Build124 Pg33 Feature NZS36042011AndMore

BUILD 124 June/July 2011 33

nZs 3604:2011 timber-framed buildings is one of the primary standards used in the new Zealand construction industry. Its release earlier this year introduced many changes that all designers and builders need to be familiar with.

In thIs sectIonWhy revise nZs 3604?

Key changes in nZs 3604:2011

Revised wind and earthquake bracing

snow loading

corrosion zones

timber treatment has just got simpler

Roof framing and trusses

engineered wood products now included

nZs 3604 committee recognisedBUILD 124 June/July 2011 33

nZs 3604:2011 And MoRe

34 BUILD 124 June/July 2011

NZS 3604:2011 AND moRE

Why RevIse nZs 3604?A standard represents the thinking on a subject at the time it was written. As soon as it is published, the world moves on. Recently, nZs 3604:2011 timber-framed buildings was released, which considered the many changes since 1999.By Roger Shelton, BRANZ Senior Structural Engineer

standards can only ever be a snapshot in time. The knowledge on the subject grows, construction practices change, other standards

are revised and occasionally a major event occurs that causes a stop and rethink. All of these happened in the case of the review of NZS 3604:1999 Timber framed buildings.

NZS 3604 was last revised in 1999 and then amended in 2000 and again in 2006 to incorporate the revised timber grading regime introduced by Amendment 4 to NZS 3603 Timber structures standard.

Many changes since 1999Since 1999, a number of significant events have happened in the New Zealand construction environment that have impacted on NZS 3604.

AS/NZS 1170:2002 INTRODUCEDThe most important was the introduction of the joint Australia/New Zealand loading standard, AS/NZS 1170:2002 Structural design actions, to supersede NZS 4203, the loading standard on which NZS 3604:1999 is based. The new standard was cited by the New Zealand Building Code clause B1 Structure in 2008.

The change of loading standards had wide-ranging implications on the technical basis and all the selection tables in NZS 3604. To avoid NZS 3604 designs getting too out of step with the equivalent specific designs done to AS/NZS 1170, it became essential to update NZS 3604.

DEVELOPMENT OF E2/AS1When NZS 3604 was last revised in 1999, E2/AS1 was very limited and didn't include some commonly used exterior cladding systems and weathertightness detailing. To fill that gap, a small range of details was included in Section 11 Building envelope. The subsequent publication of E2/AS1 (Third Edition) in 2005 expanded and superseded these details, creating many contradictions.

The reveiws of NZS 3604 and E2/AS1 at the same time has allowed rationalisation, by

removal of all weathertightness details from NZS 3604 Section 4 Durability and Section 11 Building envelope.

Construction practices changedChanges in construction practice over the last 10–12 years since the last revision have rendered many aspects of the standard out of step with the industry.

Changes in limited technical reviewIndustry feedback and surveys by Standards New Zealand indicated that the standard wasn’t broken, and only minimal changes were required. The guiding principle for the technical drafting committee therefore became a ‘limited technical review’ only.

The main areas of change in NZS 3604:2011 are summarised on the next page.

Darfield earthquake consideredThe 4 September 2010 earthquake in Canterbury occurred during the final stages of the revision so the initial findings on building performance were considered. Generally, timber buildings performed well in terms of life safety, although the cost of damage was very high.

This is perhaps an expected result for an event that was between 60% and 80% of the design value of an ULS (ultimate limit state – a return period of one in 500 years) event in Canterbury.

LIQUEFACTION NOT ADDRESSEDMany foundations (especially concrete slabs) were badly affected by liquefaction. The resultant lateral spreading often caused irreparable damage to a superstructure that would otherwise have performed well. There was a lot of discussion about how to address this in NZS 3604:2011.

In the time available before publication, it was not possible to include soundly based solutions for foundations on sites potentially subject to liquefaction. One of the major stumbling blocks is how to reliably identify a site that needs an enhanced foundation to cope with liquefaction, because such a solution will inevitably add significantly to the cost of constructing a timber-framed building. This has not yet been resolved but solutions are starting to emerge.

Changes likely after February quakeThe 22 February earthquake happened just after publication. This was a much larger event with more significant ground shaking, significantly above Code levels. It’s too early to predict the effect of these events on the New Zealand Building Code and related standards such as NZS 3604, especially in view of the government’s intention to hold a Royal Commission of Inquiry into building performance. It is fair to assume that there is likely to be an amendment of NZS 3604 in the not too distant future.

CHRISTCHURCH SEISMIC HAZARD LEVEL INCREASED

An early outcome of the February event has been the decision to increase the seismic hazard level for Christchurch from 0.22 to 0.3. Fortuitously, this is still within the band used for zone 2 in NZS 3604 Section 5, so no adjustments will need to be made to 3604 designs in respect of bracing demand.



