Background and Overview
At the beginning of the century, a new building crisis emerged in NZ with relatively new houses leaking, structural rotting and everyone ducking for cover. In response, the Building Act 2005 was introduced that is overkill and makes it very difficult to secure consent to build.
In 2011 the Christchurch earthquake struck an area thought to be the least likely to suffer earthquake damage. In response, seismic rules have been tightened.
More recently, the health crisis brought about by damp, cold, mouldy homes requires much stricter standards for insulation.
All of these performance standards must be considered when seeking pre-manufactured housing from overseas.
Note that while the information here often refers to timber frame buildings, it provides a useful overview to understand the structural issues in NZ which has:
- Earthquakes anywhere
- High winds often
- Driving rain
- Snow in the higher elevations and the further south one goes
- Salt water corrosion exposure closer to the coasts - including blown salt water
For engineering purposes, the life of the building is 50 years. This is especially relevant for matters like the steel framing. Note the Exposure section below that sets different standards depending on how close to the sea.
Health and safety during the life of the building: Some materials give off emissions or allow run-off or leaching of chemicals that can be harmful to the health of building occupants. Adequate ventilation can mitigate some of the effects of gas emissions, but materials should generally be selected to minimise adverse effects to occupants.
Structural capability: Materials must be selected or designed for their ability to support the loads imposed by the building over the whole life of the building. An appropriate structural system and correct selection of structural materials can reduce excess material use and waste and increase the building’s adaptability for other uses.
Durability: A highly durable material may provide the most sustainable solution if it reduces maintenance or replacement requirements but a material should also be appropriate to the expected life of the building. Durability considerations should include:
- the actual or serviceable life of the building
- maintenance requirements
- the minimum statutory requirements for the building element.
Maintenance: Design buildings using materials that are readily and easily maintained. Generally, elements with higher maintenance requirements are likely to have lower initial costs but they may also have higher whole-life and environmental costs. The level of maintenance of a building element may also be determined by the performance requirements of the Building Code, particularly with regard to durability and weathertightness.
Moisture resistance: Selected materials must be protected from moisture. Some materials have a natural moisture resistance while others must be fully protected from moisture.
Material deterioration/decay: Some materials deteriorate rapidly, particularly in a moist environment or if they are continuously wet, generally due to the growth of moulds or fungi, or corrosion of some materials, so it is essential that materials selected have the durability required for their area of use.
Thermal performance: Building design and material selection must contribute to good thermal performance and reduced energy demand by including insulation and thermal mass in the building. Building Code clause H1 Energy efficiency sets out minimum requirements for thermal performance but BRANZ recommends that the minimum requirements are exceeded wherever practicable.
Sound insulation: Building design and material selection must contribute to the sound insulation of the building, both from exterior noise and sound transmission within the building.
Fire performance: Building design and material selection must be in accordance with the requirements of Building Code clause C Protection from fire including fire compartment separations, allowing the occupants safe escape from the building and allowing fire service personnel safe access to the building. Materials must be selected for ignitability, surface spread of flame, fire loading, and fire resistance and stability.
New Zealand is divided into four earthquake risk zones for the purpose of determining the bracing requirements of buildings to resist earthquakes.
Earthquakes can bring rapid, violent shaking both sideways and/or up and down, or slower rolling movements.
In some areas – especially hillsides – unstable ground may slide and rocks may fall.
On flat ground where there is a high water table and the soil is low-density sand or silt, liquefaction can take place. Liquid is forced to the surface, carrying sand and silt with it; land can slump; surface soil close to sloping ground (such as stream banks) can spread, with cracks opening up.
Depending on the location, earthquakes can bring other hazards such as tsunamis.
There are strong rules around house construction to help houses better cope with earthquakes, and some of these rules were strengthened following the Canterbury earthquakes of 2010/2011.
Under NZS 3604:2011 Timber-framed buildings, bracing must be provided for all buildings, with greater bracing being required for buildings:
- in a higher earthquake zone
- where heavy roof and/or wall claddings are specified.
Earthquake bracing demand is covered in section 5.3 of NZS 3604:2011. The section includes maps showing earthquake zones 1–4.
Earthquake bracing is provided by lengths of wall where the cladding and/or lining works with the timber frame to form a bracing panel. NZS 3604 ensures an even distribution of bracing elements, so the building is not expected to twist significantly in an earthquake.
