The two systems explained
A foul drain carries waste water from toilets, sinks, baths and appliances, while a surface water drain carries rain. A surface water drain carries rainwater from roofs, paths and driveways. Some areas have separate public sewers for each.
Others have a combined sewer that carries both. Which you have depends on the age of your area. Ask your water company if you are unsure.
Some homes have a mix of both. For example, a house may have a separate rainwater drain at the front and a combined drain at the back. Because changes over the years can cause surprises, a survey gives the best answer.
How to tell which system your property has
Look at where your downpipes go, because they show how rain leaves the house. If they join a gully or a pipe that runs to a soakaway or a surface water sewer, the system is separate. If they join the same drain as your toilet, it may be combined.
Check any drainage plans you have. A camera survey can also trace the pipes. Ask a drainage engineer, since drains are hard to judge from above.
Why it matters: misconnections
A wrong connection sends rainwater into the foul drain or waste into the surface water drain. Rainwater in a foul drain can overload it in storms. Waste in a surface water drain can pollute streams.
For this reason, connections matter. Check the plan before you connect new pipes, because mistakes are costly to fix.
Common misconnections in older properties
Extensions and conversions often add pipes without care, so wrong connections are common. A new kitchen or bathroom may connect to the wrong drain. Downpipes may join foul pipes by mistake.
Old homes may also have had changes over many years. Records can be missing, so a survey helps.
Finding and correcting misconnections
A camera survey and dye tests can show where each pipe goes, since they trace the flow. Once found, a misconnection should be corrected. Ask a drainage engineer to advise.
Tell your water company if you find a wrong connection. They may help, since misconnections can affect the sewer.
New drainage connections: getting it right
Plan new drainage before you build, because mistakes are costly to fix. Connections to public sewers usually need the water company’s agreement. Building control may need to know too.
Read our guide on plumbing a kitchen extension for related advice. Use a qualified professional, because mistakes are hard to put right.
Related guides and local help
If you want to know more, read Why Drains Smell Worse in Summer and How to Stop It. You can also read 10 Hidden Plumbing Problems in Victorian Homes to Look For and CCTV Drain Survey: What to Expect and When You Need One, because they cover closely related problems.
These guides help too: Soakaway Problems: Why They Fail and What to Do About It, Emergency Plumber Costs: What Affects the Price and Tips, Septic Tank Maintenance: A Complete Guide for UK Homeowners, Plumbing Emergency? Here Is Exactly What to Do Right Now and 7 Signs Your Restaurant Drain Needs a Maintenance Contract. Each one explains a related topic in plain English.
Need help near you? Read our drain jetting page first, or see Drain Jetting in Bolsover and Drain Jetting in Bolton. You can also see Blocked Drains in Cudworth, since every service page has a free quote form. We pass each request to a local drainage engineer.
