How to Design a Deck in NZ
- TopBuilders
- 3 hours ago
- 3 min read
Thinking about adding a deck to your home? It’s a classic Kiwi upgrade and a smart way to enjoy more of our outdoor lifestyle. But before you start digging holes or ordering timber, it’s worth getting the design right. This guide on how to design a deck in NZ walks you through each step to make sure your new outdoor space is both safe and stunning.
Before You Design Your Deck
Start by checking in with your local council. Most decks under 1.5 metres high don’t need consent, but site boundaries, pipes, and power lines might still affect your build. You’ll also need to comply with the NZS3604 Building Standard, which applies to all residential timber decks.
Design Your Deck Frame
Once you’ve sorted consents, grab some graph paper, a tape measure, and a pencil. The first thing to sketch is the overall shape and size of your deck. In our example, the deck runs 5.7m wide and 3.3m deep. We’re using 140mm x 32mm pine decking boards with a 3mm gap between each, which works out to 23 full-width boards across the shorter side. That means our frame needs to be about 3270mm deep to keep things clean and symmetrical.
Choose Your Decking Boards
Deck boards run across the joists, so it makes sense to plan their direction early. Joists are the horizontal framing members that support your decking. The thickness of your decking will determine how far apart the joists can go. At 32mm thick, we can space our joists up to 600mm apart—but we’re choosing 450mm for extra strength and to allow a longer joist span.
Work out Joist Size & Span
We’re going with 140 x 45mm joists, which at 450mm centres can span up to 2.35 metres under a 2kPa load. That span means we only need two rows of bearers under our joists, reducing the number of post holes we’ll need to dig. You can also cantilever your joists (let them hang over the bearers a bit) to stretch the look of your deck and minimise material waste. Our design includes a 900mm cantilever at each end, which is well within the 1100mm limit for our joist size.
Let’s work out how many joists we need. Our width is 5700mm, divided by 450mm max spacing, giving us 12.6 gaps. Rounding up to 13 spaces, we need 14 joists total. Simple enough.
Now to support those bearers. NZS3604 allows you to cantilever your bearers 200mm past the last post. In our case, we have a 5.7m long bearer. Take off 400mm for the overhang and we’re left with 5.3m to support. Divide that by four and you’ll need five piles, spaced around 1325mm apart—well within the 1.65m limit for a 140 x 90 bearer.
Next is lateral support. To keep your deck rigid and safe, you’ll need to add nogs between the joists, boundary joists at each end, and rows of nogs every 1.8m along the bearer lines.
Determine Deck Bracing
Then we get into bracing. Because NZ is earthquake-prone, decks need to resist lateral loads. The standard requires a minimum of four braced or anchor piles, symmetrically placed. Our deck is 23m², with a bracing demand of 7.5BU/m². That totals 172.5 bracing units. Since each anchor pile provides 120BU, technically two would do—but the code says four is the minimum, so we’re sticking with that.
Deck Piles & Foundation
Let’s talk foundations. Post holes need to be at least 450mm deep into solid ground. For standard piles, you’re looking at a minimum diameter of 290mm. For braced piles, you’ll go wider and deeper—up to 400mm diameter and 900mm deep.
Calculate Deck Materials
Once you’ve calculated everything and drawn your plan, the last step is to figure out how much material you’ll need. Multiply the number of boards by their length, and don’t forget to add 10% for waste. In our case, that works out to about 180 lineal metres of decking. You’ll also need around 672 screws, plus concrete for the post holes.
If You're Thinking About How to Design a Deck in NZ
Designing your own deck isn’t just about compliance. It’s about avoiding surprises halfway through the build, saving money on materials, and ending up with a space that works for your home and lifestyle. And if it ever starts to feel a bit too technical, there’s no shame in calling us to give you a hand with the drawings or building your deck.