Drainage

Drainage

Foul drains
recommended minimum gradients
Peak flow Pipe size Minimum Maximum
l/s mm gradient capacity
l/s
< 1 75 1 : 40 4.1
< 1 100 1 : 40 9.2
> 1 75 1 : 80 2.8
> 1 100 1 : 80 * 6.3
> 1 150 1 : 150 † 15.0
Pipe Ø Gradient Pipe Ø Gradient
50 1 : 500 150 1 : 2160
75 1 : 860 175 1 : 2680
100 1 : 1260 200 1 : 3200
125 1 : 1680 225 1 : 3720
* Minimum of 1 WC † Minimum of 5 WCs
Land drains
in normal soils – minimum gradients
Traps
minimum sizes and seal depths
Appliance Ø trap Seal depth Ø trap Seal depth
mm mm mm mm
washbasin 32 75 waste disposer 40 75
bidet 32 75 urinal 40 75
bath* 40 50 sink 40 75
shower* 40 50 washing machine* 40 75
syphonic WC 75 50 dishwasher* 40 75
*Where these fittings discharge directly into a gully the seal depth may be
reduced to a minimum of 38 mm.