For structural reasons little or no fall can usually be provided and it can be said that any built-in fall merely assists in draining the gutter. Therefore, with the numerical value of incline (I) being zero, the normal hydraulic formulae cannot be used to determine the dimensions of level gutters. If conditions existing at the outlet or point of free discharge are used, a rational approach is possible if a fundamental fl ow formula is applied. It can be shown that the depth of water at the point of free discharge, i.e. maximum fl ow for a given depth will be two thirds of the depth upstream but to provide a safety margin, design for depth of fl ow at ‘still’ end to be twice the depth at the point of free discharge. In addition, allow for ‘freeboard’* of 50 to 60 mm and minimum gutter width of 250 mm. *Freeboard-height above water level to top of gutter edge when fl owing at maximum rated depth.