How does basement drainage work?

Basement drainage collects water from the floor (via channel drains or point drains) and directs it to a sump chamber. A submersible pump then lifts the water through a rising main to the above-ground drainage system. A non-return valve (NRV) prevents backflow. The system is designed per BS EN 12056-4 (pumping installations for gravity drainage systems).

What size sump chamber do I need?

A minimum 25-litre sump chamber is recommended for drainage-only basements. If the basement has a bathroom (WC, shower, basin), a 50-litre or larger chamber is needed to handle the peak discharge. Package pumping stations for basement bathrooms typically come with integrated chambers of 50-100 litres.

Do I need a non-return valve on a rising main?

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 cycle continuously. The NRV should be accessible for maintenance and positioned above the sump but below the connection to the gravity drain.

Can I have a bathroom in a basement?

Yes, but because the drainage is below the sewer level, you need a pumped drainage system. Options include a macerator pump (for a single WC + basin) or a package pumping station (for a full bathroom suite). The rising main connects to the above-ground gravity drain at a suitable point. Building Regulations approval is required for the drainage connection.

What diameter rising main do I need?

For drainage-only sump pumps, 32mm rising main is usually sufficient. For basement bathrooms with a macerator, 40mm is typical. For larger package pumping stations handling multiple sanitary fittings, 50mm or larger may be needed. The diameter depends on the pump flow rate and the acceptable friction loss over the pipe length.

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Basement Drainage Calculator

Calculate channel drain layout, sump collection, pump capacity, rising main sizing, and non-return valve requirements for basement below-ground drainage.

Basement Perimeter

Internal wall perimeter for channel drain

Floor Area

Basement floor area

Channel Drain TypeLinear (perimeter) or point (floor gulley)

Pump TypeSelect pump type for your installation

Pump Head

Vertical lift to above-ground drain

Basement Bathroom

WC/shower requires larger pump and sump

How We Calculate This

This calculator designs the below-ground drainage layout for a basement, including collection drains, sump chamber, pump, and rising main to the above-ground drainage system per BS EN 12056-4.

Channel drain layout

Linear channel drains are installed around the perimeter of the basement floor, collecting water from the Type C cavity membrane system and directing it to the sump. Alternatively, point drains (floor gulleys) can be used at a rate of approximately 1 per 15m² of floor area.

Pump and rising main

The pump must lift water from the sump to the above-ground drainage connection. The rising main diameter and pump capacity are matched to handle the expected flow rate with adequate head. A non-return valve on the rising main prevents backflow when the pump is off.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.