Key chAnGes In nZs 3604:2011 nZs 3604:2011 will be cited later this year, so you need to know what the changes are.Trevor Pringle, BRANZ Principal Writer

While many current projects will be designed in accordance with the 1999 version of NZS 3604, new work should be designed following

the 2011 version. Do not use information from both the old and new versions – it must be one or the other. Once the 2011 version is cited in the compliance documents in mid 2011, the 1999 version should not be used.

The 2011 version is a completely new document with many changes, so it is not possible to purchase replacement pages – a new standard must be purchased from Standards New Zealand.

Some information removedAll information relating to the building envelope, that is, wall and roof underlays and claddings, has been removed from Section 11. Cladding-specific information will now be in the revised E2/AS1, which is due to be released in July 2011. However, NZS 3604:2011 states that the durability of the ‘closed’ timber framing covered by the standard requires the effective installation of roof and wall claddings (including windows and doors) and interior linings to prevent moisture entry. All references to the BCA processes have also been removed.

Section 18 Building product appraisals and BIA accreditations, Section 19 Statutory information and Section 20 Industry information have been removed. All other sections remain with the same numbering and colour coding.

The specific fixing requirements for roof edge zones have also been removed.

Some changes or revisionsSome information has been changed, including: ❚ alignment with AS/NZS 1170 in all the span selection tables

❚ references to other standards updated ❚ an increase in the maximum snow loading covered by the standard to 2 kPa

❚ a revision of the corrosion zone definitions in Section 4 – geothermal zones now require specific design to address the selection of metal components to resist corrosion

❚ a revision of the illustrations, with additional figures included such as isometric drawings showing the definitions of spans and loaded dimensions

❚ introduction of a new single timber grade, structural grade (SG), to cover both visual and machine stress-graded timber – the term ‘No. 1 framing grade’ is removed

❚ SG8 as the default timber grading to simplify the span tables (span tables for grades SG6 and SG10 are included as appendices at the end of each section)

❚ how ‘good’ ground is to be determined ❚ revision of Figure 3.1, which illustrates the relationship of a foundation to sloping ground

❚ simplified wind bracing tables – the default table gives the requirements for a high-wind zone, and for other wind zones, the high-wind zone is multiplied by a factor to give the requirements for that wind zone, e.g. 0.7 to give the requirement for a medium-wind zone

❚ revision of the earthquake zone boundaries ❚ revision of the requirements for the maximum BU/m for wall bracing elements and the minimum capacity of internal and external bracing lines.

❚ revision of foundation edge details, particularly the location of bottom plate fixings where the edge is formed by masonry header blocks

❚ revision of the roof truss section, with design responsibility placed on the truss supplier.

Some new informationThere have been some additions to NZS 3604:2011 including: ❚ LVL and glulam as a direct substitution for the equivalent SG timber

❚ an extra-high wind zone (maximum design wind speed of 55 m/sec)

❚ introduction of soil classifications for earthquake bracing demand in Section 5

❚ a DPC is now required between all timber and concrete/concrete masonry

❚ new clause 4.4.4 to cover the durability of fixings in ACQ and CuAZ-treated timber

❚ requirements for the design of cantilevered decks including the design of the balustrade loads and fixing of balusters.


one of the primary reasons for updating NZS 3604 was that the loading standard NZS 4203 was superseded by the joint loading standard AS/NZS 1170:2002 Structural design actions, which was cited in New Zealand Building Code clause B1 Structure in

December 2008.

Wind demandThe existing wind zones have been retained, but an extra-high wind zone has been added to increase NZS 3604 coverage of more exposed building sites without having to use specific engineering design. These include the hilltop locations and coastal areas where Kiwis are increasingly building to get views and water access.


RevIsed WInd And eARthqUAKe BRAcInGAs/nZs 1170:2002 has had a big influence on the bracing demand provisions of nZs 3604:2011. however, there are also changes throughout the bracing section, and this article looks at the more significant ones.By Roger Shelton, BRANZ Senior Structural Engineer

A conscious decision was made to keep the maximum wind speeds for each zone the same as before, so as to avoid redesign of standard components such as windows. It also allows a product or system with an Appraisal for a certain wind zone to remain unchanged.

There are minor changes in the determination of the topographic class, and the ground roughness categories have been reduced from three to two. These changes were partly a result of the new loading standard and also the drive to simplify the process.

Earthquake demandThe changes here are more significant and result mainly from changes in the loading standard.

The maps of the earthquake zones have been significantly changed (see Figure 1). These reflect our greater knowledge of the overall seismicity of New Zealand, which is improved with every event. It is worth noting that the base maps were drawn before the recent Canterbury earthquakes and may be subject to revision as the data is processed. Both maps show the increasing influence of the major faults, for example, the Alpine fault in the South Island.

SUBSOIL CLASSIFICATION ADDEDA new provision is the introduction of subsoil classification. This was done with some reluctance by the committee because it is not intuitive and adds complication to the design process. However, there is overwhelming evidence from past earthquakes of the strong correlation between building damage and subsoil type.

