Foul Water Drainage Requirements

What This Requirement Covers

Approved Document H1 requires that adequate provision is made for the drainage of foul water (waste from WCs, basins, baths, showers, sinks, and other sanitary appliances) from buildings to a public sewer, private treatment system, or cesspool. The drainage system must be designed and installed to convey foul water without leakage and without creating health hazards.

Key Requirements

Above-Ground Drainage

1. Waste pipes from individual appliances:

- Basin: 32 mm minimum diameter - Bath, shower, sink: 40 mm minimum diameter - WC: 100 mm minimum diameter - Washing machine, dishwasher: 40 mm minimum diameter

1. Maximum trap-to-stack distances (to prevent siphonage of the trap seal):

- 32 mm waste: 1.7 metres - 40 mm waste: 3.0 metres - WC: 6.0 metres (with minimum gradient of 18 mm/m)

1. Soil and vent stacks: Minimum 100 mm diameter for stacks serving WCs
2. Stacks must be vented at roof level (minimum 900 mm above any opening window within 3 metres) or connected to an air admittance valve (AAV) in appropriate circumstances

Below-Ground Drainage

1. Minimum pipe diameters: 100 mm for drainage serving WCs; 75 mm for drainage serving only waste appliances
2. Minimum gradients:

- 100 mm pipe: 1:40 for up to 5 WCs; 1:80 minimum where flow from at least one WC is discharged - 150 mm pipe: 1:150 minimum

1. Pipes must be laid on a firm, uniform bed (granular material or concrete) with appropriate support
2. Inspection chambers (manholes) must be provided at changes of direction, gradient, or pipe size, and at the head of each drain run
3. Maximum distance between access points: 22 metres for 100 mm drains, 45 metres for 150 mm drains

Connection to Public Sewer

1. All new developments must connect to the public sewer where one is available within a reasonable distance
2. Connection requires approval from the water and sewerage company under Section 106 of the Water Industry Act 1991
3. The connection must be made by an approved contractor

Building Over or Close to Sewers

1. Building over or within 3 metres of a public sewer requires approval from the sewerage company
2. A build-over agreement sets out conditions for protecting the sewer during construction
3. Building Control may require a CCTV survey of the sewer before and after construction

Practical Compliance Tips

1. Plan the drainage layout early to minimise pipe runs and ensure adequate gradients
2. Use a drainage layout drawing showing pipe sizes, gradients, and inspection chamber positions for the Building Control application
3. Ensure all below-ground joints are properly made and tested before backfilling (air or water test)
4. Provide adequate access for maintenance; rodding points and inspection chambers must be accessible
5. Do not connect surface water drainage to the foul sewer unless specifically approved by the sewerage company
6. Check with the water company about any sewer that may run through or close to the site before designing the foundation layout
7. Use flexible couplings where drains pass through or under foundations to accommodate differential movement