Revision of Standard for timber-framed buildings

STANDARDS NEW ZEALAND MEDIA RELEASE

19 August 2009

Revision of Standard for timber-framed buildings NZS 3604 – update

As advised in December 2008, Timber-framed buildings NZS 3604:1999, the foremost Standard in New Zealand for the building and construction industry in New Zealand, is being revised. The project 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.

Standards New Zealand notified key stakeholders about the revision in late 2008 and sought feedback. Many comments were received and these have been passed to the technical committee. 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.

NZS 3604 is an important Standard in the building and construction industry, setting a minimum standard for the design and construction of timber-framed houses in New Zealand. About 90% of timber-framed houses in New Zealand feature elements of NZS 3604 in their design and construction.

Architects, designers, builders, engineers, the building industry, and regulators use NZS 3604 for practical guidance on how to design and build houses to meet New Zealand Building Code requirements, without the need for specific engineering design. Timber-framed houses designed to NZS 3604 give consumers assurance that their house will meet legislative requirements.

Technical review of NZS 3604

There is wide acknowledgement and agreement that building practices have moved on significantly and NZS 3604 needs to be updated. The Department of Building and Housing and the Earthquake Commission have sponsored the revision of NZS 3604 and it has been agreed the revision of the document will proceed as a ‘limited technical review’.

The ‘limited technical review’ will reflect the needs of industry, changes in materials and industry practice, and changes in related Standards including those setting requirements for loadings. It will also incorporate content on bracings and durability.

Leadership group

Many different stakeholders are involved in the NZS 3604 revision process. Standards New Zealand has formed a leadership group of senior people in the building industry to maintain an overview of the entire review process for NZS 3604.

‘The leadership group is needed to provide guidance from all relevant industry sectors,’ says Derek Baxter, Chair. ‘This means the technical committee can focus entirely on technical issues surrounding the revision of the Standard.’

Technical committee

The technical committee has been formed in accordance with Standards New Zealand's Standards development guidelines to enable the many different stakeholders to be involved in the NZS 3604 revision process.

‘The technical committee is primarily responsible for focusing on a nd reviewing technical issues surrounding the revision of thedeveloping the revised draft,’ says Don Bunting, Chair of the technical committee.

The technical committee is responsible for considering all recommendations for changes to NZS 3604 proposed by the work groups. Through consensus-based discussion, this committee makes the final decisions on what changes will be made to NZS 3604 prior to final preparation. Once the draft is completed, it will be released for public comment.

The technical committee includes 22 industry representatives, nominated for their expertise in areas directly related to the revision of NZS 3604.

Work groups

There are five industry-specific work groups currently working on specific assignments. These groups are made up of members of the technical committee, organised by their particular area of interest and expertise, working under the guidance and direction of the technical committee. Primarily they are responsible for identifying issues within their specific areas that need updating in NZS 3604, and for recommending changes to the technical committee for a final decision.

Work group meetings are conducted independently of technical committee meetings. Recommendations for changes to NZS 3604 are tabled and debated by the relevant work group to reach a consensus. When consensus is reached, a change recommendation is scheduled and tabled at the next technical committee meeting. If consensus is not reached on a particular issue at work group level, the issue is debated further until a consensus is either reached or the matter is recorded and dropped.

If the technical committee is unable to reach consensus on a recommendation that has been tabled by a work group, the matter is referred back to the recommending work group for further discussion and debate.

Loadings work group

This group is responsible for recommending changes to NZS 3604 to reflect AS/NZS 1170, the loadings Standard. AS/NZS 1170 is referenced in the Compliance Documents to the New Zealand Building Code effective 1 December 2008.

Some of the recommendations made by the loadings work group are to:

-make changes to seismic maps to provide better clarity -change the soil types in earthquake clauses to align with AS/NZS 1170 -add an extra high wind zone -add gradients – currently there are three gradients in NZS 3604: gentle, moderate, and steep. Three additional gradients have been recommended – low, mild, and very steep.

Bracing work group

Recent research undertaken by BRANZ has identified areas in NZS 3604 section 5 – Bracing, that need updating. While the content is still accurate, problems have been identified with readability and there is a perception that some content is too open to interpretation, which can cause problems for territorial authorities.

Some of the recommendations made by the bracing work group are:

-amalgamate test method BRANZ P21 and technical recommendation TR10 into a single updated wall bracing test method -limit bracing unit ratings on timber floors to 120 BU/m -limit BU ratings on concrete floors to 150 BU/m -strengthen the bracing distribution rules – it has been agreed that the current distribution rules (internal lines 70 BU, external lines 10 x L BU) are inadequate. The group recommends that minimum requirements be increased to 15 x L BU for external walls with no wall less than 100 BU (including internal walls). In addition, the group recommends a demand-linked minimum, equivalent to D (demand) dividing by 2 n where n is the number of bracing lines in the direction being considered. Diaphragms have been recommended to be supported by elements rated no less than 100 BU or 15 x L BU, whichever is greater.

Durability work group

The durability section needs to be updated to align with the recent changes to the New Zealand Building Code and other Standards: in particular, the maps and recent product developments content need to be reviewed.

Some of the recommendations made by the dura bility work group are to:

-remove all building envelope flashings and so on, as they are to be covered in the Compliance Document E2/AS1. Section 4 – Durability is to cover structural details only -keep and update corrosion map figure 4.1 to include major cities and towns to make it more reader friendly. Add a note stating the map is indicative only and must be read in conjunction with AS/NZS 2312 clauses for atmospheric corrosivity categories B, C, and D -change zones 2 and 3 to Zone B -change zone 1 to Zone C -change seaspray zone to Zone D.

Roof framing work group

Building practices have moved ahead significantly in the past 10 years and NZS 3604 needs to be updated to reflect current technical knowledge and industry practices – section 10, Roof framing is an example of this. Roof trusses are no longer designed by the designer; they are currently designed and manufactured by producers and fall outside the scope of NZS 3604. Although roof truss design falls outside the scope of NZS 3604, it is recognised as a major component of timber home design and construction today. The roof framing work group believe this should be acknowledged in the revised Standard.

Some of the changes recommended by the roof framing work group are to:

-remove roof space bracing for all hip and light gable roofs because it is difficult to achieve space bracing in line with NZS 3604 -add a clear definition of hips and gables to NZS 3604 -revise truss to top plate fixing to one type, for example, two skewed nails and two wire dogs as standard -acknowledge trusses as a proprietary system and that providing it complies with NZS 3604, and all supporting documentation including design statements is provided, it should be acceptable -acknowledge proprietary garage beam lintels and other proprietary lintel products in NZS 3604. It was recommended that these could be covered in the foreword.

Design clarity work group

Primarily responsible for reviewing the figures in NZS 3604 for appearance and clarity, this group is in the early stages of reviewing the figures and has not yet made any recommendations for change to the technical committee.

Note: The recommendations discussed in this media release are by no means a full representation of the issues discussed to date by the technical committee or the work groups formed to undertake the project to review NZS 3604:1999 Timber-framed buildings. Nor are they a final representation of decisions made to date. They are provided as background information to introduce readers to the work groups.

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