The principle is that a building on a rock site will suffer much less damage in an event than one with a deep layer of soft soil overlying the bedrock.

As yet, there is no simple way to determine the soil parameters leading to this classification, and the services of a geotechnical engineer will be required to interpret the existing data where the significance of the proposed building justifies the expense. There are a number of agencies working on this and it is expected that local Territorial Authorities will have access to this information in due course.

Meantime, the earthquake demand tables in NZS 3604 are derived for the most conservative solution (soil type D or E), and reduction factors are provided for those who go to the trouble to classify their sites.

If wind demand governs the design of the building (as it does for most cases), then nothing further needs to be done.

Figure 1: The new earthquake zones. For detailed maps, see NZS 3604:2011 Figure 5.4.

Zone 1

Zone 2

Zone 3

Zone 4



snoW LoAdInG

t he snow loads quoted in NZS 3604:2011 are ‘ground snow loads’. Snow loads on a building’s roof can be considerably less than this, depending on roof pitch and shape. Snow loads in the 1999 version of NZS 3604 are 0 kPa, 0.5 kPa and 1.0 kPa.

Research to see if change neededTo assess the impact of AS/NZS 1170:2002 and revisit those loads, BRANZ carried out a study to assess the impact on NZS 3604-designed buildings over the whole country.

Several locations and a range of building activity were chosen. Various options were considered to avoid overdesign, while maximising the scope and coverage of the document throughout the country. The effects of the options for snow loading on various roof members were also investigated.

The study revealed that, up to 1.0 kPa of snow, wind loads, dead loads and concentrated live loads dominated over snow loads on the sizing of members. In other words, up to 1.0 kPa of snow, there was very little reduction in member spans.

Changes to cut-offs and some spansThe conclusion reached was that cut-off values of 1.0 kPa, 1.5 kPa and 2.0 kPa was the optimal arrangement for NZS 3604:2011. Only high-altitude towns such as Lake Tekapo, Omarama, Mt Cook village and Arthur’s Pass are outside the scope of NZS 3604:2011.

A new provision has also been added to account for the extra depth of snow that drifts onto a lower roof abutting an upper wall (see NZS 3604:2011 Figure 15.2). This results in span reductions for some members (such as purlins and rafters) in the higher snow load regions.

Depending on snow density, a ground snow load of: ❚ 1 kPa is equivalent to a depth of 350 mm of snow ❚ 1.5 kPa to 520 mm of snow ❚ 2 kPa to 700 mm of snow.

the citing of As/nZs 1170 structural design actions in december 2008 introduced a number of changes to snow loading on buildings, and these have resulted in changes in nZs 3604.By Roger Shelton, BRANZ Senior Structural Engineer

NZS 3604:2011 Timber-framed buildings and other standards mentioned in Build, can be purchased from Standards New Zealand by phoning 0800 735 656 for members or by visiting the website www.standards.co.nz.



coRRosIon ZonesIn the 2011 version of nZs 3604, some changes have been made to the corrosion map, mainly with the atmospheric corrosivity classification system. By Zhengwei Li, BRANZ Corrosion Scientist

t he severity of atmospheric corrosion is now categorised into five groups based on the corrosion rates of mild steel given in AS/NZS 2312:2002 Guide to the protection of structural steel against atmospheric corrosion by the use of protective coatings and ISO

9223:1992 Corrosion of metals and alloys – Corrosivity of atmospheres – Classification.

Exposure zones alignedThese standards define the zones as category A (very low), category B (low), category C (medium), category D (high) and category E (very high). Category A is not applicable in New Zealand.

To align the standards, the exposure zones in NZS 3604 have been redefined: ❚ Sea spray zone is now zone D. ❚ Zone 1 is now zone C. ❚ Zones 2 and 3 are now zone B.

Now the classification of New Zealand atmospheric environments is aligned with related standards, including AS/NZS 2312 and AS/NZS 2728, except they are now referred to as zones instead of categories.

Sea spray zone now zone D, or E in some placesThe sea spray zone, including all offshore islands and the area within 500 m of the coastline, is defined as zone D in NZS 3604:2011.

However, in areas very close to beachfronts subjected to rough seas and surf beaches, the atmospheric corrosivity can be very severe due to the heavy deposition of airborne sea salt particles. These areas are classified as category E in AS/NZS 2312 and ISO 9223 but are not shown in the map in NZS 3604:2011 because the standard requires identical corrosion protection for metallic components within both zones D and E.

Geothermal zone out, geothermal hot spots reclassifiedThe geothermal zone (zone 4) used in NZS 3604:1999 is not in NZS 3604:2011. Most geothermal areas now fall into zone B of NZS 3604:2011. This does not imply that the selection of metallic components for buildings within these areas should always follow the recommendations for a mildly corrosive atmosphere.

