The NZ Metal Roof and Wall Cladding Code of Practice is a comprehensive design & installation guide, and a recognised related document for Acceptable Solution E2/AS1 of the NZ Building Code.
When calculating roof drainage where significant inconvenience or injury to people or damage to property, including building contents, is unlikely—e.g., due to an overflow of external eaves gutters— the Average Recurrence Interval (ARI) used must be 10 years.
When calculating roof drainage where significant inconvenience or injury to people or damage to property, including building contents, is likely—e.g., due to an overflow of internal gutters—the Average Recurrence Interval (ARI) used must be 50 years.
A higher level of rainfall should be allowed when designing for higher risk situations.
The rainfall intensity in New Zealand is determined by the rainfall during a ten minute period, measured in millimetres per hour.
Although a rainfall of (for example) 17 mm in ten minutes (that equals 100 mm/hr) is unlikely, the equivalent of 3 mm in two minutes is likely and is, therefore, used as a basis to avoid flooding.
The MRM COP publishes 10 min rainfall for instensity maps for both islands as a quick reference. Rainfall Intensity information for a specific site can be obtained using the High Intensity Rainfall Design System (HIRDS) tool from NIWA at: https://hirds.niwa.co.nz/
The 5.3A New Zealand Rainfall Intensity Map shows areas with a 10% probability that rainfall will exceed the specified amount over a ten-minute duration for an ARI of fifty years.
During rainstorms, long periods of steady rainfall are interspersed with heavy downpours for short periods and the roof-drainage system should be capable of handling the peak intensities without flooding.
On large low pitched roofs, there is a considerable time-delay between the onset of rain and when the water drains at the downpipe. This time lag changes the rate of flow capacity needed for the gutter and downpipe to drain without overflow.
Gutter overflow is acceptable on eaves-gutters or freely discharging downpipes if they are designed to do so, but it is unacceptable from internal gutters or downpipes.
When the site rainfall intensity shown on the 5.3A New Zealand Rainfall Intensity Map is greater than 100 mm/h, the gutter and downpipe cross-section areas must be proportionally increased as prescribed in 5.3.2 Capacity Calculations.