Site Analysis

The amount, direction and intensity of rainfall on a site affects several aspects of a building’s design, including roof design and construction, cladding choices and construction, flashings, and stormwater drainage.

Rainwater harvesting may also be considered as part of the design brief.

Rainfall intensity

Rainfall varies with season and wind direction and location, so average figures can be misleading. Many parts of the country can get periods of intense rainfall far above average. When you’re assessing a site or designing a building, it’s prudent to be aware of past extreme weather events. A building should be designed to be able to cope with the maximum expected rainfall intensity.

Climate change may result in more extreme weather events and designing for higher rain intensities should be considered as part of the design brief.

Rainfall direction

When considering the weathertightness of any building, remember that different parts of the building will have different demands on the details from wind and rain. While there may be a prevailing wind, the most intense rainfall may come from another direction.

With strong winds, rain can be driven horizontally and in exposed situations up the building face.

Finding information

The National Institute of Water and Atmospheric Research (NIWA) provides climate station data for its 30 climate stations throughout New Zealand, including wind speeds, number of gale days per month, wind roses and more. The NIWA website also has a free climate overview, and NIWA can provide site-specific information on enquiry or using its Climate Explorer web tool. Climate information is also available from the MetService website.

Rainfall intensity curves should 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.

In the past, using a rainfall intensity of 100 mm/hour over a 10 minute period has proved an adequate design figure for external gutters in most parts of New Zealand. Examples of regions where higher rainfall intensity design figures need to be used are Arthur’s Pass, Haast, Milford Sound, Fiordland, around Mount Taranaki and the Kaimai ranges.

However, it is important to ensure that water can be effectively drained away during heavy rain because that is when demand is greatest (see hazards for information about flood risk).

For your clients

To help your clients understand the impact of climate on design, refer them to www.smarterhomes.org.nz/siting-landscaping/climate.