Foul Drain
/ faʊl dreɪn /
Also known as: foul water drain, soil drain, sewage drain, foul sewer
Definition
A foul drain is an underground pipe carrying foul water (sewage from toilets, grey water from sinks, baths, showers, and washing machines) from a building to the public foul sewer or to a private treatment system. Must be kept separate from surface water drains in a separate system (required for all new developments). Standard domestic size is 100mm diameter UPVC or vitrified clay, laid at a gradient of 1:40 to 1:80 (25-12mm fall per metre) to maintain self-cleansing velocity. Governed by Building Regulations Approved Document H. Misconnection to surface water drains is an offence under the Water Industry Act.
In practice
Foul drain blockages are a common building maintenance issue. Most domestic blockages occur either at the gully or trap nearest the point of use, or at a change of direction in the underground drain. Blockages are commonly caused by wipes, sanitary products, or cooking fat in the drain. Domestic blockages are cleared with drain rods, a high-pressure water jetter, or electro-mechanical cleaning equipment. A recurring blockage in the same section of drain suggests either a displaced joint, root ingress, or a collapsed section - a CCTV drain survey will identify the defect.
Before building an extension, conservatory, or new structure near an existing drain, the drain position must be identified - either from drain run records (which may be available from the local authority or water company) or by tracing from known inspection chambers. Building over or close to a drain without consent from the water company (for public sewers) or without appropriate protection measures (concrete encasing, bridging structures) can lead to drain damage during construction and subsequent structural problems.
Building Regulations
Approved Document H (Drainage and Waste Disposal) covers the design and construction of foul drainage from buildings. Key requirements include minimum pipe diameters and gradients, bedding specifications, access provision, and the requirement to keep foul and surface water separate in separate drainage systems. New drainage connections to the public sewer require consent from the water company under Section 106 of the Water Industry Act 1991. Any drain installed as part of building work must be notified to Building Control and inspected before being covered over - Building Control will inspect the gradient, pipe size, bedding, joints, and access provision before authorising backfilling.
Full Building Regulations guidance