Nowadays, many light timber-framed buildings are not designed completely to NZS 3604 because homeowners want large windows on one side of the house to enjoy a view. This often requires special bracing elements designed by a structural engineer. The rest of the building can still be designed and constructed to the standard.
Simple houses constructed to NZS 3604 performed well in the Canterbury earthquakes, but houses with a mixed bracing system often had significant damage.
BRANZ conducted a research project funded by the Earthquake Commission around houses with mixed bracing systems. Based on the study, a simplified approach has been proposed for engineers to reduce the likelihood of significant earthquake damage where there is mixed bracing. You can read more about the BRANZ study here.
Another key standard regarding earthquakes is NZS 1170.5:2004 Structural design actions – Part 5: Earthquake Actions – New Zealand.
Wind regions and zones
NZS 3604 Timber framed buildings requires that buildings are designed to withstand the winds that they are likely to be subjected to. Bracing requirements must be calculated separately for both wind and earthquake loadings; the bracing must be designed for the greater of the two loads. The amount of bracing required to resist the lateral wind loads on the building under NZS 3604 is based on the design wind speed.
NZS 3604 divides New Zealand into two wind regions (A and W) and several lee zone areas – these are areas where the landforms create localised wind acceleration resulting in higher wind speeds than the rest of the region.
It also classifies wind zones into categories, set out in the following table, according to maximum ultimate limit state speeds.
Wind zone classifications (from NZS 3604 Timber framed buildings)
|Classification||Maximum ultimate limit state speed|
|Low||Below 32 m/s|
|Very high||50 m/s|
|Extra high||55 m/s|
|Specific design (SD)||Over 55 m/s|
Table 5.1 in NZS 3604 provides steps to determine wind zone. From Table 5.1, determination of the wind zone for a particular site requires the following steps:
- Determine the wind region.
- Determine whether in a lee zone.
- Determine ground roughness.
- Determine site exposure.
- Determine topographic class.
BRANZ MAPS is a free-access online tool that provides information on the wind region and wind zone for any given address in New Zealand.
Exposure (Blowing Salt)
The amount, direction and intensity of rainfall on a site will affect aspects of a building design, such as roof form, flashings, stormwater drainage, rainwater harvesting and cladding type. Obtaining rainfall data for the region should be part of the preliminary design brief.
On this page:
- locating information.
Rainfall intensity varies throughout the year and from season to season, so average rainfall figures can be misleading. Some parts of the country get periods of intense rainfall that can be far higher than the average (taken over a longer period of time) would suggest. Building design should be able to cope with the maximum expected rainfall.
When assessing a site or designing a building, check the degree and frequency of past extreme weather events.
In most parts of New Zealand, a rainfall intensity of 100 mm/hour over a 10 minute period is generally an adequate design figure for external gutters. Regions where higher rainfall intensity design figures must be used are Arthur’s Pass, Haast, Milford Sound, Fiordland, Mount Taranaki and the Kaimai ranges. (Although internal gutters should be avoided where possible, where they are used BRANZ recommends that a rainfall intensity of 200 mm/hour for a 10 minute period should be used as a design figure.)
Different parts of a building may require different levels of weathertightness detailing against wind and rain because of the at-risk features incorporated into the design (see E2/AS1 for the weathertightness risk matrix) . Higher levels of weathertightness detailing are required on building faces exposed to high winds that will drive rain horizontally or even vertically up a building face.
Sunlight adds dollar value to a house
A New Zealand study by Motu Economic and Public Policy Research Trust, Valuing Sunshine, looked at the value buyers place on the amount of sunshine houses receive. They found that each additional hour of direct sunlight exposure for a house per day, averaged across the year, added 2.4% to its market value.
Building material durability
UV radiation affects the durability of many materials. Colours fade, plastic-based materials may become brittle, timber moves and twists, and expansion and contraction from heating and cooling places stress on many materials, so the effect of UV radiation over a building’s lifetime must be considered.
Select materials with a higher UV index number (when available) as they are more resistant to UV degradation (such as fading).
A. General Provisions - it is likely the classification will be Detached Dwelling
A1 Classified uses: Housing, communal residential, communal non-residential, commercial, industrial, outbuildings and ancillary buildings.
A2 Interpretation: The Building Code provides a list of defined words for interpretation.
