Merbau — Density, Movement, Workability & Cost

Rain on a fresh merbau (Intsia bijuga) deck leaves a visible, hard-to-remove red-brown stain on the concrete or stone underneath it, a genuinely common and specifically documented complaint from homeowners who installed the decking without knowing the water-soluble tannin would leach out that aggressively during its first season or two of weather exposure.

Very hard, very stable, and full of visible mineral

At 51 lb/ft³ and 1,840 lbf on the Janka scale, merbau is dense and durable, with yellow mineral deposits scattered through its pores that show up as flecks on a finished face and will nick a blade on contact — a real, if minor, working nuisance to plan around rather than a defect to reject a board over.

Movement, and one of the most stable species this site tracks

Radial shrinkage of 2.7% against a tangential figure of 4.6% gives a 1.7 ratio, and the very low absolute numbers — 7.8% total volumetric shrinkage — put merbau among the more stable species this site covers, appropriate for its heavy use in decking where dimensional predictability under weather matters enormously. A 6"-wide deck board seasoning outdoors from a wet 14% down to a drier 8% loses about 0.055" (1.4mm) across its width; run a narrower or wider board through the wood movement calculator. A 1"-thick, 6"-wide, 8-foot deck board weighs about 17 lb; the wood weight calculator covers other decking sizes.

Working the oily surface, and managing the tannin bleed

Gluing benefits from a solvent wipe first, given the naturally oily surface; without that step, bond quality is merely adequate rather than reliable. The tannin-bleed issue is entirely manageable if addressed before installation: seal the wood before it meets weather, and the staining problem largely goes away. Skip that step and expect a season of genuine bleeding onto anything porous below or beside the installed boards. Indoors, away from that specific chemistry, merbau finishes evenly with no particular complications.

Durability, cost, and a real sourcing concern

Merbau's heartwood is very durable and highly resistant to both termites and marine borers, which is exactly why it competes directly with ipe and teak for decking and boatbuilding. At roughly $14 a board foot, it lands in this site's premium tier. Decking, flooring, exterior joinery, boatbuilding, and heavy construction round out its markets. It carries an IUCN Vulnerable rating and a well-documented history of illegal logging in Papua and Indonesia specifically — this is a species where insisting on verifiable chain-of-custody documentation from a supplier is a genuinely meaningful precaution, not just a formality.

The tannin-bleed decision, made explicit

Anyone specifying merbau for a deck or patio installation over concrete, stone, or a light-coloured surface needs to plan around the bleed issue before installation, not after — a penetrating sealer applied to all faces before the boards ever meet rain is the standard, effective preventive measure, and it's far easier than trying to remove staining from concrete after the fact.

What people actually ask about merbau

Does merbau's tannin bleed stop completely after the first season, or does an unsealed deck keep staining whatever's underneath indefinitely? It slows substantially but doesn't necessarily stop entirely — an unsealed board keeps some tannin near the surface for years, so continued light staining under sustained wet conditions is possible well beyond the first season, which is exactly why sealing before installation matters more than waiting it out.

Is merbau as durable as ipe for a deck that sees heavy foot traffic? Both are very durable and widely used for exactly this purpose; merbau is generally somewhat less dense and slightly easier to work than ipe, while ipe holds a modest overall durability edge in the most demanding sustained exposure.

Does an IUCN Vulnerable rating like merbau's carry any CITES trade restriction, or is it purely an advisory conservation status? IUCN ratings and CITES appendices are separate systems — a species can be Vulnerable on the IUCN list without being CITES-listed at all, meaning no international permit requirement automatically follows from the rating itself, which is exactly why chain-of-custody documentation matters as a voluntary sourcing check rather than a legal mandate for merbau.

Table 4-4 of the USDA Forest Products Laboratory's Wood Handbook (FPL-GTR-190, 2010) lists this species as "Merbau (Intsia bijuga and I. palembanica)" and is where the shrinkage figures come from; The Wood Database's 12%-moisture-content testing gave density and Janka hardness.