Basement Drainage Calculator

A Type C drained (cavity) system to BS 8102:2022 collects groundwater behind a cavity-drain membrane and channels it — via a perimeter drainage conduit or floor channels — to a sump. A submersible pump then lifts the water through a rising main to the above-ground drainage system, with a non-return valve (NRV) to stop backflow. For Grade 3 habitable spaces, BS 8102:2022 expects design by a competent person plus a standby pump, a high-water alarm and, where possible, a gravity overflow.

It depends on whether you are pumping groundwater or foul water. For a groundwater cavity-drainage sump, use the proprietary chamber supplied with the pump — these are typically around 60 to 130 litres or more (e.g. Delta/Newton). For foul water (a basement bathroom), Approved Document H 2015 paragraph 2.39 requires the effluent receiving chamber to hold the 24-hour inflow to cover any disruption to service, taken at 150 litres per head per day. So a 2-person bathroom needs at least about 300 litres of storage, not 25 to 50 litres. This calculator sizes the foul chamber as occupants × 150 litres; always confirm against the pump-station manufacturer’s data and BS EN 12056-4.

Yes, a non-return valve (NRV or check valve) is essential on the rising main to prevent water flowing back down into the sump when the pump stops. Without an NRV, the pump would short-cycle. The NRV should be accessible for maintenance and positioned above the sump but below the connection to the gravity drain. An anti-flooding/anti-backflow device may also be needed where the connection is below the flood level of the upstream sewer (Approved Document H).

Yes, but as the drainage is below the gravity sewer you need a pumped system — a macerator pump (for a single WC plus basin) or a package pumping station (for a full bathroom suite). Critically, under Approved Document G (2015 edition, 2024 amendments, for use in England) paragraph 4.25 a macerator-served WC is only permitted where there is ALSO a WC discharging to a gravity drainage system; the pump must meet BS EN 12050-1:2001. The rising main connects to the above-ground gravity drain at a suitable point, and the drainage work is notifiable under the Building Regulations (Part H).

For a macerator the discharge is small-bore — 22mm or 32mm depending on the model (e.g. Saniflo/Uniflo); 22mm is commonly upsized to 32mm over longer horizontal runs. A groundwater sump or package pumping station handling free-flow foul typically uses a 32mm to 50mm rising main. There is no single fixed size: BS EN 12056-4 sizes the pipe so the velocity stays at or above the self-cleansing minimum (around 0.7 m/s) while the pump still meets the required total head, so always check the chosen pipe against the pump curve and run length. The diameters here are indicative starting points, not a code-mandated result.