It has been shown experimentally that mild steel and galvanised steel have much higher corrosion rates in areas adjacent to geothermal hot spots than in other inland regions. Geothermal hot spots are defined as being within 50 m of a bore, mud pool, steam vent or other sources. While they are mainly found in Rotorua-Taupo, they may occur elsewhere.

However, since the atmospheric corrosivity is highly variable within these areas, the geothermal zone has been removed and is to be considered as a micro-climate and subject to specific engineering design (SED).

Micro-climate consideredNZS 3604:2011 recognises that micro-climate, as well as macro-climate, affects the corrosion performance of a metal and the durability of a structure. Components corrode at quite different rates when installed at different locations on the same building because design features can significantly influence the local climate surrounding a component. For example, a sheltered surface may accumulate more airborne salt particles due to the lack of rain washing, making a mildly corrosive environment more aggressive toward metals. Pollutants, such as oxides of nitrogen and sulphur released by industry or agriculture, can also accelerate the deterioration of metals.

When assessing the aggressivity of a given environment, if micro-climatic factors are found to be significant and outweigh macro-climatic parameters, NZS 3604:2011 recommends a more severe corrosion category is used for materials selection (see clause 4.2.4).



tIMBeR tReAtMent hAs jUst Got sIMPLeRthe treated timber framing system that came into effect on 4 April this year allows a single hazard class, h1.2, to be used for all enclosed radiata pine and douglas fir framing.By John Harper, Senior Advisor – Building Standards, Department of Building and Housing

changes to the system for treated timber framing are contained in Building Code Acceptable Solution B2/AS1. The previous system required up to

four ‘minimum levels of treatment’, including untreated, H1.1, H1.2 and H3.1, depending on where in the building the timber was to be used. In addition, there were some timber uses and locations that were either unclear or omitted, and this caused ongoing confusion within the industry.

One exception to H1.2 for framingMost users of timber can now buy H1.2 framing and use it anywhere enclosed in a building for frames, trusses, floor joists, flat and sloping roofs, skillion roofs and so on. There is, however, one exception – cantilevered deck joists and associated framing must be H3.2 (see Figure 1). All other deck framing and enclosed balustrade and parapet framing can be H1.2 treated.

These changes follow 3 years of work with the sector, completion of research projects and consultation and evaluation by the Department of Building and Housing.

No change to NZS 3640 or 3602 yetFor the more regulatory minded, these changes are modifications to the referenced standards in Acceptable Solution B2/AS1 for timber – NZS 3640:2003 Chemical preservation of round and sawn timber and NZS 3602:2003 Timber and wood-based products for use in building. The standards themselves have not been amended. B2/AS1 simply modifies the requirements of the referenced standards in the Acceptable Solutions for durability of timber.

The standards will be updated in due course, but in the meantime, users and specifiers will need to work with both the New Zealand standards and the Acceptable Solution B2/AS1 for all treated timber information (see Table 1).

A few special provisionsThe changes relate only to radiata pine and Douglas fir framing timber that is enclosed by cladding. This includes all roof and wall framing plus intermediate and ground floor framing, provided it is not in ground contact and is protected from the weather. All this framing and

many of the secondary elements, such as valley boards and ceiling battens, can be H1.2 treated (except cantilevered decks). The following are special provisions included in the changes.

UNTREATED DOUGLAS FIR FRAMINGBecause Douglas fir has been shown to have some natural durability over that of radiata pine (though not as good as H1.2 treated framing), untreated Douglas fir has been included for use in defined ‘low-risk’ designed houses. This will be of benefit to those wishing to have chemical-free construction.

Provided all the conditions are met for the design of a house, untreated Douglas fir framing can be used (see Table 2).

FARM BUILDINGSThe changes also clarify the use of untreated framing for farm buildings. Untreated radiata and Douglas fir framing can be used if framing is: ❚ protected from direct wetting, and ❚ has no internal linings, and ❚ is not in ground contact, and ❚ is not used in a building with living accommodation.

Figure 1: New treatment requirements for timber framing.

Cantilevered enclosed balcony and balustrade. Enclosed balcony and balustrade with living space below. Enclosed balcony and balustrade open below.

H1.2 H3.2 H5

H5 post if in contact with ground

Key:


Table 1: Treatment requirements for framing and other timber uses.

LEVEL SPECIES BUILDINg ELEmENTS

Floor framing protected from weather but exposed to ground atmosphere

H1.2 Radiata pine Douglas fir

Jack studs, subfloor braces, bearers, wall plates, floor joists to the subfloor, blocking, walings and battens, nogs and diagonal boards.


Interior solid wood flooring for ground floors.

Enclosed roof framing and trusses


Sarking and framing not protected from solar-driven moisture through absorbent cladding materials.


Enclosed flat roof framing and associated roof members.


Enclosed skillion roof framing and associated roof members.


Valley boards and boards supporting flashings or box gutters, and flashings to roof penetrations and upstands to roof decks.