A3 Building importance levels: Sets levels to describe risk and structure factors for the purposes of clauses C1 – C6 Protection from fire.
B1 STRUCTURE: Buildings will withstand likely loads, including wind, earthquake, live and dead loads (people and building contents).
C1 Objectives of Clauses C2 to C6 Safety objectives for people, other property and firefighting applied to clauses C2 to C6 of the Building Code.
C2 Prevention of fire occurring: Safe design and installation of fixed appliances using controlled combustion and other fixed equipment.
C3 Fire affecting areas beyond the source: Fire affecting areas beyond the source: vertical or horizontal fire spread, Material Group Numbers, surface finishes.
C4 Movement to a place of safety: Fire warnings, visibility of escape routes (smoke obscuration), automatic fire sprinkler systems, means of escape.
C5 Access and safety for firefighting operations: Access and safety for firefighting operations: access, hazards information and unobstructed paths.
C6 Structural stability: Structural stability during fire: buildings remain stable during fire (likelihood of failure or collapse).
D1 Access routes: Safety of entry/exit to the building and the safety of any internal or external stairs.
D2 Mechanical installations for access: Safety provisions for people using or servicing mechanical installations in buildings, such as lifts or escalators.
E1 Surface Water Disposal of rainwater from external surfaces and confirmation surface water cannot enter the building. See E1 Acceptable Solutions
E2 External moisture External roof, wall claddings and external openings will prevent external moisture from causing undue dampness or damage. See E2 Acceptable Solutions
E3 Internal moisture Surfaces in wet areas must be impervious, easily cleaned, and have ventilation to meet conditions for health and safety.
F1 HAZARDOUS AGENTS ON SITE Identifying and neutralising any hazardous agents or other contamination of the building site.
F2 HAZARDOUS BUILDING MATERIALS Safety for glass and glazing methods, asbestos and materials that give off noxious fumes.
F3 HAZARDOUS SUBSTANCES AND PROCESSES Safety from hazardous substances where stored, handled or used or where hazardous processes are undertaken.
F4 SAFETY FROM FALLING Safe design of all barriers inside and outside the building.
F5 CONSTRUCTION AND DEMOLITION HAZARDS Providing protection of people and other property during construction and demolition.
F6 VISIBILITY IN ESCAPE ROUTES Safety features for escape routes during failure of the main lighting.
F7 WARNING SYSTEMS Provides early warning systems to alert people to an emergency.
F8 SIGNS Providing identification of escape routes, hazards, emergency-related safety features and accessible facilities.
F9 RESTRICTING ACCESS TO RESIDENTIAL POOLS Restricting access by young children to residential pools.
G1 PERSONAL HYGIENE Providing sufficient sanitary fixtures (toilets, showers and basins) for sanitation.
G2 LAUNDERING Providing sufficient laundry facilities.
G3 FOOD PREPARATION AND PREVENTION OF CONTAMINATION Providing sufficient safe and hygienic facilities for food storage and preparation
G4 VENTILATION Requires ventilation to all occupied spaces.
G5 INTERIOR ENVIRONMENT Habitable spaces with sufficient space for activity, accessible facilities and controlled internal temperature.
G6 AIRBORNE AND IMPACT SOUND Prevention of undue noise transmission in building elements between occupancies or common spaces in household units.
G7 NATURAL LIGHT Providing sufficient natural light for occupied spaces and appropriate visual awareness of the outside for occupants.
G8 ARTIFICIAL LIGHT Requires buildings to have sufficient artificial light to safeguard people from injury.
G9 ELECTRICITY Requires the safe use and distribution of electricity.
G10 PIPED SERVICES Requires the safe distribution of hot, cold or toxic substances.
G11 GAS AS AN ENERGY SOURCE Requires the safe installation of gas-fuelled appliances.
G12 WATER SUPPLIES Requires the safe supply, storage, reticulation and delivery of hot and cold water.
G13 FOUL WATER Requires the safe disposal of foul water to prevent illness and the loss of amenity due to odour and accumulated matter.
G14 INDUSTRIAL LIQUID WASTE Requires the safe and hygienic collection, treatment and disposal of industrial liquid waste to avoid contamination.
G15 SOLID WASTE Provides for the safe and hygienic collection, holding prior to disposal of solid waste.