Getting the timber sizing right for your deck is one of the most important structural decisions you'll make. Use undersized joists or bearers and your deck could bounce, sag, or worse — fail under load. This guide walks you through the key factors that determine what size timber you need, based on NZ conditions and the NZ Building Code.
Why Timber Size Matters
Deck joists and bearers are the structural backbone of your deck. Joists are the horizontal members that run between bearers and directly support the decking boards. Bearers are the larger members that span between posts and carry the joists. Both need to be sized correctly for the loads they'll carry — people, furniture, BBQs, and the weight of the timber itself.
In New Zealand, residential decks must comply with NZS 3604, the timber-framed buildings standard, and the NZ Building Code. If your deck is more than 1.5 metres above ground, or attached to the house in a way that affects the structure, a building consent is likely required — check with your local council or visit MBIE Building Performance for guidance.
Key Factors That Determine Timber Size
There's no single answer to "what size do I need" — it depends on several variables:
- Span – the distance the joist or bearer must bridge between supports
- Spacing – how far apart joists or bearers are placed (centres)
- Load – residential decks are typically designed for a live load of 1.5 kPa (about 150 kg per square metre)
- Timber species and grade – H3.2 treated radiata pine is the most common choice for NZ outdoor decks
- Whether the deck is freestanding or attached to the house
Typical Joist Sizes as a Guide
The table below gives general guidance for H3.2 treated radiata pine joists at common spacings. These are indicative only — always verify against NZS 3604 span tables or get your plans checked.
| Joist Spacing | Joist Span up to ~1.8 m | Joist Span up to ~2.4 m | Joist Span up to ~3.0 m |
|---|---|---|---|
| 400 mm centres | 100 × 45 mm | 140 × 45 mm | 190 × 45 mm |
| 600 mm centres | 140 × 45 mm | 190 × 45 mm | 240 × 45 mm |
Note: These are approximate starting points. Actual sizes depend on your specific timber grade, load, and configuration. Always cross-reference with the NZS 3604 span tables.
Typical Bearer Sizes as a Guide
Bearers carry more load than joists, so they're generally larger. A common rule of thumb is that bearer depth increases with span and the number of joists it supports.
| Bearer Span (post to post) | Indicative Size (H3.2 radiata pine) |
|---|---|
| Up to ~1.8 m | 140 × 45 mm (single or doubled) |
| Up to ~2.4 m | 190 × 45 mm or 190 × 90 mm |
| Up to ~3.0 m | 240 × 45 mm doubled, or engineer-specified |
For longer spans or heavier loads, a structural engineer should specify the bearer size. Doubled-up bearers (two pieces bolted together) are common and can be more practical than sourcing large single sections.
Timber Treatment: H3.2 Is the Minimum
For any timber in an outdoor deck that's exposed to the weather, H3.2 treated timber is the minimum required by the NZ Building Code. Timber in contact with or close to the ground requires H4 or H5 treatment. Using the wrong treatment level will void your consent and lead to premature rot.
You'll find treated structural timber at Bunnings, Mitre 10, and PlaceMakers. Always check the treatment stamp on the timber before purchasing.
Joist and Bearer Spacing Tips
- Joist spacing is typically 400 mm or 600 mm centres for residential decks. Closer spacing (400 mm) gives a stiffer feel underfoot and is recommended for diagonal decking layouts.
- Bearer spacing (post to post) is usually kept to 1.8–2.4 m to keep bearer sizes manageable.
- Keep joist cantilevers (overhangs past the last bearer) to no more than about one-quarter of the back span — check NZS 3604 for exact limits.
Do You Need a Building Consent?
Many decks in NZ require a building consent, particularly if they are:
- More than 1.5 m above the ground at any point
- Attached to the house structure
- Over a certain area (rules vary by council)
Even for exempt decks, the work must still comply with the Building Code. BRANZ publishes excellent guidance on deck construction best practice that's worth reading before you start.
When to Call a Professional
Deck framing is restricted building work (RBW) if it is attached to the house or requires a building consent. In these cases, the work must be carried out or supervised by a Licensed Building Practitioner (LBP). Even for simpler freestanding decks, if you're unsure about sizing, spans, or consents, it's well worth getting a qualified carpenter involved.
A good carpenter will size your joists and bearers correctly from the start, saving you costly mistakes. You can find licensed local carpenters and deck builders on tradedirectory.co.nz.