Gradients

Foul water

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

A 110mm foul 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).

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 to 1:50 (20mm per metre).

If designing a complete system, guidance should be taken from BS EN 752-4: 1998 & BS EN 12056:2000, or contact us for technical advice.

Note: If you are installing a new soil stack or drainage system, or significantly altering an existing system, you will need to inform the Building Control Department at your local Council. You will need to demonstrate that the work carried out conforms to Building Regulations: England & Wales or Scotland