What is the difference between a foul drain and a surface water drain?
Most houses have two separate underground drainage systems doing completely different jobs. How to tell which is which, and why connecting the wrong thing to the wrong one matters.
- Difficulty
- beginner
- Time
- 15 min
- Read
- 4 min
Short answer
The foul drain takes everything from inside the house — toilets, basins, baths, sinks, washing machines — and goes to the sewer for treatment. The surface water drain takes rainwater only, from roofs and paving, and often runs straight to a watercourse or a soakaway. Never connect waste water to a surface water drain.
Almost every drainage question a householder has — where a blockage is, who is responsible, whether a new bathroom can connect somewhere, why the garden floods, why a stream near the estate runs grey — depends on this distinction. It is also the thing most people do not know their house has, because both systems are buried and both look identical from a manhole cover.
Step by step
- Learn what the foul system takes.Everything that leaves the inside of the house: toilets, basins, baths, showers, kitchen sinks, washing machines, dishwashers, and boiler condensate. It runs to the public sewer and is treated before it is returned to the environment.
- Learn what the surface water system takes.Rainwater only. Roof gutters and downpipes, patio and driveway drainage, and sometimes land drainage. It goes either to a separate surface water sewer, to a soakaway on your own land, or directly to a watercourse — and in the last two cases it is not treated at all.
- Know that some systems are combined.In older parts of many towns and cities both flows go into one pipe and one sewer, and everything is treated. Combined systems are why heavy rain causes sewer overflows, and it is why new development is required to keep the two separate.
- Work out which you have.Lift the chambers and look. Foul chambers have channels carrying visible waste and usually smell. Surface water chambers are clean, often dry between rainfalls, and connect back to downpipes and gullies rather than to the soil stack. Following a downpipe to see where it goes is the simplest test of all.
- Follow the soil stack.The large pipe from the toilets, which vents above roof level, always joins the foul system. Tracing where it leaves the building and which chamber it enters identifies your foul run immediately.
- Check for misconnections.A washing machine plumbed into an outside gully that turns out to be surface water, a new shower connected to the nearest available drain, a downpipe from an extension joined to the foul run. All are common, all are wrong, and the first two put untreated waste into a river.
- Understand what it means for blockages.Foul blockages back up into the house and are urgent. Surface water blockages flood the garden or the drive and are usually not. Knowing which system is blocked also tells you who to call and whether it is likely to be the water company's problem.
- Get it right on any new work.Any new bathroom, kitchen, utility or extension connects waste to the foul system and rainwater to the surface water system. Building control checks this. A misconnection found later is the homeowner's to put right, and water companies do trace them.
Tips
- Pour a bucket of water down a downpipe and watch which chamber it appears in. It is the cheapest drainage survey there is.
- Modern chamber covers are sometimes marked FW for foul water and SW for surface water. Older ones are not, but a look inside always tells you.
- Draw a rough plan of your drainage as you work it out and keep it. It is worth a great deal at the point something blocks or an extension is being designed.
- Some water companies charge for surface water drainage on the bill, and offer a rebate if your rainwater goes to a soakaway rather than their sewer. Worth checking.
Questions people ask
Can I connect a downpipe to the foul drain?
Generally not, and on a separate system it is not permitted — rainwater in a foul sewer overloads treatment works in heavy rain. On a genuinely combined system it may already be the arrangement, but new connections should be discussed with the water company or building control.
Where does my surface water actually go?
One of three places: a surface water sewer, a soakaway in your own ground, or straight to a ditch, stream or river. If it is the third, everything that enters that drain reaches a watercourse untreated, which is why misconnections matter.
How do I report a misconnection?
Your water company handles misconnections and has a duty to investigate. If it turns out to be on your property, correcting it is your responsibility, but they will usually advise on what is needed.