New Zealand has unique and often extreme weather patterns. We have comparatively high average wind speeds, and some areas have high rainfall. Wind-driven rain puts the exteriors of our buildings under considerable load. Buildings must be specifically designed to take into account the physical surroundings and local climate of a site. A building that is inappropriate for the site is more likely to be a weathertightness failure.
High wind speeds are relatively common, and wind can drive rain through a building’s exterior cladding.
New Zealand is divided into two wind regions. Almost the whole country is designated ‘A’ except those parts of the North and South Islands adjacent to Cook Strait, which are ‘W’.
In addition to identifying the appropriate wind region, the site’s wind direction, speed and frequency should be established. This information will influence overall design, roof and wall cladding selection and detailing, building entry locations, and window size, placement and detailing.
The topography of land surrounding a specific building site must be considered by the designer.
Topography has a significant effect on wind speed and direction, with the presence of mountains creating higher wind areas as well as low wind locations in the lee. Wind increases in speed when it is:
- forced to divert around or over something in its path
- squeezed through a narrow gap
- compressed by landforms such as the sides of valleys and high ground
There will be more wind close to a large expanse of open area such as parks, beaches or farmland.
The average rainfall in New Zealand is reasonably moderate at around 1000 mm per annum. Many regions are subject to significantly more - for example, the Waitakere Ranges of Auckland get 2000 mm and Hokitika has experienced annual rainfall of over 2800 mm.
Wind can drive rain in many directions, including upwards, and with significant force. Building exteriors need to be designed and built to ensure that rainwater is drained or deflected off exterior surfaces and deflected away from critical junctions or openings.
Climate change may bring more frequent extreme weather events. New houses, with an expected life of over 50 years, must be designed to cope with these.
New Zealand temperatures are moderate, and we do not have massive variations in temperature within seasons, although reasonably high (35°C in summer) and low
(-10°C in winter) extremes are possible.
The average annual sunshine hours for New Zealand are relatively high at around 2000 hours, and this is associated with generally very high levels of UV rays. This has a significant impact on the stability and durability of claddings, particularly on the north and west faces of buildings, where surface temperatures can easily reach 50°C and may get to 70°C on dark-coloured finishes. Deterioration caused by UV light can affect sealants, paints and finishes that are a critical to weathertight performance.
Changes in temperature affect the thermal movement of the materials, and this movement needs to be allowed for in construction detailing, particularly with movement control joints, fixings, sealants and material laps. Designers need to give consideration to:
- the specification of materials that are affected by temperature
- the effect of colour on performance.
In some areas of the country, building design and construction needs to take account of snow. Snow can build up on roofs and against exterior walls, and can remain in these locations for long periods of time. Claddings need to accommodate this and the increased potential for water entry from melting snow.
Our long, narrow islands and coastline means many areas are subject to marine influences. Salt has a corrosive effect that will damage metals and surface finishes.
In areas of the country subject to geothermal activity, sulphur can corrode metal building materials.
On sites subject to higher risks of corrosion, appropriate materials must be selected.
Updated: 17 May 2016