Choosing the Right Timber for a Sydney Deck
Spotted gum, blackbutt, merbau — what actually performs in Sydney's coastal climate, and where to spend.
Picking the right deck timber in Sydney isn't only an aesthetic decision. Salt air off the Harbour, intense UV through summer and sudden southerly downpours all push timber to its limits. Choose well and your deck will look better with every year. Choose poorly and you'll be replacing boards inside a decade. This guide is what we tell our clients on site, distilled.
What Sydney's climate does to timber
Three forces dominate: UV, moisture and salt. UV greys the surface and breaks down lignin. Moisture cycles cause cupping and checking. Salt — particularly through the Eastern Suburbs and Northern Beaches — accelerates corrosion of fixings and can attack untreated softwoods.
The good news: a handful of Australian hardwoods are built for exactly this environment. The trick is matching species to position, finish and budget.
Species at a glance
| Species | Hardness (Janka) | Best for | Notes |
|---|---|---|---|
| Spotted Gum | 11.0 kN | Feature decks | Beautiful colour variation, very durable |
| Blackbutt | 9.1 kN | Wide consistent decks | Pale, even tone; bushfire-rated |
| Merbau | 8.6 kN | Budget hardwood | Tannin bleed in first season |
| Modwood / composite | — | Low-maintenance | No oiling; less character |
Hidden-fix vs top-screwed
- Hidden-fix gives a clean, splinter-free surface
- Boards can move without splitting around screws
- Easier to refinish in future
- Higher install cost
- Requires precisely milled boards
- Board replacement is more involved
Finishing — oil, stain, or leave it grey
We almost always recommend a penetrating oil for the first three years. After that, some clients let the deck weather to a silver-grey — a striking look on spotted gum, particularly against the sandstone and weatherboard homes common across the Inner West and North Shore.
A note from the workshop
"The board is only as good as the joist beneath it. We oversize joists and use stainless fixings on every coastal job — it's the part you'll never see, and the reason the deck lasts."
Common questions
How long should a hardwood deck last in Sydney?+
Properly built with hardwood, stainless fixings and regular oiling, expect 25–30+ years of structural life — even in salt-exposed suburbs.
Do composites really last longer than hardwood?+
They need less maintenance but don't last meaningfully longer than a well-built hardwood deck. The trade-off is character.
Can you build a deck over an existing slab?+
Yes — we use adjustable pedestals or low-profile sleepers. We'll assess drainage and ventilation on site.
