It is a common assumption that all the water leaving a property, whether from a downpipe or a toilet, ends up in the same place. In reality, UK drainage is usually split into two types of system, and whether storm drains and sewers are connected depends on how your area was built. Understanding the difference can help explain how water is managed on your property, and why certain drainage work is required for new developments. This guide covers what each system does, how to connect them, and how to find out which one you have.
What is a Storm Drain?
A storm drain, also called a surface water drain, carries rainwater away from roofs, driveways and paved areas. This water has not been used in the home, so it does not usually need treatment. In a separate system, surface water is directed to a soakaway, a watercourse, or a dedicated surface-water sewer rather than to a sewage treatment works. Storm drains are designed to handle sudden, high volumes of water during rainfall, which is why they are often larger than the pipes carrying waste from inside a building.
What is a Foul Sewer?
A foul sewer carries wastewater from inside the home, including from toilets, sinks, baths, showers, and washing machines. This water needs treatment before it can be safely returned to the environment, so foul sewers lead to a sewage treatment works rather than directly into a river or the ground. Because the flow is steady and predictable, foul pipes are generally smaller than storm drains, and keeping rainwater out of them helps the system work as intended.
Combined and Separate Systems
In many older parts of the UK, particularly in towns and cities built before modern standards, storm water and foul water share a single combined sewer. This was common practice for many years, and in these areas the answer to whether storm drains and sewers are connected is yes, because they are the same pipe. It means treatment works have to process rainwater alongside wastewater, and during heavy rain the system can be overwhelmed. Combined sewers are sometimes fitted with overflows that release diluted wastewater into a watercourse to relieve pressure, which is one reason separate systems are now preferred.
Newer developments are generally required to use separate systems under current Building Regulations, keeping surface water and foul water apart. Many properties sit somewhere in between, with one system serving the road and another serving the property, so it is not always obvious which type you have.
Where Does Rainwater Go in a Separate System?
Building Regulations Document H sets out a preferred order for dealing with surface water. Where the ground allows, rainwater should be managed by infiltration, which means directing it to a soakaway that returns it to the ground. If that is not possible, it can go to a watercourse, and only if neither is available should it be connected to a sewer. This is why soakaways and storm crates are common features of new builds and extensions, as they retain rainwater on site and relieve pressure on the public network.
Can You Connect a Storm Drain to a Sewer?
In a separate system, surface water and foul water are not supposed to be connected, and connecting them without permission can cause real problems. Rainwater entering a foul sewer adds volume the system was not designed to carry, which can lead to flooding and overflows. Foul water entering a storm drain can pollute a watercourse. These wrong connections are surprisingly common in older properties and after building work, and they often go unnoticed until something goes wrong. Any new connection to a public sewer needs the water company’s approval first.
How to Find Out Which System You Have
The simplest starting point is your water company, which holds records of the public sewers in your area and can often tell you whether they are combined or separate. Drainage plans from when the property was built or extended can also help, and the inspection chambers in your garden or driveway can give clues about where the pipes lead. If the picture is still unclear, a CCTV drain survey lets an engineer follow the pipes and establish exactly where each one goes, and a dye test can be used to confirm where a particular drain connects.
Why This Matters for Your Property
If you are extending a property, converting a building, or dealing with drainage that is not behaving as expected, knowing whether you have a combined or separate system affects the solution. A soakaway is typically used to manage surface water in a separate system, while a new connection to the foul sewer may be needed for wastewater from an extension. Getting it wrong can lead to flooding, pollution issues, or problems with water company approval, all of which are expensive to fix after the event.
Not Sure Which System You Have?
Older properties in particular are not always clear about how their drainage is set up. Dorset Drainage Services can carry out a CCTV drain survey to establish exactly how your property’s drainage is connected, and advise on the right solution across Dorset, including Bournemouth and Poole, whether that is a soakaway, a new sewer connection, or another fix entirely.
Frequently Asked Questions
Do all properties have a separate storm drain?
No, many older properties are served by a combined sewer, where rainwater and wastewater share the same pipe. Newer properties are generally on separate systems.
Where does the water from my gutters go?
It depends on the property. It may go to a soakaway in the garden, to a watercourse, to a surface water sewer, or into a combined sewer in some older areas.
Can I fit a soakaway instead of connecting to the sewer?
Often, yes, where the ground can absorb the water. A soakaway is generally the preferred option for rainwater, and a percolation test shows whether the ground is suitable.
Unsure How Your Drainage Is Connected?
Contact Dorset Drainage Services today to find the right solution for your property. Call +44 1258 841208 or email [email protected].
