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 roof drainage system for a building consists of four separate parts:
- gutter or spouting;
- outlet, sump, rain-water head;
- downpipe; and
N.B. Ground drainage is outside the scope of this Code of Practice.
This section specifies good trade practice for the design of roof drainage systems including eaves, valley and box gutters, sumps, rain-water heads and downpipes based on the Average Recurrence Interval (ARI) (see 8.2.) and the applicable catchment area calculations.
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 the contents of buildings.
Any seepage can lead to dampness that encourages the growth of moulds, some of which are detrimental to health. Flooding, not necessarily related to the intensity of rainfall or the design of the drainage system, is often caused by gutter or spouting blockages as a result of inadequate regular cleaning and inspection. Drainage systems, as described in this section, will not perform as required without on-going maintenance. See 16 Maintenance .
Roof drainage design requires consideration of the following:
- rainfall intensity;
- catchment area;
- cross-sectional gutter area;
- sump design;
- the cross-sectional area of downpipes;
- water disposal from downpipes;
- roof cladding profile capacity;
- roof pitch; and
- penetrations which obstruct water flow.