One of the biggest costs to homeowners is flooding from the drainage system, causing nearly 50% of the total damage recorded in flood events. This guide aims to give you an overview of the things to consider when you may be at risk of drainage flooding. This maybe due to an extension or planning permission application, or you are buying a property in a flood risk area. The drainage system is something that is often overlooked but can require relatively simple solutions to fix.

What is the drainage system?

There are two types of drainage: foul and surface water.

  • Foul drainage – carries waste from your property to the main sewer system for treatment. This includes toilets, sinks, showers, baths and washing machines. Foul drainage is split into above ground pipe work, known as sanitary pipework, and below ground, referrred to as foul sewers.
  • Surface water drainage – carries water from hard surfaces, such as roof drainage and rainwater pipes. There are underground surface water drainage known as surface water sewers.

If your property was built after the 1920’s, these two drainage systems should be seperate. You should check whether you have a misconnection, as failure to do so can lead to prosecution.

What is the difference between sewers and drains?

A drain is a conduit to allow liquid to flow, where as a sewer specifically refers to a system of pipes to remove human foul drainage.

Where is an underground sewer or drain located near my house?

Your local authority will provide a sewer map to assist you with your nearest sewers. In addition, utility companies often have services available to request information on drains and sewers, for example, if you live in London, you can request a property search from Thames Water. We would recommend speaking with a drainage expert or architect if you are planning on building an extension. In addition, you can check the waste disposal guidance on the link below.

Guidance on Waste Disposal

What is the size of pipe for various applicances?

Pipes need to be sized for the flow of water, to minimise the risk of blockage and to allow air movement. Advice on pipe sizes is given in Approved Document H. Sanitary pipework should be designed with access hatches, or manholes to provide for access to deal with blockages. Rodding eyes and access chambers should be used to enable all parts of the underground drainage to be cleared and to allow removal of blockages.

What is the maximum fall of foul water drainage?

The typical size of foul pipes for homes in the UK is 110mm. A 110mm foul drain taking the discharge of less than 1 l/s should be laid at a 1:40 (25mm per metre) fall. A foul drain taking the discharge from a minimum of one WC can be laid at 1:80 (12.5mm per metre).

How can I protect my property from drainage flooding?

The key for any flood protection in the drains is to act as a one way valve, allowing water to leave the property but not come back up the drains pipes. This is also known as a non return valve. A non return valve can in be retro-fitted to your property, but the type of valve will depend on a number of factors.

The first step is to determine where you can install a drain non return valve. The easiest place to start is to look around your property for an inspection chamber or manhole, and lift the lid. If you are unable to remove lid yourself, please contact a drainage expert to help you. If you have an inspection chamber, count the number of in pipes into the chamber and assess the size of each pipe. In this case, you can install a retro fit non return valve. No tools are required and can be installed in minutes by simply pushing the valve into the inlet into the inspection chamber. Do not install on the outlet of the chamber. You can watch a video on Youtube on how to install a drain 110mm non return valve.

If you do not have an inspection chamber, then you will need to install an inline non return valve. The type of inline non return valve will depend on whether you are preventing storm or saline water. Although inline non return valves operate with a single flap like a retro fit non return valve, the issue is they require expertise to install. The works may involve digging down, cutting the pipe and installing the valve.

For washing machines and other appliances a smaller 40-50mm McAlpine Non Return Valve would be required. This can be abit tricky to do yourself, but if you watch some videos it is possible to do.

What size of non return valve should I install?

Where can I buy non return valves?

If you have any further questions about the type of non return valve you require to protect your property, contact us on 01484 505509 or email to