All roof trusses, including gable-end trusses, roof framing, ceiling and eaves framing, purlins and battens.

Enclosed wall framing protected from the weather


Framing and other members within or beneath a parapet.


Framing and other members within enclosed decks or balconies (see H3.2 for cantilevered decks).

H3.2 Radiata pine Framing and other members within enclosed cantilevered decks (including joist trimmers, nogs and blocking).


Framing and other members supporting enclosed decks or balconies (including cantilevered decks).

H3.1 Radiata pine Battens used behind cladding to form a cavity (H3.1 treatments can be either solvent-based or boron. H3.1 boron treatments supplied grey primer-painted).


All other exterior wall framing and other members including exterior and boundary joists.

Internal wall framing


Internal walls.

mid-floor framing


All mid-floor framing, including boundary joists, ceiling framing and ceiling battens and double top plates.

Interior flooring

H1.2 Pinus species Douglas fir

Interior flooring.

other framing

None Radiata pine Douglas fir

Wall framing and roof framing (including trusses) protected from the weather, in unlined and unoccupied farm buildings and outbuildings, except buildings with high internal humidity, such as saunas, spas and so on.

H3.2 Radiata pine Framing exposed to the weather and above ground.

H4 Radiata pine Framing such as fence posts and landscape timbers that is exposed to the weather and is in contact with the ground.

H5 Radiata pine Framing such as house piles, poles and crib walling that is exposed to the weather and is in contact with the ground.

Note 1: For structural use of other species, refer to NZS 3602:2003 Tables 1 and 2.

Note 2: For non-structural use of radiata pine, Douglas fir and other species, refer to NZS 3602:2003 Table 3.

Note 3: A higher treatment level also satisfies the level specified in this table.

Refer to NZS 3602:2003 for other framing choices, such as larch or macrocarpa.

SOLVENT-BASED TREATMENTS REMOVEDSolvent-based treatments are no longer required for framing. Previously, some parts of the building, such as enclosed flat roof framing, required H3.1 treatments using light organic solvent preservatives (LOSP). Now that H3.1 is no longer required, LOSP treatments can be avoided.

The changes also confirm the removal of LOSP treatments from the H1.2 category. At present, this leaves only boron (pink-coated) framing for H1.2. It is expected that a future amendment of NZS 3640 will consider other H1.2 treatment options such as water-based azoles. In the meantime, pink framing will be the norm on construction sites.

Higher treatment levels OKSince at least 2003, NZS 3602 has stated that timber treated to a higher level than the minimum satisfies the minimum. This is a statement of the obvious and not an endorsement for more chemical use.

The research and history of use now strongly indicates that boron-based treatments with average cross-section concentrations of 0.4% mass/mass boric acid equivalent (BAE) or above are adequate to cope with fungal and insect risks for normal framing use and construction practices in New Zealand. H1.2 treatments offer ‘opportunity time’ for construction and detecting and repairing leaks throughout a building’s life. No treatment is capable of permanently protecting framing that remains continuously wet.

An owner or designer who chooses higher levels of treatment for framing is making a choice – these levels are not needed to comply with NZS 3602. They may be chosen in response to durability risks in some exceptional types of buildings, but there are implications and costs. These include the increased environmental effects from timber residues around the home the generally higher costs of purchase and waste disposal and the potential health and safety effects (and costs) for those working with treated timber especially hazard classes H3 and above (see Safety and health on page 42).

For a good summary, see A quick guide to timber treatment for enclosed framing from the Department of Building and Housing website, www.dbh.govt.nz, then Publications.


Table 2: Low-risk house conditions when untreated Douglas fir can be used for framing.

All the following conditions must be satisfied:

Is a stand alone, single household unit of no more than two storeys (as defined in NZS 3604) that is designed and constructed to NZS 3604.

Is situated in wind zones no greater than ‘high’ as defined in NZS 3604.

Has a building envelope complexity no greater than ‘medium risk’ and a deck design no greater than ‘low risk’ as defined by the risk matrix in the Acceptable Solution E2/AS1.

Has drained and vented cavities complying with E2/AS1 behind all claddings.

Uses roof and wall cladding systems and details meeting E2/AS1.

Has a risk matrix score of no more than 6 on any external wall face, as defined in E2/AS1.

Has a simple pitched roof with hips, valleys, gables or monopitches, all draining directly to external gutters.*

Has a roof slope of 10° or more.

If it has a skillion roof, the roofing material is corrugated iron or concrete, metal or clay tiles for adequate ventilation.

Has eaves 450 mm wide or more for single-storey houses and eaves 600 mm wide or more for 2-storey houses.

*The roof does not have internal or secret gutters, concealed gutters behind fascias or any roof element finishing within the boundaries formed by exterior walls (e.g. the lower ends of apron flashings, chimneys, dormers, clerestorey, box windows).

