Designing and building your own deck

Building your own deck falls into the top end of do it yourself work and is a satisfying achievement for any home owner. Designing a deck that meets all requirements of the NZ Building Code is a complicated and daunting task. We have broken down each part of the process we go through when designing a new deck build.

Spending time during the planning stage will help make the project run smoother, allow for accurate costing and help with ordering materials.

Country and Coast Construction - Wellington Builders
Nearly finished framing out a large deck in Wellington

Heres some helpful tips to consider when designing and building a timber deck compliant with NZS 3604.

Size, shape, height and style are all different factors of deck design and effect each component.

If you need help designing a deck Country and Coast can help design and supply materials for you to build.

Basic components of a deck


Piles are the posts that get concreted into the ground and support the weight of the deck structure. Ordinary pile footings for decks must have a minimum depth of 200 mm.

Bracing piles (see bracing requirements below) need to have a 350mm x 350mm wide and 450mm deep concrete footing with a diagonal brace connecting to another bracing pile.

All piles must be lifted off the bottom of the hole to allow at least 100mm of concrete under the pile.


Bearers are the timbers that sit on top of the piles and provide the main support for the joists. The size of the bearers is determined by the spacing between the piles.

Country and Coast Construction - Wellington Builders
Basic components of a deck


Joists sit on top of the bearers running perpendicular to the decking. They are the timber in which the decking is nailed or screwed to. The type of decking and space between bearers determine the size and spacing requirements for floor joists.


Decking is the finished surface of the deck. There are many types of decking available from wide boards to skinny boards, softwoods and hard woods, thick and thin. Common sizes of decking boards are 90x19mm, 90x32mm, 140x19mm and 140x32mm.

Some technical stuff

To design your deck properly loaded dimension and bracing units need to be taken into account. Although it seems confusing it can be worked out relatively quickly.

Most people will chose to gloss over the technical stuff and jump straight to choosing decking.

Loaded dimension

First we will work out the loaded dimension as it determines what size and spacing some of our components need to be. These numbers will help us when selecting bearers and determining pile spacings.

Loaded dimension is a measure of the weight of construction supported by the timber member or beam. For more information check out BuildmagazineLoadedDimension.pdf

As the bearer is designed to support the weight of the deck we work out LD using the formula

(Span 1 + Span 2) ÷ 2 for a middle bearer and

Span 2 ÷ 2 + Span 3 for a bearer with joists cantilevering past.

Country and Coast Construction Wellington
Using the same diagram we can help explain loaded dimension.

Loaded Dimension can be a bit confusing to get your head around. If in doubt exaggerate this number and you should be safe.

Bracing requirements when designing a deck

To work out the amount of bracing you need, refer to NZS 3604 Table 5.8 for single-storey buildings. Use the section for light subfloor cladding, light wall cladding, and light roof options for a 0 – 25° pitch roof.

Because the deck is not subject to the same windage as a full-height building, use half the demand (in bracing units per m2) given in the table, then multiply that figure by the soil class for the appropriate earthquake zone. Bracing lines must be at 5m maximum centres for subfloors.

In most cases a deck requires 7.5 bracing units per m2 (BU/m2).

A rule of thumb to exceed the bracing requirements set out in NZS3604. Bracing piles need to be incased in a minimum 350x350mm footing 450mm deep with diagonal braces spanning between.

The standard says the bracing units must be spread evenly across the deck where possible.

Next we look at starting with decking.