Approved Document H (Table 6) gives the flattest gradients at which foul drains should normally be laid: 1 in 40 for 75mm or 100mm pipes with a peak flow of less than 1 litre per second, 1 in 80 for 75mm or 100mm pipes with a peak flow of more than 1 litre per second (a 100mm pipe should serve at least one WC), and 1 in 150 for 150mm pipes serving at least five WCs. If a pipe is laid too flat, solids can slow down, become stranded and block the pipe.
All above and below ground horizontal drainage pipes should be laid to an adequate gradient. A gradient of 1 in 80 is suitable for commencing calculations for pipe schemes.
The fall or rise in a pipe may be defined as the vertical amount by which the pipe drops or rises over a given distance. The distance can be between sections of pipe or between manholes.
A gradient may be defined as fall divided by distance.
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