Rainwater heads are situated outside the building envelope and sumps are internally located.
They both serve to increase the head of water entering a downpipe, and to provide an overflow capacity to safely discharge water when downpipe capacity is compromised or exceeded. The overflow should be obvious so discharging water warns the occupant that downpipe capacity has been exceeded or the primary downpipe is blocked.
Rainwater heads must be at least as wide as the gutter and have an overflow (normally a weir type). The cross-sectional area of the overflow must be at least equal to that of the required downpipe size for the catchment being served. The lower edge of the overflow must be at least 25 mm below the sole of the gutter, and the upper edge must be at least 25 mm below the upper edge of the gutter.
Internal sumps must have overflows. These are often a secondary pipe overflow, with the outlet height positioned above the level of the primary outlet. Internal sumps must also be sealed against the sides and sole of the gutter.
An internal sump should have a guard that prevents debris from blocking the outlet. A removable aluminium expanded-metal box can be fitted at 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, so it is preferable to balloon-type guards. A leaf guard should have a horizontal surface area of at least four times the size of the downpipe outlet area and should be installed at roughly mid-height of the sump depth. Gratings can cause sump blockage, and this can reduce the outlet capacity.
Gratings or guards should be designed so that any debris will float, and hail, or obstructions, such as a tennis ball, will not wedge and block the guard. Gratings or guards should be cleared of accumulated debris regularly as part of normal maintenance.
Overflows must discharge clear of the building to clearly show that downpipe capacity has been exceeded; it should be an obvious indication that the gutters need maintenance.
The overflow opening of a rainwater head from an external gutter must have a cross-sectional area equal to that of the downpipe. The bottom of the overflow must be no higher than 25 mm below the bottom of the spouting.
Where the position of an outlet of a parapet wall gutter is on an outside wall, any scupper outflow should discharge into a rainwater head.