COP:roof-drainage; sumps

5.8 Sumps 

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.

 

 

 

 

 

 

 

5.8.1 Rainwater Heads 

Rainwater heads should be placed on the outside of the building to ensure that the gutter will not overflow. Both rainwater heads and sumps can reduce turbulence and provide a head of water to maximise downpipe flow.

 

 

5.8.2 Outlets and Overflows 

The type of outlet affects the flow rate of water from a gutter. Weir outlets are recommended as the best way to ensure positive flow from gutters or spouting with limited fall; they also provide an automatic overflow, because the spouting or gutter is not sealed to the downpipe or rainwater head.

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.

The overflow opening of a rainwater head from an internal gutter must have a cross-sectional area equal to that of the gutter, and the overflow weir must be 25 mm below the sole of the gutter. See 5.8.1A Rainwater Heads.

Overflows must provide a conspicuous warning that maintenance is required and must discharge clear of the building.

 

 

Where the position of an outlet of a parapet wall gutter is on an outside wall, any scupper overflow cut through the wall should discharge on to the same property.

For parapet wall gutters, where the position of an outlet is within the building, a minimum of two downpipe outlets from the sump should be used, joined separately to the downpipe beyond any offset. Alternatively, a separate overflow pipe of equal capacity to the outlet can be used with the top 25 mm below the sole of the gutter and run to discharge onto the ground. See 5.8.2A Outlet and Overflow Types.

Downpipes that discharge over a gully trap or ground sump provide an automatic overflow if the drain becomes blocked. See 5.8.2C Gully Trap.

Using an oversize drain without any sealing can also act as an overflow by providing an air break. See 5.8.2D Air Break.

Spouting can be made fail-safe for overflowing by providing a rainwater head as shown in 5.8.2A Outlet and Overflow Types.