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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.
On this page:
- locating information
- managing stormwater.
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.
NIWA’s High Intensity Rainfall Design System (HIRDS) provides rainfall intensity estimates for any location in New Zealand based on historical data. It also gives projections of future rainfall intensities for various climate change scenarios.
Regional, city or district councils may also be able to provide rainfall data for their area, and Appendix A of Acceptable Solution E2/AS1 provides 10 minute duration rainfall intensities for various locations.
BRANZ MAPS also provide rainfall intensity data 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.
In November 2020, E1/AS2 was introduced. This Acceptable Solution states that AS/NZS 3500.3:2018 Plumbing and drainage Part 3: Stormwater drainage (with certain modifications) is an Acceptable Solution for the design and installation of surface water drainage systems.
(The 2018 version of AS/NZS 3500 is still the one currently in use as means of demonstrating compliance with the Building Code, even though a new version of the standard was published in May 2021. The 2021 version is expected to come into effect as a means of compliance in November 2022.)
For more information, see the section on Stormwater control and landscaping.
Updated: 10 June 2021