Timber-framed buildings Standard NZS 3604 myth busting

The project to review the foremost Standard in New Zealand for the building and construction industry, Timber-framed buildings, NZS 3604:1999, is now well underway. The draft revised Standard is planned for public comment release in late 2009. Publication is scheduled for late 2010/early 2011.

A leadership group and a technical committee have been formed to review the Standard, and five industry-specific work groups have been set up to review specific sections of the Standard.

This article clarifies the relationship between NZS 3604 and the New Zealand Building Code (NZBC) by exploring two myths: 'Some people say NZS 3604 is the Building Code', and 'Some building consent applicants say they always need a producer statement when using NZS 3604'.

Relationship between NZS 3604 and the New Zealand Building Code

NZS 3604 provides an Acceptable Solution in the Compliance Documents of the NZBC. This means designs and plans in accordance with NZS 3604 are deemed to comply with the NZBC and must be accepted by building consent authorities (BCA). This provides the sector with the most cost effective, efficient way for designing, building, and inspecting a house, which meets the performance requirements of the NZBC.

Myth busting

Myth buster #1 – 'some people say NZS 3604 is the Building Code'

This is not correct. NZS 3604:1999 provides an Acceptable Solution in the Department of Building and Housing Compliance Documents for Clause B1, B2, E1, E2, G12, and G13 of the NZBC. The Compliance Documents outline specific requirements to comply with the NZBC and by inference, the Building Act 2004.

NZS 3604 is one way of meeting the NZBC structural requirements. It is not the only way – Alternative Solutions are often applied.

Following the requirements in NZS 3604 enables a house to be designed without the need for specific engineering design (a professional engineer is not required). NZS 3604 is a cost-effective solution that can be applied when designing a house. Providing your design complies with NZS 3604, building consent authorities are required to accept the design as complying with the NZBC structural requirements, and must issue a building consent.

There are limitations to NZS 3604. Today's consumers often have expectations that cannot be met within the scope of NZS 3604. For example, many houses today have extra-large rooms that often have joist or rafter spans that fall outside the scope of NZS 3604. Another example is the design of trusses, which is also outside the scope of NZS 3604 and requires specific engineering design.

Myth buster #2 – 'some building consent applicants say they always need a producer statement when using NZS 3604'

Providing a design complies with the requirements specified in NZS 3604, specific engineering design calculation or other design justification is not required. However, today's houses are often only partially designed using NZS 3604. For the parts of the design that don't follow NZS 3604, design justification would be required. For example, many houses today use a proprietary concrete slab system instead of the traditional slab-on-ground system that is in NZS 3604.

A proprietary slab system is outside the scope of NZS 3604; therefore its structural adequacy needs to be demonstrated/justified to the BCA at building consent stage. This justification may be through an engineering calculation submitted by a professional engineer or it may be by appraisal or other means, but it must be sufficient to satisfy the BCA on reasonable grounds that the proposed building work will comply with the NZBC.

Other examples where design justification is required include the following.

  • Many houses require a steel beam over the garage, to support the weight of the upper floor. Steel beams are outside the scope of NZS 3604 and therefore require justification to demonstrate their structural adequacy, such as a specific engineering design by a professional engineer. Such design and justification would not be limited to the beam itself, but would need to cover connections and supporting structure, as well as any necessary strengthening to the foundation so that loads can be safely transmitted to the ground.
  • When a house is designed in a location where the wind speeds fall outside the 'highest wind zone' specified in NZS 3604 (currently very high). In this case, a professional engineer will be required to review the relevant wind conditions, design appropriate bracing requirements, and provide design calculations to support the bracing design.

For information about the revision of NZS 3604, read the following Touchstone articles: leadership group,http://www.standards.co.nz/touchstone/Issue+05/Building/default.htm#1,  technical committee,http://www.standards.co.nz/touchstone/Issue+06/Building/default.htm#1 and work groups' update.http://www.standards.co.nz/touchstone/Issue+07/Building/default.htm#2.

Published in building.

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