When heavy rain overwhelms the public sewer, the water has to go somewhere — and sometimes that somewhere is back up private drains and out through the lowest fittings in the house. A non-return valve, often called an anti-flood valve or backflow valve, is a one-way flap fitted on your drain that lets your waste out but stops the sewer coming back in. We fit them where properties are at risk of surcharge. Call 07427 195142 to discuss whether one would help.
Why sewage comes back up
In a combined sewer, foul water and rainwater share the same pipe. In a storm, the sewer can fill faster than it can drain, and the level rises above the lowest private connections. Properties at the low point of a street, basements, and homes near watercourses are the usual victims. The first sign is often a toilet or shower tray filling from below during heavy rain.
If this is happening to you, it is worth reporting the flooding to Scottish Water as well, because the public sewer is their responsibility and repeated surcharge is something they need to know about.
How a non-return valve works
The valve is a flap or gate installed in the drain run, usually in or near the inspection chamber. Under normal flow it sits open and everything passes. When water tries to travel back towards the house, the flow pushes the flap shut and seals the line. It is entirely mechanical — no power, no controls, nothing to switch on when a storm is forecast.
Larger anti-flood valves for whole-property protection are designed so the flap can also be locked closed manually, and so it can be opened for cleaning and inspection.
The trade-off to understand
A non-return valve protects you from the sewer, but while it is shut, your own waste cannot get out either. During a long surcharge, using upstairs toilets and appliances can fill your own drain behind the valve. For most homes this is a manageable limitation — the valve is only closed during the storm conditions that would otherwise flood you — but it is worth understanding before fitting one. We explain how it behaves so you can decide.
Where the valve goes
The right position depends on the layout of your drains. Usually it goes on the final run before the drain leaves your property, so everything in the house sits behind it. Where a property has more than one connection to the sewer, each may need its own valve. A CCTV drain survey or drain tracing first makes sure the valve ends up where it will actually protect you, and that the pipe it fits into is in good condition.
Maintenance
The flap and its hinge need to stay clean to seal properly. An occasional inspection — lifting the chamber cover and checking the valve moves freely and is free of debris — keeps it reliable. Valves with an access point make this a quick job. If the drains are ever jetted, the valve should be checked afterwards.
Questions
Will a non-return valve stop my basement flooding from the sewer?
If the flooding is backflow from the public sewer, yes — the valve seals your drain against it. It cannot stop flooding from surface water, groundwater or your own pipework, so the cause needs confirming first.
Can I still use the toilet while the valve is closed?
Only into your own drain, which will fill behind the valve. During a surcharge it is best to limit water use until the sewer level drops and the valve reopens.
What affects the cost of fitting an anti-flood valve?
The valve size, where it can be fitted, whether a chamber needs altering or a new one installed, and whether a survey is needed first. We price it after seeing the layout.
Is sewer flooding my responsibility or Scottish Water's?
The public sewer is Scottish Water's, and repeated surcharge should be reported to them. Protecting your own property from backflow with a valve is normally the owner's choice and cost.