Drains can be overwhelmed by storm water during heavy down pours, or in flood events, which can lead to backflow in the drainage system. A drain non return valve has a single flap to prevent water backing up the drain pipe and into your property.

There are two types of non return valves for drainage: Retro-fit and In-Line.

Retro-fit – the easiest way to protect from backflow is a retro-fit non return valve, which are simply pushed into the pipe leading into the inspection chamber or into the end of drain pipe. These valves are DIY, require no expertise or tools to fit. Example retro-fit drain non return valves are:

In-line – if you cannot find the inspection chamber outside your house, you will need to install an in-line non return valve. These require a lot more work, as a hole will need to be dug before the cutting of the pipe is required. We would recommend you call a specialist to advise on the best location and works required.

Where in the drain to install a non return valve?

Inspection Chamber – every property needs to be connected to the drainage system. An inspection chamber is a manhole outside your property which allows the council to inspect the drains without requiring access to your home. The inspection chamber will be approximate 350mm wide, but can be 450mm wide at the bottom. The inspection chamber can be up to 3m but the majority of inspection chambers are easily accessible and within an arms length (12-18 inches below ground level). A manhole cover will be in place which you will need to remove to access the drain pipe. It is worth checking the size of the manhole as some inspection chambers are too small to house multiple non return valves, however this is rare.

End of Pipe – if you have a drainage pipe leading to a pond, stream or river, you may want a non return valve to go over the top of the pipe. In this situation, you can install a end of pipe non return valve. Example includes:

New chamber outside your property  if you cannot find an inspection chamber or the end of the drain pipe, you will need to find plans to understand where your drains are connecting into the drainage system. Once you have found the pipe call a drainage expert to discuss installing a new chamber for a non return valve. 

How does drain non return valve work? 

The drainage non return valve can be inserted into the foul pipe inside the inspection chamber. The valve has a single flap, which is in the closed position most of the time. When water leaves the property, the flap opens and will close again once the pressure has rescinded. The valve is designed open with just the pressure from the flush of a toilet. In a flood event, or a back up of water due to blocked rains, the valve is closed shut to prevent back flow. In addition, a stainless steel flap stops rats in the drains.

Size of Drainage Non Return Valves 

The size of the drain pipe will depend on the type of property leading to the drainage system. In the UK, the standard drain pipe sizes are:

  • Residential properties – 110mm
  • Commercial properties – 150mm

Benefits of Drainage Non Return Valve 

  • Anti-flooding solution, preventing backflow from entering your home from the sewage system.
  • Rat blocker for drains – a single stainless steel flap will stop rats climbing up the drains
  • Stop smells from the drains – with the single flap in the closed position a drain non return valve stops smells entering your property.

How to install a retro-fit non return valve

  1. Locate the inspection chamber nearest to the toilet you are looking to protect
  2. Put on rubber gloves (included with every valve)
  3. Open the inspection chamber lid
  4. Flush the toilet to check which foul pipe you want to protect
  5. Simply push the 110mm non return valve into your chosen pipe

Where to buy drainage non return valves? 

You can buy non return valves from our shop online or from major e-commerce platforms such as:

If you have any questions, contact us on 01484 505509 or email to info@thefloocompany.co.uk.