A gutter’s discharge capacity increases with the depth of water over the outlet. The best way to increase the head is to discharge the open end of the gutter into a rainwater head or sump. Swirl at the outlet reduces its performance, so positioning of the outlet is important.
Outlets must be placed at a distance less than or equal to the outlet diameter from the nearest vertical side of the sump.
Where they are connected directly to the drain, 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. All sump downpipes must be able to withstand a water pressure test with an applied head of 1.5 m of water without leakage.
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 water does not back up the downpipe.
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.
Downpipe retaining brackets, otherwise known as stand-off clips as astragals, are used to align the downpipe between the dropper and and discharge sump. Normally they are situated near both ends of the downpipe, with intermediate brackets placed in high traffic areas or where span is greater than 3 m.
Downpipes fixed at an included angle of less than 105° must have a cross-sectional area equal to that of the gutter or be sized by calculation.
Downpipes must be compatible with the roof and gutter material and must comply with the 15-year durability requirement of the NZBC.
Discharging water off an inert surface onto unpainted galvanised rainwater goods can cause corrosion. See Inert Catchment.
Horizontally run PVC downpipes and gutters require a greater provision for expansion than metal, particularly if they are painted a dark colour. Horizontally run PVC downpipes and gutters should have a maximum length of 9 m.
When rainwater is collected into a water tank, there is often not enough distance to obtain adequate fall for one downpipe outlet. In such cases, or whenever the roof design pre-empts a continuous spouting to the tank, it is possible to have several sealed downpipes (some of which can run underground) to discharge into the tank. The outlet discharging into such pipes should be a rainwater head to avoid flooding.
