Building a deck: timber, fixings and consent in NZ

Decks look simple, but they sit outside in the weather for years, carry people and often stand above the ground. Most deck failures come from the wrong timber treatment, the wrong fixings or poor drainage. This guide covers the basics to get right before you start.

Check consent and barriers first

In New Zealand, a deck where nobody can fall more than 1.5 metres is generally exempt from building consent. Above that, you will usually need consent. Exempt work still has to meet the Building Code.

If someone could fall more than 1 metre from the deck, it needs a safety barrier. Barriers have their own rules for height, strength and gaps, so check them before you price.

Also check district plan rules with the council, such as boundary setbacks, height limits and site coverage. A deck that does not need building consent can still need a resource consent.

Pick the right timber treatment

Timber in NZ is treated to hazard classes. Use the right class for where each part sits:

Part of the deck	Typical treatment
Decking boards	H3.2
Joists and bearers clear of the ground	H3.2
Piles and posts in the ground	H5

Ask the supplier to confirm the treatment on the label or docket, and keep a record. Cut ends of treated timber should be sealed with a suitable preservative, because treatment does not always go all the way through.

Use fixings that last

Treated timber and the weather are hard on metal. Fixings that rust will stain the timber and can fail.

• Use fixings rated for the treated timber you are using.
• Near the coast, use stainless steel for decking nails or screws and for exposed fixings. Salt air eats through ordinary galvanised fixings quickly.
• Use proper joist hangers, brackets and bolts for the structure, as set out in the design or the timber framing standard.
• Do not mix metals that react with each other.

Keep it clear of the ground and let it drain

Water is the enemy of a deck. Plan for it:

• Leave enough ground clearance under bearers and joists for air flow and drying.
• Leave a gap between decking boards so water runs off and the boards can swell.
• Make sure the ground under the deck does not hold water against piles.
• Do not trap water against the house. Where a deck meets the cladding, follow the flashing and clearance details carefully, because leaks here can damage the house.

Lay out the frame

• Set out piles in straight lines with the spacing from the design.
• Get the bearers level and the joists true before laying boards.
• Check the spans for the timber size and grade you are using. Undersized joists give a bouncy deck.
• If the deck is attached to the house, the fixing to the house needs to be designed and detailed properly.

Laying the boards

• Lay a test row first to check the spacing and the board layout at both ends.
• Predrill near the ends of boards to stop splitting.
• Keep fixings in neat straight lines.
• Stagger joins and land them over a joist.

Steps and edges

Steps off a deck are where people trip. Keep every riser the same height and every tread the same depth, so feet do not have to guess. Make treads wide enough for a whole foot, and add a handrail where the steps are steep or long. Around the edge of the deck, finish the board ends neatly with a fascia board, and seal cut ends before it goes on. A deck that sits low to the ground still needs air flow under it, so do not close in the sides completely.

Pricing a deck

Break the price into piles and excavation, framing, decking, barriers, steps, fixings, waste removal and labour. Add an allowance for hard digging if the ground is unknown. Write down what is included and excluded so there are no surprises. See how to write a scope of work clients understand, which uses a deck as its worked example.

Quick checklist

• Consent need checked, including district plan rules
• Barrier planned if the fall is over 1 metre
• H5 piles in the ground, H3.2 framing and decking
• Stainless or suitably rated fixings, stainless near the coast
• Ground clearance, board gaps and drainage planned
• Spans checked for the timber size