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.
Water collected by the gutters is transported to the storm-water disposal system via downpipes, and 18.104.22.168B Downpipe Size details the size of downpipes required for both external and internal gutters.
The cross-sectional area of external downpipes must be half the cross-sectional area of the external gutter.
All internal downpipes must be sealed to internal sumps by a compression ring, or similar fitting, and must have access for cleaning at the base, where they are connected directly to the drain. To comply with the 50-year durability requirement of the NZBC, all hidden downpipes must be seamless and must be able to withstand a water test with an applied head of 1.5 m of water without leakage.
Internal downpipes that are easily replaced require a 15-year durability.
Where vertical downpipes are sealed to the drain internally, they must be designed with a minimum of half the cross-sectional area of the internal gutter.
The exterior junction or manhole must be vented to enable free discharge to the ground if the drain is blocked.
Major internal building and consequential damage have been caused by failure to comply with these requirements rather than, as often assumed, because of inadequate design capacity of the roof drainage system. See 5.8.2 Outlets and Overflows.
Horizontal drains are not designed for the maximum rainfall that is required for gutters, and it is likely that their capacity will be exceeded during the life of the building.
To avoid any water back-up if the drain capacity is overloaded or obstructed, an air break should be provided for all downpipes to ensure that drain water does not back up the downpipe.
Ground outlets should be built up to avoid debris and surface water entering the drain.
All exterior downpipes must discharge freely over a grated gully trap or into an oversize pipe which must be a minimum of 50 mm above the adjacent ground level.
Sealed water systems such as siphonic tank systems must have an overflow capacity of 200 mm/hour.
Downpipes fixed at an included angle of less than 105°, must have a cross-sectional area equal to that of the gutter.
Downpipes must be compatible with the roof and gutter material and must comply with the 15-year durability requirement of the NZBC.
Galvanised steel downpipes should not be used to discharge rainwater from AZ coated or painted roof cladding.
When using galvanised rain-water goods with other materials, inert catchment corrosion should get special attention. (see section 2.6).
Horizontally run PVC downpipes should have a greater provision for expansion than metal, because they absorb heat, particularly if they are painted a dark colour. They should have a maximum length of 9 m before discharging into a rainwater head that will act as an overflow. PVC downpipes and spouting are also prone to damage by hail.