If there is one part of a deck you never want to get wrong, it is what sits underneath it. A beautiful merbau or spotted gum deck with crooked footings will sag, twist and eventually fail. The good news is that getting footings right in Sydney is not complicated, but it does require understanding your soil, your slope and what the local council expects.
Why Deck Footings Matter More Than the Boards Above
Deck boards are what you see and walk on, but the footings and substructure carry every kilogram of weight, from people and furniture to wind loads and the deck itself. Poorly planned footings lead to uneven surfaces, bouncing, rotting posts and in worst cases, structural collapse. In Sydney’s reactive clay soils, the ground swells in wet weather and shrinks in dry spells, so a footing that seems solid in spring can shift by winter.
The fix is straightforward: dig deep enough, use the right post material, and set everything in concrete below the ground movement zone. A well-built footing should outlast the decking above it.
Common Footing Types Used in Sydney Decks
Concrete Piers with Embedded Posts
This is the most common residential deck footing in Sydney. A hole is augered into the ground, a treated pine or steel post is dropped in and concrete is poured around it. For most Sydney backyards with stable clay or sandy loam, a 400 mm diameter hole going 600 to 900 mm deep is typical for low-level decks.
Stirrup-Style Post Shoes (Galvanised Brackets)
A cleaner alternative embeds a galvanised steel bracket in the concrete, and the timber post bolts on above ground. This keeps timber out of direct soil contact, dramatically reducing rot risk. We use these frequently in Sydney’s wetter suburbs like Drummoyne and Lane Cove where moisture lingers around ground level.
Screw Piles (Helical Piers)
Increasingly popular for tricky sites, screw piles are twisted into the ground by machinery and provide immediate load-bearing capacity without waiting for concrete to cure. They are excellent for decks built over rock, near retaining walls or in tight access areas where an auger truck cannot reach. The trade-off is cost, but they can significantly speed up your build timeline.
Sydney Soil Types and How Deep to Go
Sydney does not have deep frost lines like southern Australia, but footing depth still matters. The main concern is reactive clay, which is common across western suburbs like Penrith, Kellyville and Parramatta. In these areas, footings should extend below the active soil layer, typically 600 mm or deeper, to avoid the seasonal shrink-swell cycle pulling your deck out of level.
In sandy coastal areas like Dee Why, Manly or Bondi, drainage is excellent but lateral stability can be poor. Deeper and wider footings or screw piles can prevent sideways movement during storms or heavy foot traffic.
Practical Footing Specs for a Standard Low-Level Deck
- Hole diameter: 300 to 450 mm
- Depth: 600 to 900 mm (deeper on reactive clay or sloping ground)
- Post size: 90 x 90 mm or 100 x 100 mm treated pine (H4 rating for in-ground contact)
- Spacing: 1.5 to 1.8 metres between posts along the bearer
- Concrete: Minimum 20 MPa mix
For raised or second-storey decks, engineering requirements change significantly. Posts may need to be 145 x 45 mm or larger, and footings deeper and wider. This is where a structural engineer’s specification becomes essential, particularly if the deck is over one metre off the ground.
Council Approval in NSW
In New South Wales, many low-level decks (under 25 square metres, less than one metre above ground and not attached to a dwelling requiring structural modification) fall under exempt development. However, raised decks, those near boundaries or decks attached to a house typically need a Complying Development Certificate or a full DA through your local council.
Setback rules also apply. In many Sydney councils, a deck must be at least 900 mm from a side boundary unless fire-resistant construction is used. Always check your specific council’s requirements, as rules vary between Inner West, Northern Beaches and Hills District councils.
Drainage and Ventilation Under the Deck
The area beneath your deck is just as important as the footings themselves. Water pooling around post bases causes premature rot, even with H4-treated timber. Ensure the ground slopes away from the deck, install gravel to improve drainage, and for low-level decks, leave at least 150 mm of clearance between the ground and bearers for airflow.
Good ventilation also prevents mould and timber decay in the substructure. If you are building over a concrete slab, consider pedestal systems that keep framing elevated above any moisture that seeps through.
Steel vs Timber Posts: Which Lasts Longer?
H4-treated timber posts are cost-effective, easy to cut on site and familiar to most builders. Galvanised steel posts are pricier but virtually immune to rot and termites, making them ideal for decks near the coast or in termite-prone areas across Sydney’s north and west. For longevity, a galvanised post shoe with a timber post above ground offers the best of both worlds, keeping the timber dry while remaining affordable.
Do Not Skip the String Line
The simplest mistake in footing installation is not setting out posts in a straight, square grid. Use a string line and builder’s square to mark every post before you dig. A footing 50 mm off-line creates headaches later when bearers do not sit flush and joists have to be packed. Take an extra hour at this stage and save yourself days of remedial work.
When to Call a Professional
DIY decking can be a satisfying project, but footings and substructure work is where most owner-built decks come unstuck. If your site has significant slope, reactive clay, rock close to the surface, or your deck will be more than one metre off the ground, engaging a licensed builder is the safer choice.
5 Star Decking holds NSW builder’s licence 395250C and has built decks across Sydney from the Inner West to the Hills District. We handle the entire process, from soil assessment and engineering specs through to the final coat of oil. If you are planning a new deck or need help assessing your existing footings, call us on 0433 334 415 orreach out through our contact page for a no-obligation quote.
Key Takeaways
- Footings should go at least 600 mm deep in Sydney, deeper on reactive clay
- Keep timber posts out of direct ground contact where possible using galvanised brackets
- Check your council’s exempt development rules before building
- Drainage and ventilation under the deck matter as much as the footings themselves
- For raised decks, consider screw piles or steel posts for added stability
A deck is only as solid as what holds it up. Get the footings right and everything above them becomes the easy part.