Timber treatments generally consist of chemicals that need to be handled safely and appropriately. Important measures to take when using treated timber, especially hazard classes H3 and above, are: ❚ reduce contact by wearing gloves, goggles and a dust mask

❚ don’t burn off-cuts or cook with them ❚ dispose of waste in an approved landfill ❚ wash your hands before using the toilet, smoking or eating

❚ wash work clothes separately ❚ ventilate work spaces as much as you can ❚ working with solvent-damp timber is not advised, so allow solvent-damp timber to properly dry off before use.

Boron has been used commercially as a timber preservative in New Zealand since the 1950s, with no known health issues for timber users or building inhabitants.

sAFety And heALth


RooF tRUssesA high proportion of residential buildings use timber nail-plated roof trusses to form the roof structure. nZs 3604:2011 includes provisions for their use.By Stephen Walker, FTMA Chairman and WPA Board member

Previous versions of NZS 3604 put most of the emphasis on close-coupled roof framing design and construction and left nail-plated roof trusses to the

realm of specific engineering design. Roof truss information was in the commentary in the 1999 standard, making it informative. NZS 3604:2011 has moved it into ‘normative’ text (where it is mandatory) and updated the requirements to reflect current industry practice.

What is current industry practice?The Frame & Truss Manufacturers Association (FTMA) Code of Practice was developed at the same time as the NZS 3604 revision. This assist-ed in determining current industry practice.

The truss documentation section of the Code of Practice sets out the requirements for FTMA members for the minimum level of documentation, specific details and identifi cation requirements to provide when manufacturing timber nail-plated roof trusses. Some of this information has been used in NZS 3604:2011.

Accredited fabricators and SEDRoof trusses used with NZS 3604:2011 remain specific engineering design and must be manufactured by an accredited fabricator. An accredited fabricator is a company accredited by a nail-plate manufacturer to fabricate roof trusses using nail plates and construction details supplied by that same nail-plate manufacturer.

This is an important inclusion as the controls in these production environments help ensure the integrity of the manufactured structural item.

Limitations when using NZS 3604Being a proprietary product and subject to specific engineering design, it is necessary to provide limitations to the use of roof trusses within a building designed using NZS 3604:2011. This is to avoid loads that may have an adverse effect on the rest of the structure.

Limitations applied to roof trusses are: ❚ a span no greater than 12 m ❚ eaves overhang shall not exceed 750 mm measur ed horizontally from the face of the support

❚ spacing no greater than:•900 mm for heavy roof claddings•1200 mm for light roof claddings

❚ resultant loads not exceeding 16 kN in either an upwards or downwards direction

❚ ground snow load may not exceed 2 kPa.Clause 10.2.2.3 sets out the three levels of documentation required for each project: ❚ Producer Statement (design) for the design software used.

❚ Design statement by the accredited fabricator listing the specific project design particulars.

❚ Manufacturing statement by the fabricator confirming that the truss installation is in accordance with the design statement and truss layout plan.

It is critical that designers receive and review roof truss documentation before completing the design of the supporting structure.

Trusses need identification NZS 3604:2011 introduces the requirement for truss identification labels as a means of determining that the roof trusses installed in a building match the documentation provided to the builder, specifier or building official.

Six trusses in every job shall be labelled or all the trusses where there are less than six in a job.

Roof truss bracing requirementsThe requirements for roof truss bracing are set out in section 10.3 of NZS 3604:2011.

Whoever designs the roof bracing should also design the bracing for the rest of the structure. Systems to resist horizontal or vertical loads should be considered using a whole-of-house approach rather than be designed in isolation.

RooF FRAMInGSelection tables

Selection tables have been rationalised and the number reduced. Light and heavy roofs are combined in one table, and the extra-high wind zone is the default require ment with adjustment factors for the other wind zones.

Also, all tables default to the SG8 timber grade. Other grades have been moved to the appendix at the back of the section. This reflects the dominance of SG8 framing timber in the market.

FixingsThe fixings have been rationalised so that the same fixing type is consistent throughout the section. A summary table is provided in NSZ 3604:2011 Table 2.2.

PurlinsThe scope of purlins on edge has been greatly increased. This was driven by the increasing use of monopitch roofs with no rafters, and purlins spanning across walls. The deeper members also accommodate the greater insulation thicknesses now required by the Building Code.

In the previous version of NZS 3604, purlin tables provided for extra fixings around roof edges and ridges, reflecting the greater wind uplift in these areas. Very little area was often left as the ‘body’ of the roof, and it proved unrealistic to expect designers and installers to determine when and where extra fixings were required. The solution has been to provide the requirements for extra fixings over the whole roof in the tables.

Roof bracingThis has been simplified with bracing demand now related only to roof weight and area – roof shape is irrelevant. Options for providing roof bracing are widened and made more flexible and include: ❚ sarking ❚ structural ceiling diaphragm directly attached to the underside of the rafters

❚ combinations of roof plane bracing and roof space bracing.

