If you’re planning a new deck in Sydney, you’ve probably spent plenty of time choosing between timber and composite boards. But the framing underneath your deck — the bearers, joists and posts that carry the whole structure — is just as important. Get the framing wrong, and you’ll end up with a bouncy, unsafe deck that won’t last in Sydney’s coastal climate.
In this guide, we’ll walk through how deck framing works in Sydney homes, what joist spacing you need for different board types, and why getting the substructure right matters more than any aesthetic choice on top.
What Is Deck Framing?
Deck framing is the structural skeleton beneath your decking boards. It’s built in layers, each carrying the weight above it down to the ground. The main components are:
- Posts — vertical supports set into footings, usually treated pine or steel
- Bearers — the primary horizontal beams that sit on top of the posts and span the deck’s length
- Joists — smaller beams that run across the bearers, creating the surface the boards attach to
- Noggins — short blocking pieces between joists that prevent twisting and add rigidity
- Ledger board — where the deck attaches to the house, a bearer bolted to the wall frame (for attached decks only)
Each element must be sized and spaced according to the span, the expected load, and the decking material on top. Australian Standard AS 1684 (Residential Timber-Framed Construction) and AS 3660.2 set out the engineering requirements that apply to Sydney decks.
Joist Spacing: The Key to a Solid Deck
Joist spacing is the distance between the centre of one joist and the next. Get this wrong and your deck will flex, bounce, or worse, sag over time. The right spacing depends on the type of decking board you’re using:
Timber Decking Joist Spacing
For standard hardwood timber boards (typically 90mm to 140mm wide), joists should be spaced at 450mm centres maximum. This is the industry standard for most Sydney residential decks and provides a firm, stable feel underfoot.
If you’re using thinner boards, or want a particularly rigid surface, 400mm centres will give even less flex. For wider boards (140mm+), stick to 450mm — wider boards at wider spacing tend to cup and twist.
Composite Decking Joist Spacing
Composite decking boards are more flexible than timber and require tighter joist spacing. Most manufacturers — including ModWood, Trex and Evalast — specify 400mm centres maximum, and some require 300mm centres for diagonal patterns or commercial applications. Always check the manufacturer’s technical data sheet, because not following it can void your warranty.
At 450mm spacing, a composite board can sag between joists, creating an uneven surface that looks and feels poor. The tighter spacing adds a small cost in extra timber and labour, but it’s essential.
Plywood and Tile Substrates
If you’re building a deck with tiles or a waterproof membrane on top, joist spacing typically tightens to 300mm centres, with a structural plywood substrate over the joists. This removes any flex that would crack tiles or split waterproofing membranes.
Bearer Spans and Post Sizing
The spacing between posts and the span of your bearers determines how much timber you need and how many footings you’ll dig. A typical Sydney low-level deck using treated pine H4 bearers (190x45mm) can span around 1.8 metres between posts. Hardwood bearers can span further, but cost significantly more.
For taller decks (above one metre), the engineering becomes more critical. Taller posts are more prone to buckling, and lateral wind loads increase. This is where a structural engineer’s input becomes essential, particularly for second-storey decks where the framing must support the deck, its occupants, and any future loads like furniture or planters.
Choosing the Right Framing Timber
In Sydney, the most common framing timber is treated pine (T2 or T3 for above-ground, H4 for in-ground). It’s cost-effective, readily available, and performs well when properly protected. For premium or exposed applications, hardwood framing (Spotted Gum, Ironbark) offers greater strength and natural durability, but at a substantially higher price.
One critical mistake we see in Sydney is using the wrong treatment level. Above-ground framing should be T3 or H3 treated. Posts in the ground must be H4. Using a lower treatment rating means the timber will rot prematurely, especially in Sydney’s humid, coastal air.
Ventilation and Clearance Underneath
Sydney’s climate means moisture management is critical. The space under your deck needs airflow to prevent rot and mould. As a rule, you want at least 150mm of clearance between the ground and the bottom of your bearers, with cross-ventilation from at least two sides.
If your deck is low to the ground, consider a free-standing design with a gap between the house and deck, or install ventilated skirting panels. Covered ground (like paving with gaps) also helps. Without airflow, moisture gets trapped under the deck and starts breaking down the timber from below.
Common Framing Mistakes to Avoid
From years of building and repairing decks across Sydney, here are the framing errors we see most often:
- Over-spanned joists — pushing joist spacing beyond 450mm to save timber, resulting in a bouncy deck
- Incorrect fasteners — using galvanised screws in hardwood which rust and snap near the coast
- No nodding — skipping noggins, which lets joists twist and twist the boards above
- Under-treatment — using H3 where H4 is needed for in-ground durability
- Missing weep holes — not drilling weeps in bearers bolted to a wall, so trapped water rots the identification wall
Why Your Framing Matters Long-Term
Your decking boards sit on the surface — you can replace those if they weather. But your framing is the permanent part. Replacing framing means tearing the entire deck down to the ground. That’s why getting the sizing, spacing, treatment and fastening right from day one saves thousands of dollars and years of headaches.
A well-engineered frame in properly treated pine, with correct joist spacing for your board type, will last 25+ years in Sydney’s climate. A poorly built one may start showing problems within 5.
Need Help With Your Deck Project?
Whether you’re planning a low-level garden deck or a second-storey entertaining area, getting the framing right is the foundation of a deck that lasts. At 5 Star Decking, we’ve built decks across Sydney from the Northern Beaches to the Inner West, and every one starts with a properly engineered substructure.
Give us a call on 0433 334 415 or contact us here for a free quote. We’ll assess your site, discuss your options, and design a deck with the right framing for your space, budget and materials.
5 Star Decking — NSW Builder’s Licence No. 395250C. Fully licensed, insured and serving Sydney wide.
