Bearer spans for a residential timber deck frame in NZ are governed by NZS 3604:2011 (Timber-framed buildings), and the correct span depends on several variables — including the bearer size, timber species and grade, joist spacing, and the load the deck is designed to carry. There is no single universal answer, but this guide walks you through the key principles so you can plan your project with confidence.
What Is a Bearer and Why Does the Span Matter?
A bearer is a horizontal structural member that sits on top of the piles and supports the joists above it. The bearer span is the distance between the supporting piles (or posts) beneath it. Get this wrong and you risk a deck that bounces, sags, or — in the worst case — fails structurally. Correct sizing protects both the people using the deck and the long-term integrity of your home.
NZS 3604 and Span Tables
NZS 3604:2011 is the primary New Zealand standard for light timber framing, and it includes span tables specifically for deck bearers and joists. These tables are the go-to reference for builders and designers working on standard residential decks.
The span tables take into account:
- Timber size (e.g., 100×50, 150×50, 190×45 mm)
- Timber grade (No. 1 Framing, MSG8, MSG10, LVL, etc.)
- Joist spacing (typically 400 mm, 600 mm, or 800 mm centres)
- Load width (the tributary area of deck the bearer supports)
- Deck live load (residential decks are generally designed for 2.0 kPa)
As a general guide only, a 190×45 mm SG8 bearer at 1,800 mm load width might span around 1.8–2.4 m between piles, while a 200×50 mm LVL bearer can often span further. However, you must always verify against the actual span tables in NZS 3604 or have a qualified person do this for you — do not rely on rough rules of thumb for structural decisions.
Common Timber Sizes and Approximate Spans
The table below gives a very rough indication of what is commonly used in NZ residential decks. Always confirm with NZS 3604 span tables or a licensed building practitioner before building.
| Bearer Size (mm) | Typical Grade | Approximate Max Span (guide only) |
|---|---|---|
| 100 × 50 | SG8 | Up to ~1.2 m |
| 150 × 50 | SG8 | Up to ~1.8 m |
| 190 × 45 | SG8 | Up to ~2.4 m |
| 200 × 50 | LVL | Up to ~3.0 m |
These figures are indicative only and will vary depending on load width, joist spacing, and other site-specific factors.
Timber Species and Treatment
In NZ, deck framing timbers must be treated for outdoor use. The MBIE Building Performance guidelines and NZS 3640 specify the required Hazard Class for ground contact and above-ground applications:
- H3.2 — above ground, exposed to weather (joists, bearers not in ground contact)
- H4 — in-ground contact (piles)
Commonly used species include treated radiata pine (available from Bunnings, Mitre 10, and PlaceMakers), as well as hardwoods like kwila or garapa for decking boards. Always check the treatment stamp on the timber before purchasing.
Pile Spacing and Its Relationship to Bearer Span
Bearer span and pile spacing are essentially the same thing — the distance between the tops of your piles determines how far your bearer must span. Reducing pile spacing (i.e., adding more piles) allows you to use smaller, lighter bearers. This is a useful trade-off when working with difficult ground conditions or when you want to minimise material costs.
Typically, pile spacings of 1.2 m to 2.4 m are common for residential decks, but this must be confirmed against the span tables for your specific setup.
Do You Need a Building Consent?
This is one of the most important questions for any deck project. Under the NZ Building Act 2004, many decks require a building consent. However, Schedule 1 of the Act lists certain exemptions — for example, some low-level decks may be exempt, but the rules are specific and have conditions.
Always check with your local council before starting work. Exemptions vary depending on deck height, proximity to boundaries, and whether the deck is attached to the house. You can find general guidance at MBIE Building Performance, but your council has the final say.
When to Call a Professional
While a confident DIYer can build a straightforward low-level deck, there are situations where you should engage a qualified professional:
- The deck is more than 1.0–1.5 m above ground (fall risk and structural complexity increase significantly)
- The deck is attached to the house (affects weathertightness and structure)
- You are unsure about soil bearing capacity or ground conditions
- A building consent is required (an LBP-licensed builder must carry out or supervise restricted building work)
- You want peace of mind that the structure is safe and compliant
For electrical work on or near the deck (lighting, power outlets), this is restricted electrical work and must be carried out by a registered electrician — see the Electrical Workers Registration Board for more information.
You can find qualified local deck builders and LBPs on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
