Every property in the UK produces two types of water that need draining away: foul water (the dirty stuff from your toilets, sinks, and showers) and surface water (the rainwater that lands on your roof and hard surfaces). These two types of water go to very different places, and the systems that carry them are supposed to be completely separate. When they’re not, problems follow, from sewer flooding in your street to pollution in local rivers.
Understanding which system is which might seem like a technical detail you can leave to the plumber, but it matters if you’re planning building work, investigating a drainage problem, or trying to work out why your drains keep backing up when it rains. Here’s what you need to know.
What is foul water drainage?
Foul water drainage carries contaminated wastewater away from your property. Everything that goes down the toilet, sink, bath, shower, dishwasher, and washing machine flows into the foul water system. This water contains sewage, detergent, food waste, grease, and other contaminants that make it unsafe to release directly into the environment.
Foul water drains run from your property through underground pipes to the public foul sewer, which is maintained by your local water company. From there, the sewage travels through the sewer network to a treatment works, where it’s processed, cleaned, and eventually discharged as treated water into a river or the sea. If your property isn’t connected to the mains sewer (which is the case for around 4% of UK homes), your foul water goes into a septic tank or sewage treatment plant on your own land instead.
The foul water system is a closed system by design. The pipes are sealed, the joints are watertight, and the only water entering should come from inside the property. Any extra water getting into the foul system, whether from a misconnected gutter or a cracked pipe letting groundwater in, adds volume that the treatment works has to process, and in heavy rain that extra load can overwhelm the system entirely.
What is surface water drainage?
Surface water drainage handles rainwater. When rain falls on your roof, driveway, patio, paths, and other hard surfaces, it runs off and needs somewhere to go. Surface water drains collect that runoff through gutters, downpipes, gullies, and drainage channels, and carry it away through a separate set of underground pipes.
Because rainwater is clean (or at least clean enough), it doesn’t need treating at a sewage works. Instead, surface water is discharged into the nearest watercourse, a river, stream, ditch, or pond, or into a soakaway where it percolates back into the ground. Some properties discharge surface water into a dedicated surface water sewer, which is a separate public sewer that carries rainwater to a watercourse rather than a treatment works.
The principle behind surface water drainage is simple: get clean rainwater back into the natural water cycle as directly as possible, without sending it to a treatment works that doesn’t need to process it and can’t cope with the volume during a downpour.
Why they must be kept separate
Keeping foul water and surface water apart isn’t just good practice. It’s a legal requirement for new drainage installations under Part H of the Building Regulations. The reasons are practical and environmental.
Foul into surface water: pollution
If foul water gets into the surface water system, it goes directly into a river or stream without treatment. That means raw or partially treated sewage entering a watercourse, which is both illegal and harmful. It causes water pollution, kills aquatic life, and poses a risk to public health. The Environment Agency and local water companies actively investigate and prosecute cases of foul-to-surface misconnections, and property owners can face fines and enforcement action.
Surface water into foul: flooding
If surface water gets into the foul system, the treatment works receives far more water than it’s designed to handle every time it rains. This overload is one of the main causes of combined sewer overflows (CSOs), where untreated sewage is discharged into rivers during heavy rainfall because the system can’t cope with the volume. It also causes sewer flooding in homes and streets, because the extra rainwater fills the foul sewer to capacity and the sewage has nowhere to go but back up through manholes, toilets, and drain gullies.
In England, water companies reported over 370,000 storm overflow discharges in 2023, many of which were caused or worsened by surface water entering the foul sewer network. Keeping the two systems separate is one of the most effective ways to reduce sewer overflows and the environmental damage they cause.
What is a combined drainage system?
Many older properties in the UK, particularly those built before the 1960s, have combined drainage systems where both foul water and surface water flow through a single set of pipes to a combined sewer. This was standard practice for decades, and millions of homes are still connected to combined systems today.
Combined systems work during dry weather because the foul water volume alone is well within the pipe’s capacity. The problems start when it rains. Rainwater from roofs and hard surfaces floods into the same pipes, and the combined sewer has to handle both the sewage and the storm water at the same time. During heavy rainfall, the volume exceeds what the sewer and treatment works can process, which triggers the combined sewer overflows that have become such a contentious environmental issue in the UK.
For new builds and new drainage installations, Building Regulations require foul and surface water to be drained separately. The preferred approach under the SuDS (Sustainable Drainage Systems) hierarchy is to deal with surface water on your own property first, through infiltration into the ground via soakaways, before considering a connection to the surface water sewer, and only connecting to a combined sewer as a last resort when no other option is available.
How to tell which system you have
Working out whether your property has separate or combined drainage isn’t always obvious, but there are a few ways to check.
Inspection chambers. Lift the covers on the inspection chambers in your garden or driveway. If you have two separate sets of chambers, one carrying foul water (you’ll see waste and discoloured water in the channel) and one carrying only clean water, you probably have a separate system. If there’s only one set of chambers and everything, both waste and rainwater, flows through them, it’s a combined system.
Pipe colour. Under current standards, foul water pipes are brown (sometimes marked in red), and surface water pipes are grey (sometimes marked in blue). On older properties, the pipes may all be the same colour, and the only way to tell them apart is to trace where they go.
