Drain Installation and Replacement

Most drainage installation work comes with building work:

• Extensions: New drains for a kitchen, utility room, WC or shower, and often rerouting an existing drain or moving an inspection chamber that sits where the new floor will go. See our house extensions and kitchen extension pages.
• Conversions: Connecting a new bathroom in a loft, garage or basement to the existing drains. Where a gravity connection is not possible, a pumped system may be needed, through our Saniflo installation service or, for a basement, a pumped drainage system planned with our basement conversions team.
• New builds: Complete foul and surface water drainage for new builds, from the building to the connection with the public sewer.
• Outside: New gullies, channel drains for driveways and patios, and rainwater connections.

We plan the route on the drawings before work starts, so the drains, the foundations and the floor levels all work together.

Many drains can be cleaned, patched or lined from inside without digging, through our drain repairs and lining service. Drain replacement is the answer when the pipe is too far gone:

• Collapsed sections, where the pipe has broken and soil has fallen in.
• Pitch fibre drains, used in many post-war homes, which can blister, deform and flatten with age.
• Badly displaced or broken clay pipes, often from tree roots or ground movement.
• Drains with the wrong fall, where a sag or backfall keeps causing blockages.
• Drains in the way of new building work, which need rerouting.

The decision should be based on evidence. A CCTV drain survey shows the condition of the whole run, so we replace only the sections that need it.

A gravity drain needs a steady fall so that water carries the waste with it. For a typical 100 mm household foul drain, a fall of around 1 in 40 is common, and Approved Document H allows flatter gradients down to around 1 in 80 where the drain serves at least one WC. Too flat, and solids settle and block the pipe; too steep is also avoided, as water can run away from the solids.

The fall must be continuous, with no sags. Pipes are laid on a bed of granular material, surrounded and covered before backfilling, and given extra protection where they are shallow or under a driveway.

• Inspection chambers: Access points at junctions, changes of direction or gradient, and the head of the drain, so the drain can be rodded, jetted and surveyed later. Today these are usually plastic chamber units, with covers suited to their location, including load-rated covers on a driveway.
• Pipe materials: Plastic (PVC-U) or vitrified clay pipes with flexible joints, which tolerate small ground movements.
• Foul and surface water: Waste from toilets, sinks and appliances goes to the foul drain. Rainwater should not go into a foul-only sewer. Misconnections cause pollution and overloading.
• Soakaways: Under Part H, rainwater should go to a soakaway or other infiltration system where the ground allows, before a watercourse or sewer is considered. A percolation test shows whether the ground drains well enough. Soakaways are typically kept at least 5 m from a building.

Sustainable drainage (SuDS), such as permeable paving, also applies to front gardens: see our driveways page.

Usually, yes. In England, new drains, and extending or altering underground drainage, are building work under Part H of the Building Regulations, so building control is normally notified before work starts and inspects the drain before it is covered. The drain is usually tested for leaks. Like-for-like repairs to an existing drain generally do not need approval; where it is unclear, we check with building control.

Connecting a new drain to the public sewer also needs the water company's approval, under Section 106 of the Water Industry Act 1991. In London, the sewerage company is usually Thames Water. Work in the road or pavement also needs permission from the highway authority.

A build over agreement is permission from the water company to build over, or within 3 metres of, a public sewer. It protects the sewer and makes sure the water company can still reach it.

Since October 2011, most shared and lateral drains have been public sewers, so a pipe running through your garden may belong to the water company even if it serves only a few houses. The water company usually wants a CCTV survey before work starts and often another one afterwards, and may set conditions such as protecting the pipe, moving a manhole or diverting the sewer. We check the sewer records early in the design, so the agreement does not hold up the build.