Ulin alongside Australian hardwoods

Denser wood packs more fibre into every cubic metre, which is why density quietly drives three things that matter for decking: hardness, natural decay resistance, and fire performance. A denser board absorbs impact without denting, has fewer voids for rot fungi to colonise, and ignites more slowly under radiant heat.

Janka measures the force needed to press an 11.28 mm steel ball halfway into a timber sample — a standardised proxy for how well a deck will hold up to dropped tools, dragged furniture, stiletto heels, and decades of foot traffic. Most residential flooring timbers sit between 4 and 9 kN; anything above 10 kN is premium decking territory, where differences become imperceptible underfoot.

Timber expands in wet weather and contracts in dry conditions — and it does so unevenly, shrinking roughly twice as much across the growth rings (tangentially) as through them (radially). Over years of wet/dry cycling, that differential movement opens surface checks, pulls boards into cup, and eventually produces splinter hazards underfoot. The timber doesn't fail — it remains structurally sound — but small splits and checks appear over time. On a deck those are a real consideration, not just an aesthetic one. Ulin's interlocked grain resists the movement by locking fibres in alternating directions; its low tangential shrinkage reduces the driving force in the first place. Combined with its exceptional natural durability, this makes reclaimed Ulin one of the longest-lasting outdoor timbers available anywhere in the world.

AS 5604 rates a timber's natural durability — the ability of its heartwood to resist decay fungi and insects without chemical treatment. Class 1 timbers carry extractive compounds (oils, tannins, silica) that are toxic to rot organisms and termites; Class 4 timbers have none and depend entirely on preservatives to reach any outdoor service life. The distinction matters because chemical envelopes fail over time, while natural durability doesn't.

BAL (Bushfire Attack Level) measures the radiant heat, ember and flame exposure a building face may receive, in kilowatts per square metre. BAL-29 is the threshold at which unprotected timber decking becomes a compliance question: at 29 kW/m², unpiloted ignition of timber can occur from radiant heat alone. AS 3959 Appendix F lists seven species tested and approved as Bushfire-Resisting Timbers (BRT) for use at up to BAL-29 without additional protection: Blackbutt, Kwila (Merbau), Red Ironbark, River Red Gum, Silvertop Ash, Spotted Gum, and Turpentine.

Termite resistance in Class 1 hardwoods comes from extractive compounds laid down in the heartwood while the tree was growing — tannins, oils and in Ulin's case also silica that make the timber indigestible or actively toxic to termites and decay fungi. These extractives don't wash out with rain or leach with age; a 50-year-old Ironbark bridge beam is as termite-resistant as the day it was milled. Treated softwoods, by contrast, rely on a chemical envelope — cut ends, deep checks or worn surfaces can expose untreated core.

Marine-borer resistance is a separate property from land-based decay resistance — it describes how well heartwood holds up against shipworms (Teredo, Bankia) and gribbles (Sphaeroma) that attack submerged timber in salt and brackish water. It matters for any deck within 5 km of the surf coast (salt spray), any waterfront residence, and obviously anything over water. Most hardwoods have some resistance; very few are benchmark-grade.