Code of Practice v3.0 Online
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.
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.
The objective of roof drainage systems is to maintain a weatherproof building, to minimise the risk of injury or inconvenience due to flooding, and to avoid potential monetary loss and property damage — including to the contents of buildings.
Roof drainage design requires consideration of:
This section details specific requirements for the sizing of all drainage components.
The effective catchment area for a gutter is determined not only by the plane area of the roof itself but also by the walls adjacent to the roof. When a wall is discharging on to a roof, half the surface area of that wall (up to a maximum height of 10 m), must be added to the catchment calculation.
The COP calculations are based on the plane area of the roof (which is the sloping surface area of the roof), not the plan area (which is the area covered by the roof).
Wind action can influence effective catchment area, and the COP assumes the worst case scenario, i.e., rain striking the roof at an angle perpendicular to the roof plane.
Rainfall intensity can be taken off the maps for 50-year average return intervals (ARI). When the co-ordinates of a site are known, site-specific values can be obtained using NIWA’s HIRDS tool at https://hirds.niwa.co.nz/
Use NIWA’s HIRDS tool for the most accurate rain intensity figures. The HIRDS tool shows figures for historical rainfall intensities and predicted rainfall based on the anticipated effects of climate change, expressed as Representative Concentration Pathways (RCP) levels.
The increased rainfall intensity in a worst-case scenario is typically 11 – 13% higher than historical levels, mostly occurring under the least intense (RCP 2.6) value. To ensure design calculations account for expected climate change, use the most appropriate RCP level.
Rating | Description |
---|---|
RCP 2.6 | A very stringent pathway that assumes CO2 levels peak at 2020 and go to zero by 2100. |
RCP4.5 | An intermediate scenario where CO2 levels peak by 2040, then decline due to the decreased availability of fossil fuels. |
RCP 6.0 | A stabilisation scenario, where CO2 emissions peak around 2080, then decline with the deployment of various technologies and strategies. |
RCP 8.5 | Worst-Case scenario where CO2 emissions continue to rise throughout the 21st century. Thought by some to be based on an overestimation of projected coal outputs. |
Location | Historic | RCP 2.6 | RCP 4.5 | RCP 6.0 | RCP 8.5 | % change |
---|---|---|---|---|---|---|
Whangarei | 137 | 147 | 150 | 149 | 152 | 11 |
Auckland Central | 121 | 131 | 133 | 132 | 135 | 12 |
Mt Maunganui | 149 | 161 | 164 | 163 | 167 | 12 |
Waikanae | 107 | 116 | 118 | 117 | 120 | 12 |
Christchurch | 55 | 59 | 60 | 60 | 61 | 11 |
Dunedin | 71 | 77 | 79 | 78 | 80 | 13 |
Queenstown | 55 | 59 | 60 | 60 | 61 | 11 |
Huntly | 126 | 136 | 138 | 137 | 140 | 11 |
Nelson | 114 | 124 | 126 | 125 | 128 | 12 |
Silverdale | 124 | 134 | 136 | 135 | 138 | 11 |
Rainfall intensity figures quoted on the NIWA site are for maximum intensity over a ten-minute duration. Intensity may vary within this period, and roof drains can overflow quickly when demand exceeds capacity. A 1-minute rainfall intensity can be as much as 4.2 times higher than the 10-minute intensity.
To account for short-term rainfall intensity, various factors should be applied to internal and external gutters, and to drains depending on their location and consequence of overflow. See 5.3.2.3A Short-Term Intensity Multiplication Factors.
The COP drainage calculator multiplies the ten-minute maximum intensity by a factor to allow for short-term fluctuations. This minimum factor varies by gutter location as follows.
Application | Gutter Multiplier | Downpipe Multiplier | |
---|---|---|---|
With Overflow | No Overflow | ||
Valleys | 3.1 | n/a | n/a |
Penetrations | 3.1 | n/a | n/a |
Internal Gutters Residential | 3.1 | 2.1 | 3.1 |
Internal Gutters Commercial | 2.2 | 1.5 | 2.5 |
External Gutters — no Overflow | 2.5 | 1.7 | 2.5 |
External Gutters — with Overflow | 1 | 1 | 1 |
These are minimum factors; higher factors may be applied at the designer’s discretion.
For convenience, ARI maps are included in the calculation section which includes tables for gutter and valley capacity for different rainfall intensities.
In gutters where overflow can enter the structure, it is necessary to have freeboard to allow for wave action, obstructions, and other unforeseen circumstances. 5.4.7 Gutter Capacity Calculator allow for these minimum freeboard values.
Gutter Type | Freeboard |
---|---|
Internal gutters | 30 mm |
Secret gutters | 15 mm |
Valleys > 8° | 15 mm |
Valleys < 8° | 20 mm |
Asymmetrical valleys | 20 mm |
External Gutters with Overflow | No freeboard required |
External gutters with no overflow | 15 mm |