For 150x50 deck joists in NZ, the maximum allowable span typically ranges from around 1.8 m to 2.4 m, depending on the timber grade, joist spacing, and the load the deck is designed to carry. These figures come from NZS 3604:2011 — the standard that governs timber-framed buildings and decks in New Zealand — and should always be confirmed against the actual span tables in that document or with a qualified professional.
Understanding NZS 3604 Span Tables
Most residential decks in NZ are designed under NZS 3604:2011, which includes span tables for floor joists that also apply to deck framing. The tables account for:
- Timber grade — No. 1 Framing, MSG8, MSG10, or LVL all have different allowable spans
- Joist spacing — commonly 400 mm, 450 mm, or 600 mm centres
- Load width (tributary area) — how much of the deck each joist supports
- Live load — residential decks are typically designed for a 2.0 kPa live load
As a general guide for SG8 (MSG8) timber at 600 mm centres, 150x50 joists can span roughly 1.8 m to 2.0 m. Tighten the spacing to 400 mm centres and you may get closer to 2.2–2.4 m. However, always treat these as indicative — the actual span tables in NZS 3604 are the authoritative source.
Timber Grade Matters
Not all 150x50 timber is equal. Common grades you'll find at NZ suppliers include:
| Grade | Typical Use | Relative Span Capacity |
|---|---|---|
| SG6 / MSG6 | Light framing | Lower |
| SG8 / MSG8 | General framing | Moderate |
| SG10 / MSG10 | Structural framing | Higher |
| LVL (Laminated Veneer Lumber) | Engineered timber | Highest |
For outdoor decks, H3.2 treated timber is the minimum required for joists that are sheltered from direct rain, while H4 treatment is needed for joists in ground contact or exposed to wetting and drying. Always check the treatment level stamped on the timber or ask your supplier at Bunnings, Mitre 10, or PlaceMakers.
Joist Spacing and Decking Boards
Your joist spacing also needs to suit your decking boards. Common decking thicknesses (e.g. 40 mm or 50 mm) have their own maximum spans between supports. If you're using thinner decking, you may need to reduce joist spacing to 400 mm or even 300 mm centres — which in turn affects how far your joists can span between bearers.
Cantilevers
If your deck design includes a cantilevered joist (one that extends beyond the bearer), NZS 3604 limits the cantilever to a proportion of the back-span — typically no more than one-quarter of the back-span length. Cantilevers need careful design, especially on elevated decks.
Do You Need a Building Consent?
This is an important question for any NZ deck project. Under Schedule 1 of the Building Act 2004, some low-level decks may be exempt from building consent — but the exemptions have specific conditions around height, area, and proximity to boundaries. As a general rule:
- Decks no more than 1.5 m above ground and meeting other Schedule 1 criteria may be exempt
- Decks above 1.5 m almost always require a building consent
- Decks attached to the house may trigger additional requirements under NZ Building Code clauses E2 (external moisture) and B1 (structure)
Always check with your local council before starting work. Exemptions vary and councils can have their own conditions. You can find general guidance at MBIE Building Performance.
Checking Your Specific Situation
Before you finalise your joist sizing, confirm:
- The timber grade and treatment level of the timber you're buying
- The joist spacing you plan to use
- The span between bearers in your design
- Whether the deck is freestanding or attached to the house
- The height of the deck above ground (affects consent requirements and safety barriers)
For anything outside standard NZS 3604 parameters — such as heavy loads, unusual spans, or complex designs — you'll need input from a Chartered Professional Engineer (CPEng) or a Licensed Building Practitioner (LBP).
When to Call a Professional
Deck building looks straightforward, but structural mistakes can be costly and dangerous. You should engage a qualified professional if:
- Your deck is elevated (above 1.5 m) or attached to the house
- You need a building consent — an LBP must carry out or supervise restricted building work
- You're unsure which span table applies to your timber grade or configuration
- The ground is sloped, unstable, or near a boundary
A qualified deck builder or LBP will know exactly which span tables apply and can ensure your deck is safe, compliant, and built to last. You can find experienced local deck builders on tradedirectory.co.nz. Check that any tradesperson you hire holds a current Licensed Building Practitioner licence for the relevant work category.
Licensed Building Practitioner (LBP)
