions says the stain is hard to shift.
• Coat every face of each board, cut ends included. WoodSolutions advises seasoned timber and a water-repellent finish on all of them to lessen the bleed³.
• Get the surface tannin out before the top coat. Either let the deck weather or scrub it with a tannin and oil remover, as the same coating maker suggests⁵. Send the run-off to the garden, not across the paving.
• For stains that land anyway, expect a wait. Pharma.., in the same thread, said the stain does go away eventually but looks poor for six months to a year, and that oxalic acid and a laundry stain remover "helps but doesn't really cut it"⁸.

Put pot plants on stands with drip trays, and move pots and furniture around now and then. WoodSolutions says items left in one place make the deck weather unevenly³.

Black spots are a different problem. Iron filings from an angle grinder, including the dust from cutting bricks or tiles nearby, react with the timber and leave black stains, and metal left on the deck for long periods can stain it too³. I'd finish all the metal cutting before the boards arrive.

Where does merbau come from, and is it legally sourced?

It comes from tropical forests, and the supply chain has a history worth checking. WoodSolutions says merbau "may be sourced from forests legally converted to other land uses or where there are inadequate controls"². Certified merbau is available in Australia, and WoodSolutions suggests checking with your supplier. It adds that certification and chain-of-custody systems, including DNA tracking, give assurance that the wood comes from legal or well-managed forests.

Australian law puts the duty on the importer. Under the Illegal Logging Prohibition Act 2012, anyone importing wood products has to make sure they are not bringing in illegally logged timber, and the due diligence rules changed on 3 March 2025⁷. Your own check is the paperwork. I'd ask for the chain-of-custody certificate number with the quote and look it up on the certifier's public register before paying a deposit.

How long does merbau decking last?

Above ground, a long time. The heartwood is Class 1 for above-ground durability, a life expectancy of more than 40 years, and it resists termites¹. In the ground it drops to Class 3, or 5–15 years, so merbau posts shouldn't go into the soil¹. The pale sapwood is the weak spot. Untreated, it can be attacked by borers, so I'd set aside boards that carry a strip of it.

The build matters as much as the species. WoodSolutions treats a deck less than 400 mm off the ground as a special case that needs more ventilation and drainage underneath, the narrowest boards available and gaps of 5–6 mm between them³. Where a deck can't meet those conditions, it says the service life will be shorter, whatever the species.

Owners with both timbers notice the difference. A Whirlpool member, teacup5, who has spotted gum at the back of the house and merbau out the front, wrote in December 2025 that "the merbau has held up much better in the sun/rain", although both exposed areas need another coat every six months⁹.

What upkeep does a merbau deck need?

To keep the red-brown colour, WoodSolutions says a deck should be cleaned and resealed or stained at least once a year, or as often as the coating maker says³. Pre-oiled boards still need that top coat once they're laid. One coating maker recommends a final coat over any factory pre-oil⁶.

You can skip the oil. Left unfinished, merbau weathers to grey. The durability rating belongs to the heartwood, not the finish, but WoodSolutions says a deck without regular maintenance discolours, goes rough and splintery, and can grow mould that makes it slippery or shortens the boards' life³. A grey deck still needs cleaning, and you lose the red-brown you paid for.

How much does merbau decking cost?

I don't print timber prices here, because they change too often and every width and finish is priced differently. Owner accounts give a sense of how merbau compares. Jikx, who started a Whirlpool thread in December 2025, had two quotes for the same south-facing deck of about 40 m², and the composite quote was about a quarter higher than the merbau one⁹. On the same thread, teacup5 chose merbau for the exposed front deck to save money over the spotted gum at the back. One composite maker agrees that its decks cost more than timber in the short term, and argues that the money saved on upkeep covers the difference¹⁰.

For composite board prices by state, with the date each was checked, see the prices page. Ask for timber and composite quotes on the same frame drawing. The joist spacing and the fixings can differ between the two, and you want to see where. Then add the oil and the days spent applying it, every year or so for an exposed deck, before comparing the totals.

How does merbau compare with composite decking?

Merbau wins on repairs and has a real timber grain, and composite wins on upkeep and on keeping stains off your paving. Here is the comparison point by point:

On heat, ModWood says its uncapped range sits at a similar surface temperature to timber of a similar colour, while its darker colours can get quite hot underfoot, especially when new¹¹. Does composite decking get hot goes through colour and shade. I'd still pick composite beside pale paving or a pool, and for anyone who knows they won't oil. I'd pick merbau for a covered deck where the oiling is light and the run-off lands on garden beds. The composite decking hub covers the boards it would be competing with.

Can you use merbau in a bushfire area?

Yes, as decking up to BAL-29. Kwila (merbau) is one of the seven bushfire-resisting timbers in WoodSolutions' summary of the timber options under AS 3959-2018⁴. On a deck with an open, unenclosed subfloor, that summary allows it as decking at BAL-12.5 and BAL-19 over ordinary framing, and at BAL-29 only if the supports and framing are bushfire-resisting timber too. At BAL-40 and BAL-FZ it points to alternatives to timber. Your certifier decides, and composite decking fire ratings and BAL zones covers boards with their own tested ratings.

What I'd do

Work out where the water off the deck goes before anything else. If it drains onto pale paving or a pool surround, seal or cover that first. Coat every face of the boards and expect tannin to run through the first wet weeks. Ask for the certificate number with the quote and check it. Give the deck 400 mm of air underneath if the site allows, or use the narrowest boards and wider gaps if it doesn't³. Then either book an oiling day each year for an exposed deck or decide now that you'll let it go grey. If you're still weighing merbau against composite, draw the same deck with each in the deck designer and compare what each one needs.