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Understanding all the features of a site, using and protecting the best, and minimising the impact of the worst.
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.
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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. Consider also flood risk and climate change forecasts for the region. In some regions, more rainfall or more extreme events are projected.
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.
The National Institute of Water and Atmospheric Research (NIWA) provides climate station data for its 30 climate stations throughout New Zealand, including rainfall, number of wet days per month and climate extremes.
Rainfall intensity curves may be available from the local council. Where they are not available, use the design intensities given in New Zealand Building Code Acceptable Solution E1/AS1 Surface water. These intensities, in mm/hour, are based on a 10% probability of such rainfall occurring annually for a period of 10 minutes.
BRANZ MAPS provide data on rainfall intensity for any given address in New Zealand.
Development of a site typically means larger volumes of rainwater surface runoff. In some areas, new homes cannot simply connect to a local authority stormwater system to deal with stormwater, but must manage it on site through the use of soak pits and other methods. Where this is the case, site analysis should include consideration of where a soak pit may be located on the site.
Updated: 19 November 2019