Non-return valves

The hidden route into a property

During a flood, drains and sewers can become surcharged. The pressure in the drainage network rises and water flows backwards, emerging through toilets, shower trays, sinks, floor gullies and washing machine outlets. This water is often contaminated with sewage, making the damage and clean-up far worse.

Many homeowners fit flood doors and barriers and are surprised when water still rises through the ground floor toilet. Non-return valves, also known as anti-flooding valves or backflow valves, address this route.

How they work

A non-return valve allows water to flow in one direction only. Under normal conditions, wastewater from the property flows out through the valve into the drain. If water starts to flow back, a flap or ball closes against a seat and prevents it passing. When the pressure falls, the valve reopens and normal flow resumes.

Some valves operate automatically. Others include a manual closure that can be engaged when flooding is expected, giving extra security. Where a building has a basement or floor level below the drain, a pumped system may be needed instead, because the valve alone cannot lift wastewater out.

Where valves are fitted

Valves can be installed on the main drain leaving the property, usually in an inspection chamber, which protects every connection at once. Alternatively, they can be fitted to individual appliances such as a downstairs toilet or basement sink. The right location depends on the layout of the drainage, access and whether any upstream connections, such as rainwater pipes, should continue to drain during a flood.

An installer will usually carry out a drainage camera survey first to confirm the layout, pipe sizes and condition. Cracked or displaced pipes may need repair before a valve can be relied upon.

Important considerations

When a valve closes, wastewater from the property cannot leave. Occupants should avoid using toilets, showers and appliances during a flood, otherwise water will back up inside the building. This should be explained at handover and included in any household flood plan.

Valves on shared drains need care. Fitting a valve to a drain shared with neighbours can affect them, and may require agreement from the sewerage undertaker. An experienced drainage specialist will advise on ownership and responsibilities.

Look for valves that meet the relevant European standard for anti-flooding devices in buildings, BS EN 13564, and that are suitable for foul water where applicable.

Maintenance

Non-return valves sit in the wastewater flow and can be affected by debris, fat, wipes and grit. They need periodic inspection and cleaning, typically once or twice a year and after any flood event. Choose a valve with good access for maintenance, and keep a record of servicing.

Costs

Cost depends on the valve type, the location, and whether excavation or a new inspection chamber is required. Fitting a valve into an existing accessible chamber is usually straightforward; installing a new chamber under a driveway or garden is more involved. Include the cost of a camera survey in your comparison of quotations.

Finding a specialist

Non-return valves are fitted by both drainage contractors and flood-protection installers. Search Flood Protection Finder by postcode, compare profiles and request quotations. Ask each firm whether they will carry out a camera survey and how they will provide maintenance access.