NZS 3604 Explained: When It Applies to Your Build (and When It Doesn't)

What Is NZS 3604?

NZS 3604 Timber-framed buildings is a Standards New Zealand publication that sets out prescribed, "off the shelf" ways to design and build low-rise timber-framed buildings so they meet the Building Code without needing a full specific engineering design. It is used for the vast majority of New Zealand's light timber-framed homes, which is why builders, designers and homeowners run into it constantly, even if they've never opened the document itself.

When NZS 3604 Applies

NZS 3604 is a set of rules of thumb for straightforward situations. It generally applies when a building:

• Is no more than 10 m high and no more than three storeys, with any third storey sitting within the roof space
• Is built on what the standard defines as "good ground" — ground that can carry the building's load without significant settlement
• Stays within the standard's prescribed spans, bracing, fixing and foundation details for its wind zone and other site conditions

In Canterbury specifically, the definition of good ground excludes land subject to liquefaction or lateral spread, which is one reason a geotechnical assessment matters more in some areas than others.

Why "Good Ground" Matters

Good ground isn't the same as cleared ground. Cleared ground is simply the level reached after stripping topsoil and vegetation — it says nothing about strength. Good ground is a technical standard: under NZS 3604 it needs an ultimate bearing capacity of around 300 kPa, sustained over time. Builders and assessors check for this by looking for warning signs (settlement in neighbouring buildings, fill, soft or expansive clay, peat) and, where needed, testing with a scala penetrometer. If the ground doesn't meet that bar, the foundation falls outside NZS 3604 and needs specific engineering design instead.

What NZS 3604 Doesn't Cover

This is where many builds quietly step outside the standard without anyone noticing:

• Any detail not shown in the standard. If your design doesn't match one of its prescribed details, that element needs specific design.
• Any variation to a standard detail — different fixings, different spacings — also needs specific design.
• Roof trusses. NZS 3604 gives installation rules and load/span limits, but truss design itself is always specifically engineered.
• Cladding. Cladding compliance sits under a separate Acceptable Solution (E2/AS1), and the cladding chosen can change the framing sizes, bracing and foundation setout needed — so these all need to be considered together, not in isolation.

How It Fits the Building Code

NZS 3604 is referenced, with some modifications, in Acceptable Solution B1/AS1 for Building Code clause B1 Structure, and it also provides a means of complying with clause B2 Durability for relevant building elements. Following it is one accepted path to compliance — it is not the only path, and a designer can choose an alternative method instead. Structural work on most residential buildings is also restricted building work, meaning it must be carried out or supervised by a Licensed Building Practitioner.

Why This Matters for Your Project

Whether a job can be built "to NZS 3604" or needs specific design changes the paperwork, the time and sometimes the cost of a project. A quick check early — ground conditions, building height and shape, cladding choice, any unusual spans or openings — can save a lot of back-and-forth later. If your project looks straightforward but doesn't quite fit the standard's boxes, that's exactly the kind of thing worth running past an engineer before pricing or consenting.

Not sure whether your build sits inside or outside NZS 3604? Get in touch with Fil Engineers and we can take a look.

Sources

• MBIE Building Performance – Using NZS 3604: Timber-framed buildings
• BRANZ – Good ground versus cleared ground
• MBIE Building Performance – Updating the timber-framed building standard (NZS 3604)

This article is general information, not site-specific engineering advice. Every site and building is different — talk to a structural engineer about your particular project.