Freeboard

Total depth of gutter minus the designed water depth.

In accordance to BS EN 12056-3, the minimum freeboard for valley and parapet gutters;

Table 5 - Minimum freeboard for valley and parapet gutters

Depth of gutter including freeboard, Z mm	Minimum freeboard, mm
Less than 85	25
85 to 250	0.3 Z
Greater than 250	75

In accordance to (SG Gov.) PUB Code of Practice on Surface water drainage, Freeboard refers to the depth from the top of the drain (cope/ bank) to the top of the water surface in the drain at design flow condition. Sufficient freeboard shall be provided to prevent waves or fluctuation of the water surface from overflowing the cope/ bank. Generally, a depth of freeboard equivalent to 15% of the depth of the drain is required.

References

1. BS EN 12056-3:2000
2. Code of Practice on Surface Water Drainage (p.43)