CB 460MM UG Insp Chambr Cover Squre

Below ground drainage carries foul water and surface water away from a building to the sewer, a septic tank or a soakaway. The category covers PVC-U drain pipe, bends, junctions, couplers, adaptors, rodding eyes, inspection chamber bases, risers, covers and frames, anti-flood valves, jointing lubricant, and large twin wall pipe for surface water and land drainage.

Pipe and fittings

Most domestic drains use 110mm pipe, with 160mm for larger flows and 82mm in some systems. Underground PVC-U drain pipe is made to BS EN 1401. Joints are push-fit with ring seals: cut pipe square, chamfer the end, and use the system's own lubricant. Bends and junctions come double socket or single socket, in angles from about 11° up to 87.5°. Socket depths and seals differ between makers, so keep to one system or use the correct adaptor when joining another.

Layout and access

Approved Document H asks for access points, such as rodding eyes, access fittings, inspection chambers and manholes, at or near the head of each run, at bends and changes of gradient or pipe size, and at junctions that cannot be cleared from another access point. As a guide, a 100mm drain carrying the flow from at least one WC can be laid at a gradient as shallow as 1:80. Where a drain is at risk from a surcharging sewer, an anti-flood valve helps stop sewage backing up into the building.

Large diameter and land drainage

Twin wall pipe has a corrugated outer wall and a smooth bore and is graded by ring stiffness, shown as an SN rating. Perforated versions gather ground water for land drains and soakaways.

New drainage work must comply with Building Regulations Part H. For soil, waste and rainwater pipe, see above ground drainage.