Note that roof bracing, like wall bracing, is the responsibility of the building designer.



Table 1: Fixing type and capacity reference guide (Table 2.2 from NZS 3604:2011 Timber-framed buildings, reproduced with permission from Standards New Zealand).

Fixing type

Description Alternative fixing capacity (kN)

See table

A 2/90 × 3.15 end nails 0.7 8.18

B 2/90 × 3.15 end nails + 2 wire dogs 4.7

C 2/90 × 3.15 end nails + strap fixing (see Figure 8.12) 8.5

D 4/90 × 3.15 end nails + 2 strap fixing (double stud) 16.0

E 2/90 × 3.15 skew nails + 2 wire dogs 4.7 10.1, A10.1, 10.7, A10.7, 10.11, A10.11 10.14, 10.15, 15.6, A15.6, 15.10, A15.10F 2/90 × 3.15 skew nails + strap fixing (see Figure 10.6) 7.0

G 10/90 × 3.15 nails (5 each side) 4.7 10.2, A10.2, 15.7, A15.7

H 1/M12 bolt 8.5

I 2/M12 bolts 16.0

J 2/M16 bolts 24.0

K 6/90 × 3.15 nails 3.0 10.5, A10.5

L 2/M12 bolts 9.8

M 2/M16 bolts 13.0

N 6/100 × 4.0 HDG nails (hand driven) 4.7 10.8, A10.8, 15.8, A15.8

O 2/M12 bolts (see Figure 9.3 (C)) 6.8

P 2 HDG ‘flat’ straps (see Figure 9.3 (B)) 13.7

Q 2 HDG ‘tee’ straps (see Figure 9.3 (A)) 25.5

R 1/90 × 3.15 nail 0.55 10.10, A10.10, 10.12, A10.12, 15.9, A15.9

S 2/90 × 3.15 nails 0.8

T 1/10 g self-drilling screw, 80 mm long 2.4

U 1/14 g self-drilling type 17 screw, 100 mm long 5.5

FIxInGs RAtIonALIsedFixings are the Achilles heel of timber buildings. Wind and other damaging hazards find the weakest link in the building, and failure will start there. this is almost always the fixings.By Roger Shelton, BRANZ Senior Structural Engineer

designers will, with some incentive, use selection tables to arrive at suitable sizes for most principal members. Fixings, however, are ignored or left to

the builder. Downwards gravity loads are fairly self-evident to the person on the job, but uplift due to wind loads is not particularly intuitive.

For these reasons, NZS 3604 has tables of fixings covering different situations. After several revisions, these were fragmented in NZS 3604:1999 and inconsistent through the document.

Table 2.2 has it allNZS 3604:2011 has rationalised all of the fixings used throughout the standard into Table 2.2 (see Table 1). A type E fixing, for example, is always 2 skew nails plus 2 wire dogs in the configuration shown in Figure 1, wherever it is used.

The first two columns give the fixing type and description (which is consistent throughout the standard). The third column gives the capacity of the fixing (which must be equalled or exceeded by any alternative), and the last column lists the tables where the fixing is used. Fixings within each grouping by colour shading are the same configuration, but in increasing order of capacity.

Fixing type ≠ fastenerNote that a fixing type is not the same as the fastener used to construct the fixing. Each

Note: Capacities are associated with fixing type, not fasteners. See individual selection tables for the appropriate fixing type for the application.

Figure 1: Type E fixing used in NZS 3604:2011 is always 2 skew nails plus 2 wire dogs in this configuration.

fixing type has a unique capacity depending on its configuration, load direction and in-service environment. For example, 2/M12 bolts are used in fixing types I and O. Fixing type I is used to connect a ridge beam to its supporting studs in a dry internal environment, whereas type O is used to connect a veranda beam to a post

that may be exposed to the weather and uses reduced properties appropriate for timber with a moisture content greater than 18%.

The basis of the fixing capacities and the way they were derived will be updated in the BRANZ report The engineering basis of NZS 3604, which will be published shortly.



enGIneeRed Wood PRodUcts noW IncLUdedthe new nZs 3604 includes the use of laminated veneer lumber (LvL) and glued laminated timber (glulam), now common in residential construction.By Stephen Walker, Chairman of the Frame and Truss Manufacturers Association and member of the Wood Processors Association Board

over the past 10 years, LVL and glulam products (collectively termed engineered wood products or EWPs) have become more prevalent in residential

construction, particularly in lintels, beams, rafters and floor joists where larger section sizes of solid timber have become less available or are unable to span the distances required. Currently, it is estimated that over a third of lintels used in prenailed wall frames use LVL or glulam timber.

Standard has restrictions on useBecause EWPs are mainly proprietary in nature (that is, grades are particular to one manufacturer), their inclusion within NZS 3604 needed to: ❚ fit within the scope of the existing standard ❚ not create situations where resultant loads may compromise solid timber components able to be selected from existing tables

❚ recognise the specific engineering design required to produce selection charts or software.

