NSW Deck Building Standards: The Construction Code, Explained by a Carpenter

We've covered the approval question elsewhere — whether your deck is exempt, needs a Complying Development Certificate, or a full DA. This post is about the other half: once you've got the green light, how does the deck actually have to be built to meet the code?

Because "approved" and "built to standard" are two different things. A deck can be signed off on paper and still fail the construction rules — the structural span tables, the timber durability grades, the stair geometry, the termite provisions — and a certifier will pull you up on any of them at inspection.

Here are the building-code rules we build to on every deck, in plain English.

Structural: footings, bearers and joists

A deck is only as good as what's holding it up. The construction side is governed by two standards:

• AS 2870 (Residential slabs and footings) covers the footings — the posts and piers in the ground.
• AS 1684 (Residential timber-framed construction) covers the bearers and joists above, and gives the span tables that tell you how far a piece of timber can span before it needs another post under it.

The practical version: your post spacing, bearer size, and joist spacing have to match the span table for the timber you're using and the live load the deck will carry. You can't just copy a mate's deck — a spotted gum bearer spans differently to a treated pine one, and a deck 2 metres off the ground has different footing requirements to one sitting on the ground.

A deck also has to be designed to hold people, not just look pretty. The structural actions standard (AS 1170) sets the minimum live load — the deck has to carry a crowd, furniture, and movement, not just a barbecue and a couple of chairs.

Timber durability: the right wood in the right place

Outdoor timber rots if you use the wrong grade. The durability grades are set by AS/NZS 1604, and they're labelled H1 through H6. The two you'll hear about on a deck:

• H3 — for timber above ground but exposed to the weather. This is your bearer, joist, and fascia grade for most decks.
• H4 — for timber in contact with the ground, or a post embedded in concrete. This is your post grade.

Timber within 150mm of ground level has to be preservative-treated — that's the line. Below it, ordinary timber won't last. We use H4 treated pine or a durability-class 1 hardwood (spotted gum, ironbark) for posts, and H3 framing for the rest.

Decking boards are graded separately by their natural durability class (1 being the most durable). Spotted gum and ironbark are class 1; merbau (kwila) is class 2 and is the common budget board. All of them are fine for an exposed deck — the difference is how often you'll be re-oiling and how many decades before they soften.

Termite protection: don't give them a bridge into your house

Termites are a real issue across the Northern Beaches, and a deck bolted to your house can give them a hidden highway straight in.

The termite standard is AS 3660.1. The rules that matter for a deck:

• Posts and bearers near the ground have to be termite-resistant or treated.
• If the deck is attached to the house, you keep the inspection gap — the gap between the deck and the wall that lets a termite inspector see what's going on. We never block weep holes or bridge the slab edge.
• A termite barrier on the house has to stay intact. If the deck compromises it, the barrier has to be extended or the deck redesigned.

A cheap deck that costs you your termite warranty isn't cheap.

Stairs: the geometry the code insists on

Deck stairs aren't a free-for-all. The National Construction Code (Part 3.9.1) sets the dimensions, and a certifier checks them:

• Riser (the step height): no more than 190mm, no less than 115mm.
• Going (the step depth): no less than 240mm, no more than 355mm.
• The comfort check: twice the riser plus the going has to land between 540mm and 700mm.
• No more than 18 risers in a single flight without a landing.
• Every riser in the flight has to be identical — you can't have an 180mm step and a 200mm step in the same staircase.

The reason a lot of DIY decks get a defect notice is the last step. The landing step is often a different height to the rest, and that's the one that trips people — literally and on the inspection report.

Balustrades and handrails

If someone could fall more than 1 metre off your deck, you need a balustrade, and it has to be at least 1 metre high (865mm on a stairway). Openings can't let a 125mm sphere through — that's the gap a small child could slip or get stuck in.

If the deck is more than 5 metres off the ground, any glass in the balustrade has to be laminated safety glass, not single-pane toughened — the NCC 2022 change, so the glass fails safe if it breaks.

We build stainless wire rope, glass, timber, and steel balustrades. The right one depends on your height, your view, and your budget, and the engineering has to stack up for all of them.

Bushfire zones: the rule that catches Northern Beaches blocks

If your lot is mapped as bushfire-prone — Terrey Hills, Ingleside, Duffys Forest, Church Point, Oxford Falls and the foreshore pockets — your deck has to meet a Bushfire Attack Level (BAL) rating. The higher the rating, the more the materials are restricted:

• Decking within 300mm horizontally and 400mm vertically of a window or glazed door has to be non-combustible or bushfire-resistant.
• At higher BAL ratings the frame and board selection narrows — steel framing and the denser hardwoods become the safe options, and some composites don't qualify.

We check the bushfire map for every lot before we quote. It's a five-minute check and it changes the materials list.

Slip resistance on stairs and ramps

Outdoor steps and ramps need to be slip-resistant when wet. The slip standard is AS 4586, and on a coastal deck salt spray and rain make it a real consideration, not a box-tick. We select board profiles and finishes that hold grip wet — grooved or reeded boards laid the right way, and anti-slip inserts on stairs where the slope is steep.

What we do

Every deck we build is designed and certified to the construction code — not just approved. We work out the footing and span design against AS 2870 and AS 1684, the timber and post grades against AS/NZS 1604, the termite provisions against AS 3660, and the stairs, balustrade and bushfire rules against the NCC. Then the certifier signs off the finished build.

That's the difference between a deck that passes and a deck that lasts.

Planning a deck? Let's look at your block first

Book a free site measure and we'll tell you what the code requires on your specific site — the footings, the timber, the stairs, the balustrade, and the bushfire rating — before you commit to anything.

Call us on 0482 088 410 or get a free quote online.

MJ Leckie Carpentry — Licensed Carpenters, NSW Licence 353438C