Merbau — Intsia bijuga
A workhorse exterior hardwood specified across Australia for the better part of three decades. Class 1 above-ground durability (more than 40 years), Joint Group JD2 seasoned, and dimensional stability that's exceptional even for a dense tropical hardwood. Also known as Kwila, Ipil and Vesi across the Pacific.
Merbau products
Pool-compliant screens, standard batten screens, and bulk Merbau battens for fences, feature walls and joinery. Decking and structural sections available by enquiry.
Technical ReferenceSummary
Merbau (Intsia bijuga) — known regionally as Kwila in PNG, Ipil in the Philippines, and Vesi in Fiji — is the most widely specified imported hardwood in Australia for exterior applications. It grows across Southeast Asia, the Pacific islands, and into far north Queensland.
Its heartwood is classified Class 1 — Highly Durable above ground (more than 40 years per WoodSolutions, AS 5604), with moderate marine borer resistance (21–40 years). Structurally it is SD3 seasoned (S2 unseasoned), Joint Group JD2 seasoned (J2 unseasoned), F-grade F27. Density 860 kg/m³ air-dry. Heartwood is termite-resistant; the sapwood is Lyctus-susceptible but the heartwood is not. BCA Group 4 fire rating, BAL 12.5/19/29 approved. Two trade-offs to plan for: Merbau heartwood contains water-soluble tannins that can bleed onto adjacent surfaces, and the timber requires corrosion-resistant fasteners.
Class 1 of 4
Character & Appearance
Heartwood is yellowish-brown or orange-brown when freshly cut, deepening to a pale to dark reddish brown with age and UV exposure. Sapwood is white, pale yellow or buff and is visually distinct from heartwood. The colour-deepening is what gives Merbau its signature warm, settled look on decks and feature walls a few seasons in.
Grain is usually interlocked or wavy, producing a ribbon figure on quarter-sawn boards. Texture is coarse but even. The timber has an oily, slightly aromatic odour when cut — the natural oils contribute to its durability and dimensional stability. Gum and tannin bleed is possible after install — see Specification Notes for detailing.
Species & General Properties
Mechanical & Physical Properties
| Property | Unseasoned | Seasoned |
|---|---|---|
| Density | 1,150 kg/m³ | 860 kg/m³ |
| Janka hardness | 7.6 kN | 8.5 kN |
| Modulus of rupture (bending) | 118 MPa | 147 MPa |
| Modulus of elasticity | 15 GPa | 18 GPa |
| Maximum crushing strength | 59 MPa | 82 MPa |
| Strength Group | S2 | SD3 |
| Stress Grade (typical, Structural No.1) | F27 | F27 |
| Joint Group | J2 | JD2 |
| Shrinkage (Total) | Value |
|---|---|
| Tangential shrinkage | 2.50 % |
| Radial shrinkage | 1.20 % |
| Unit movement — tangential | 0.30 % |
| Unit movement — radial | 0.19 % |
Recommended Applications
Merbau's combination of Class 1 above-ground durability, top-tier JD1 joint group, low shrinkage, and high marine borer resistance makes it one of the most versatile exterior hardwoods on the Australian market. It performs as well around pools and on coastal decks as it does on inland fence screens.
Specification Notes
- Natural durability ratings apply to heartwood only. Specify "heart-only" or "heart-in" if appearance and durability of sapwood is a concern.
- Recommended spec wording: "Merbau (Intsia bijuga) heartwood — AS 5604 Class 1 above-ground durability, Strength Group SD3 seasoned, Joint Group JD2 seasoned, F-grade F27 seasoned."
- Tannin bleed: heartwood contains water-soluble tannins that can leach onto adjacent paving, render and concrete. Pre-oiling and pre-wetting reduces this. Avoid installing directly over light-coloured pavers without a sealer.
- Fasteners: use corrosion-resistant stainless steel or hot-dipped galvanised fasteners. Plain steel and brass will react with the natural tannins and stain the timber.
- Lyctus susceptibility: sapwood is susceptible. Specify heartwood or treated sapwood for internal applications where Lyctus is a concern.
- Bushfire performance: BCA Group 4 fire rating. Approved under AS 3959 for use in BAL 12.5, 19 and 29 bushfire-prone zones.
- For UV-exposed applications, oil within 2 weeks of install then 12-monthly maintenance to slow the natural silver-grey weathering.
- Sourcing: Kosny sources Merbau from legal and traceable supply chains. Certified material is recommended for projects requiring chain-of-custody documentation.
References
Speccing Merbau for a project?Merbau for a project?
Pool screens, decking, bulk battens or structural sections — we hold and source Merbau in commercial quantities. Send us your project specs and we'll quote with lead times and tannin/fastener guidance.