Gradients

Foul water drainage systems are generally designed to run at a maximum of theree quarters full bore. Pipe gradients should be established such that the velocity does not fall below 0.70 l/s to ensure adequate self-cleansing.

A 110mm fould drain taking the discharge from a single stack should be laid at a 1:40 (25mm per metre) fall. A foul drain taking the discharge from more than one stack can be laid at 1:80 (12.5mm per metre).

Gullies incorporating in foul water or combined drainage systems must have a 50mm minimum water seal.

The following table is taken from BS EN 752-4:1998 and provides guidance on minimum gradients for different size drains.

(a) Peak flow based on probability flow calculation method
(b) Minimum 1 WC
(c) Minimum 5 WCs

Surface water drainage systems may be designed to run full bore. Drains taking multiple rainwater pipes can be laid to a fall of 1:100 (10mm per metre). Drains from one rainwater pipe should be laid at 1:50 (20mm per metre).

However for a larger or adoptable system please refer to the Underground Drainage Design and Installation Guide or contact our technical hotline for more information.

Also see the Sustainable Drainage Systems section.