NZS 3604 is a prescriptive standard so it is necessary to provide restrictions in order to remain within the scope of the document. Clause 2.3.9 does this by restricting EWPs to only include LVL and glulam products manufactured using radiata pine or Douglas fir and nominating their respective manufacturing standards (AS/NZS 4357 Structural laminated veneer lumber and AS/NZS 1328 Glued laminated structural timber).

Timber treatment of EWPsFor timber treatment, NZS 3604:2011 calls up NZS 3602:2003 Timber and wood-based products for use in building.

Although NZS 3602 allows untreated LVL for subfloor framing, several of the NZS 3602 tables do not have LVL and glulam options. Where the

level of treatment for EWPs is not listed in NZS 3602, a conservative approach has been taken in NZS 3604:2011 by requiring the same level of treatment that would be required for kiln-dried radiata pine to comply with NZS 3602.

When read in conjunction with the recent amendment to B2/AS1, it would appear that a minimum level of H1.2 would apply for LVL in many cases. Specifiers and builders should note, though, that few producers of LVL provide an H1.2 option at present, so the H3.1 LOSP treatment is most likely to be provided when treatment is required.

Use by substitution or specificationEWPs can be used in two ways within NZS 3604:2011 – as a direct substitute for solid timber grades like SG 6, 8 or 10 (clause 2.3.9.5) or as a proprietary grade (clause 2.3.9.6).

DIRECT SUBSTITUTIONWhen using the direct substitution method, EWPs must: ❚ be the same finished size as the member being substituted

❚ have the grade verified as meeting the

requirements of an SG grade and be marked in the same manner as an SG grade in accordance with NZS 3622.

While few manufacturers of EWPs are likely to do this, there is one example in the marketplace of LVL studs provided as being SG 8.

PROPRIETARY GRADEOtherwise a proprietary grade can be used. This can be a different size from the solid timber member being substituted and have different engineering properties (usually better) as long as: ❚ it is a framing member for a building within the scope of NZS 3604:2011

❚ any loadbearing reaction from the member is not greater than 16 kN in either an upwards or downwards direction

❚ the selection charts or software used have been engineered as a minimum in accordance with B1/VM1.

The loadbearing reaction figure is a conservative approach – 16 kN was near the upper limits of loads generated by the solid timber members already selectable in NZS 3604 and likely to be substituted with EWPs, even though other framing members generate higher resultant loads. It was also the figure recommended for the loadbearing reaction limit for roof trusses.

Documentation will need to be provided to support the charts or software used to select proprietary grades (see commentary clause 2.3.9.6). This should include a producer statement (design) from a chartered professional engineer covering the engineering basis of the selection charts or software used.

EWPs for future revisionsThe introduction of EWPs into NZS 3604:2011 provides more options for users of the standard. Future revisions may seek to clarify or add to the use of EWPs and possibly include composite EWPs such as ‘I beam’ joists.



nZs 3604 coMMItteeRecoGnIsedthe committee that updated nZs 3604 timber-framed buildings has been awarded the standards new Zealand 2010 committee of the year Award.

commerce Minister Simon Power and Building and Construction Minister Maurice Williamson honoured the committee members and their employers at awards breakfasts held in Auckland, Wellington and Christchurch.

NZS 3604 is one of the primary standards used in the New Zealand construction industry. The limited technical review was needed to update the standard to reflect the needs of industry, changes in materials, industry practice and related standards, including loadings.

A technical committee comprised of 22 industry representatives (see below) with expertise in areas directly related to NZS 3604 was appointed to conduct the review. Under the leadership of architect and committee Chair Don Bunting, the committee established five industry-specific work groups. A leadership group advised the committee and provided policy guidance.

Once cited in B1/AS1 later this year, the NZS 3604:2011 committee’s work will provide the industry with a means of complying with the New Zealand Building Code for timber-framed buildings. This means designs and plans in accordance with NZS 3604:2011 will be deemed to comply with the Building Code and must be accepted by Building Consent Authorities. This provides the sector with a cost-effective and efficient way to design, build and inspect a house that meets the performance requirements of the Building Code.

BRANZ Senior Structural Engineer Roger Shelton, also received additional recognition with a Standards New Zealand 2010 Meritorious Service Award for his input.

The NZS 3604 technical committee: (back row, from left) Eddie Bruce, Colin Hill, Colin Clench, Jamie O’Leary, Don Bunting, Ian Garrett, Richard Merrifield, Allan Walters, Doug Gaunt and Graeme Lawrence; (front row, from left) Warwick Banks, Hans Gerlich, Ernest Lapish, Michael Middlebrook, John Yolland, Stuart Hayman, Craig Watkin and Roger Shelton. Not present: Bruce Anderson, David Barnard, Mark John Ash, Stephen Walker and Scott Gibbons.


Documents

Build124 Pg33 Feature NZS36042011AndMore