Water company records. Your local water company has sewer maps that show whether the public sewer in your street is foul only, surface water only, or combined. This tells you what your property should be connected to, though it doesn’t guarantee that the connections on your property are correct.
CCTV survey and dye testing. If there’s any doubt about what’s connected to what, a CCTV drain survey combined with dye testing is the definitive answer. A drainage engineer runs dye through each outlet in your property (flushing coloured dye down toilets, sinks, and downpipes in turn) while watching the inspection chambers to see which system each connection enters. It’s the only way to be sure, and it’s how water companies investigate suspected misconnections.
What is a misconnection?
A misconnection is where a pipe has been connected to the wrong drainage system, either a foul water outlet connected to the surface water drain or (more commonly) a rainwater downpipe connected to the foul sewer. Misconnections are surprisingly common, particularly on properties that have been extended, had plumbing work done, or had new bathrooms or kitchens added over the years.
The most frequent misconnection is a washing machine, dishwasher, or bathroom waste pipe that’s been connected to a surface water drain rather than the foul sewer. This sends contaminated water straight into a river or watercourse. It’s often done accidentally during building work, especially when the builder doesn’t check which system each pipe belongs to before making the connection.
Water companies across the UK run misconnection tracing programmes, and if they find a misconnection on your property, they’ll require you to have it corrected at your own expense. If you’re buying a property, a drainage survey before exchange can identify any misconnections and give you the chance to negotiate the cost of fixing them before you complete.
Regulations and requirements
Part H of the Building Regulations (Approved Document H in England and Wales) sets out the requirements for both foul and surface water drainage on new installations and alterations.
For foul water, the regulations require adequate pipe sizing, proper gradients for self-cleaning flow, access points at key locations, and connections to the public foul sewer or an approved private treatment system. For surface water, the regulations follow the SuDS hierarchy, requiring you to consider these options in order of preference:
- Infiltration into the ground (soakaways, permeable paving) where soil conditions allow
- Discharge to a watercourse (river, stream, ditch)
- Connection to a surface water sewer
- Connection to a combined sewer (only as a last resort, and the water company must agree)
If you’re building an extension, converting a garage, or adding hard landscaping that increases the amount of rainwater runoff from your property, you’ll need to show how the additional surface water will be drained. In many cases, this means installing a soakaway or permeable paving rather than simply connecting another downpipe to the existing system.
A drain mapping survey can establish exactly how your current drainage is arranged, which is useful information to have before you start any building project that involves connecting to or altering the existing system.
Common problems
Drains backing up when it rains
If your foul drains back up during heavy rainfall, rainwater is almost certainly getting into the foul system somewhere, either through a misconnected downpipe, a cracked pipe that lets groundwater in, or because your property is connected to a combined sewer that can’t cope with the volume. A CCTV survey can identify the source and help you plan the fix.
Smells from outside drains
Surface water drains shouldn’t smell. If you’re getting a sewage odour from a gully or drain that’s supposed to carry only rainwater, there’s likely a misconnection somewhere upstream that’s sending foul water into the surface water system. Dye testing will pinpoint it.
Pollution in garden or driveway
If water pooling on your property looks discoloured or smells, it could be foul water surfacing because of a blockage or failure in the foul drainage system. This is a health risk and needs investigating promptly. Blocked drain clearing and a survey will identify what’s happening and what needs fixing.
Frequently asked questions
Can surface water drain into the foul sewer?
For new installations, only as a last resort when all other options (soakaway, watercourse, surface water sewer) have been ruled out, and only with the water company’s agreement. On older properties with combined systems, surface water already drains to the foul sewer, and while it’s legal because it predates current regulations, water companies encourage separation wherever practical to reduce sewer overflow events.
How do I know if I have a misconnection?
You might notice sewage smells from surface water drains, discoloured water in gullies that should only carry rainwater, or foul drains backing up when it rains. The only definitive way to check is a dye test or CCTV survey carried out by a drainage professional. If you’ve had recent building work or plumbing changes, it’s worth checking that everything has been connected to the correct system.
Do I need separate drains for my extension?
Yes. Any new drainage installation must separate foul and surface water in line with Building Regulations Part H. Your extension’s roof water should go to a soakaway, watercourse, or surface water sewer, not into the foul system. The waste from any new kitchen or bathroom connects to the foul drain. Building Control will check this as part of the sign-off process.
Who pays to fix a misconnection?
The property owner. If a misconnection is found on your property, it’s your responsibility to have it corrected by a qualified drainage contractor. The water company can issue a formal notice requiring the work to be done within a specified timeframe. Getting it fixed promptly avoids enforcement action and, more importantly, stops polluted water entering local watercourses.
What’s the difference between a foul sewer and a combined sewer?
A foul sewer carries only foul water (sewage and household wastewater) to the treatment works. A combined sewer carries both foul water and surface water in the same pipe. Combined sewers are more prone to overloading during heavy rain because they have to handle both the sewage and the storm water, which is why new installations require separate systems.
Need help with your drainage?
Whether you need to trace a misconnection, map your existing drainage system, or install new foul and surface water drains for a building project, Clark Drainage can help. We carry out CCTV drain surveys, drain mapping, and new drainage installations across the UK, and our engineers can assess your system and advise on the best way forward.
Call us on 0800 085 1219 for a free quote, or visit our contact page to book a survey.
