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 discharge capacity of a gutter increases with the depth of water over the outlet, and the best way to increase the head is to discharge the open end of the gutter into a sump. Swirl at the outlet will reduce its performance, so the positioning of the outlet is important.
Sumps must have a grating or leaf-guard, have a flat base and must project above the calculated level of flow.
Overflows must provide a conspicuous warning that maintenance is required and must discharge clear of the building. Sumps must be the same width as the gutter and have a depth of 300 mm. Internal sumps must have an overflow as shown in 5.8B Internal Sump with Overt Overflow, otherwise they will act as an overflow and cause damage to the inside of the building. Outlets must be placed at a distance less than or equal to the outlet diameter from the nearest vertical side of the sump.
Sumps must have a grating or leaf-guard, but must not be placed directly in the outlet. The grating must have a flat base and not be tapered. See 5.8C Sump Leaf Guard.
The overflow from an internal gutter can be an unsealed joint at ground level between the downpipe and the drain. See 5.8.2 Outlets and Overflows.
Gratings can cause sump blockage and can reduce the outlet capacity.
Gratings or guards should be designed so that any debris will float, and hail or obstructions such as a ball will not wedge and block the guard. Gratings or guards should be cleared of accumulated debris regularly during normal maintenance.
An internal sump should have a guard that prevents debris from blocking the outlet. An aluminium expanded metal, removable loose fitting box can be fitted as shown in 5.8C Sump Leaf Guard a minimum of 40 mm below the sole of the gutter. Because the top is flat, it is unlikely that the entire surface area of the outlet can become blocked as is the case with balloon type guards.