How to plan a timber deck in Australia — footings, framing and boards

A deck build has a lot more going on underneath than the finished boards let on. Footings, framing, drainage falls, fixings, and — depending on your site — a retaining wall before you even start thinking about decking boards. Skip the planning and you end up ordering materials twice, or worse, finding out your footings aren't deep enough once the frame's already up.

Here's the order I'd plan it in, with what to work out at each stage. Tick off each step below as you go — it'll keep track for you.

Step 1 — Footings, before anything else

Footings carry the whole structure, so this is the step to get right before you order a single decking board. For a standard ground-level or low deck, timber or steel posts sit on concrete footings — either poured into an augered hole, or set on a precast concrete stump or bracket footing.

A few things that determine your footing depth and size:

• Soil type: reactive clay soils common in parts of South East Queensland need deeper footings than sandy or rocky ground — a geotechnical report is worth it for larger decks on clay.
• Deck height: the higher the deck, the more load and wind uplift the footing has to resist.
• Frost and termite considerations: less of an issue in most of Queensland than southern states, but termite protection at the footing-to-post junction still matters everywhere in Australia.

As a rough general guide, a standard residential deck post footing is around 300–450 mm diameter and 450–600 mm deep for a low deck, but this varies enough by site and span that it's genuinely worth checking against AS 1684 span tables or getting a builder/engineer to confirm for anything beyond a small, low deck. This isn't a step to guess on — footings are the one part of the deck you can't easily fix after the fact.

Step 2 — Bearers and joists

Once footings are set, the frame goes on top: bearers run across the tops of the posts, and joists run across the bearers to support the decking boards. Timber sizes and spacings are governed by span tables (AS 1684 for timber-framed construction) — the further apart your posts, the deeper your bearer needs to be.

A few practical points that catch people out:

• Joist spacing is usually 450 mm centres for standard timber decking, but check your specific board's span rating — composite boards often need closer spacing (300 mm) than timber.
• Fall for drainage: build in a slight fall (around 1:100) away from the house so water doesn't pool or run back toward the building.
• H3 treated timber minimum for any framing exposed to weather — H4 for anything in contact with the ground, like posts going into footings.

This step doesn't have a dedicated calculator on this site — span and joist sizing really does need to be checked against the current span tables or your local building surveyor for anything beyond a small, low deck, since it's a structural decision.

Step 3 — Decking boards, screws and fixings

With the frame done, boards are the visible, satisfying part — but there's still maths to get right so you're not short mid-job or stuck with an odd board width at the far edge.

• Board gaps: leave a consistent gap (usually 5–8 mm for timber, check manufacturer spec for composite) between boards for drainage and airflow, and slightly wider again for green/unseasoned timber that will shrink as it dries.
• Fixings: use stainless steel decking screws (grade 316 if you're anywhere near the coast — standard 304 will rust in a salt air environment far faster than people expect). Hidden fixing clip systems give a cleaner look but cost more and take longer to install than face-fixing.
• Board direction: boards typically run perpendicular to the house for a standard look, or on a diagonal for a more dynamic pattern — diagonal patterns use more material and offcuts, so budget extra waste.

Step 4 — Is your site sloped? Check for a retaining wall first

If your deck is going on a sloped block, it's often cheaper and more practical to build a retaining wall and level the ground underneath rather than building a very tall deck frame with long, exposed posts. A retaining wall also gives you usable space underneath the deck instead of an awkward crawl space.

As a rough guide, if you're looking at post heights over about 1.5–2 m on the low side of the slope, it's worth costing both options — a taller deck frame versus a retaining wall plus a lower, simpler frame. Retaining walls over about 600 mm–1 m generally need engineering and, depending on your council, a permit.

Step 5 — Stairs, balustrades and curved detailing

Any deck more than one step above ground level needs stairs, and under the National Construction Code, a deck more than 1 m above the ground generally needs a compliant balustrade — check your local council's specific requirements before you finalise the design, since this affects your frame height and layout decisions from Step 2 onwards, not just the finishing stage.

If you're after a curved stair stringer, curved deck edge or a bent timber feature (arbour, curved fascia board), that's done with kerf bending — a series of parallel saw cuts that let a straight board follow a curve without steaming or laminating.

Permits — when you need one in Queensland

Generally, a low deck (under about 1 m high, not attached to habitable rooms in a way that changes fire ratings) can often be built as exempt or self-certified work in Queensland, but this changes quickly once you're attaching to the house structurally, going higher, or the deck is near a boundary. Rules vary by council and by exact deck height and attachment method — always check with your local council or a building certifier before starting, rather than assuming your deck is exempt.

What not to skimp on

• Footings. Already covered above — this is structural, not decorative. Don't guess.
• Fixings near the coast. Standard 304 stainless will rust visibly within a couple of years in a salt air environment. Pay the extra for 316 grade if you're anywhere near the coast.
• Drainage fall. A deck with no fall pools water and rots faster, even with treated timber. Build the fall in from the start — it's much harder to fix afterward.
• Buying enough decking in one batch. Same issue as any timber product — colour and grain vary between batches, and running short mid-job means a visible mismatch.

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