Designing a deck – Starting from the top

When designing a deck at home it is important to plan out the whole project first. The timber chosen for a deck has flow on effects. The thickness of the decking can determine the joist spacings – the joist spacings can determine the maximum bearer spacings – the bearer size can determine the maximum pile spacings.

We can start by planning out the decking first before moving on to the structural components of a hypothetical 4.3m x 6m deck. We have pulled some relevant information from NZS3604.

In most cases this example deck can be built by anyone without building consent. This example deck is only 900mm above the ground so will not need a safety barrier or handrail as the fall height is less than 1m.

As you will see there are a few different ways to frame out a deck. The main option we have gone with may not necessarily be the best but it uses the same size bearers and joists to help simplify things.

Country and Coast Construction - Wellington Builders
This diagram from NZ3604 helps show the framing components of a deck. We can use it to add some theoretical measurements.

We have broken the process down into its main components to try and simplify the process a little. Each component has a flow on effect to the next component.

The key areas to focus on are:

  • Type and thickness of decking
  • How the decking is laid out
  • Joist size and spans
  • Bearer size and spans
  • Piles spacing and bracing


There are many types of decking available in NZ. Each has its own unique look, as well as certain benefits and disadvantages. Softwoods such as pine are usually less dense than exotic hardwoods and therefore are easier to cut and dont require pre drilling before being screwed/nailed down.

Some common type of decking are:

New Zealand timber

  • Treated Pine
  • Macrocarpa

NZ timber is cheaper, readily available and sustainable. Available in 19mm and 32mm thicknesses. These types of decking are often oiled or stained to add colour and help protect them for years to come.

Country and Coast Construction - Wellington Builders
140mm x 32mm smooth pine decking


  • Kwila
  • Purple Heart
  • Mahogany
  • Garapa
  • Vitex

Hardwoods are durable and look fantastic but are more expensive and require more maintenance. Many hardwoods will ‘bleed’ colour out over the first year and become sliver or grey. To protect their colour use oils or stains.

Country and Coast Construction - Wellington Builders
Purple heart decking

Be sure to check your hardwoods come from sustainable forests. Exotic hardwood decking is usually only available 19mm thick.

Composite decking

Composite decking as the name suggests is made up from a variety of different materials. They are usually eco friendly and sustainable made from a combination of recycled plastics. Composite decking is the most expensive option however requires very little maintenance.

Laying the decking

Planning how to lay out your decking first helps determine the over all size of the deck and where to frame out the joists.

Decking can be laid in a couple of different ways. One way is to create a picture frame (or boarder)around the perimeter of the deck. This is more visually appealing but takes more time and skill as well as addition joist framing to support the boarder.

Design tip – design your deck to use all full width boards as you dont want to rip down any decking.

Country and Coast Construction - Wellington Builders
Vitex deck with a negative detail and cladded balustrade

The alternative is to run all the decking boards right to the edge of the deck where they can all be cut in a straight line. For the purpose of this example we are running our boards right to the edge.

Design tip – if you are planning to install base boards around your deck framing then your decking should overhand your boundary joists by the same thickness of the base board. e.g. if you have a 20mm thick base board then overhang your decking 20-25mm past your boundary joist.

The type of decking chosen will determine the size and spacing of the joists, bearers and piles.

NZS 3604 – 7.4.3 states “The thickness of the decking shall be not less than:

  1. (a)  32 mm for 600 mm joist centres; or
  2. (b)  19 mm for 450 mm joist centres.

Next we move on to Piles, Bearers and Joists