Surface Water Drain
/ ˈsɜːfɪs wɔːtə dreɪn /
Also known as: rainwater drain, storm drain, surface water sewer, stormwater drain
Definition
A surface water drain carries rainwater and clean surface runoff (from roofs, driveways, and hard standings) away from buildings and land. In a separate drainage system (required for all new developments), it is entirely separate from the foul drain and discharges to a watercourse, surface water sewer, or sustainable drainage system (SuDS) - not to the sewage treatment works. The preferred hierarchy under Approved Document H is: infiltration on site first (soakaway), then discharge to watercourse, then surface water sewer, and finally combined sewer as a last resort. Mixing foul water into the surface water drain is an offence under the Water Industry Act.
In practice
Converting a front garden to a driveway (a very common domestic project) creates new impermeable hard standing that generates surface water runoff. Under the Town and Country Planning Act, paving a front garden with impermeable materials requires planning permission if the area is over 5m2 - but using permeable paving (block paving or gravel on a permeable sub-base, allowing water to soak through to the ground) does not require permission. This distinction reflects the national policy push towards on-site infiltration to reduce downstream flooding.
When surface water soakaways are relied upon to dispose of roof drainage, a soakaway percolation test must be carried out to confirm the ground is permeable enough to accept the required flows. In heavy clay soils, soakaways do not work and an alternative disposal route is required. The soakaway must also be positioned away from any building foundations - typically a minimum of 5m from the nearest foundation and 2.5m from the boundary. Building Control will check the soakaway design and test results as part of new build or extension drainage approval.
Building Regulations
Approved Document H (Part 3, Rainwater drainage) requires rainwater from roofs and paved areas to be managed in a way that avoids flooding and does not overload the sewerage system. The preferred method of disposal is infiltration to ground on site. Where this is not practicable, discharge to a watercourse or surface water sewer is required. For new major developments, the SuDS national standards (Schedule 3 of the Flood and Water Management Act 2010 as enacted in Wales; under separate national planning policy guidance in England) require SuDS to be designed into new schemes. All new drainage works must be notified to Building Control under Part H, and drainage connections to public sewers require Section 106 consent from the relevant water company.
Full Building Regulations guidanceSee also