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What Is Surface Water Drainage?
Surface water is rain: what falls on your roof, your drive and your garden and has to go somewhere. It is a separate system from the one carrying waste out of your house, and keeping the two apart is not a technicality. Getting it wrong causes flooding somewhere downstream and is one of the more common drainage mistakes on domestic property.
The Short Answer
Surface water drainage takes rainwater from roofs and hard surfaces to a surface water sewer, a soakaway or a watercourse. Foul drainage takes waste from toilets, sinks and appliances to treatment. On a separate system those are two different pipe networks; on an older combined system they share one. Which you have decides what you are allowed to connect where, and it is worth establishing before any work involving a downpipe or a new hard surface.
Two Systems, Two Jobs
The distinction is worth being precise about because the words get used loosely.
Surface water is clean rainwater. It comes off roofs into gutters and downpipes, off driveways and patios into channels and gullies, and out of saturated ground. It needs no treatment. It needs to go somewhere that can take a very large volume very quickly during a downpour and almost nothing the rest of the time.
Foul water is everything from inside the building: toilets, basins, baths, kitchen sinks, washing machines. It needs treatment before it can be returned to the environment, and it arrives at a fairly steady rate.
On a modern separate system these are two independent pipe networks leaving the property, and they stay separate all the way to their destinations. On an older combined system, which a lot of the country still has, one pipe carries both to treatment.
That difference is why a rule that sounds bureaucratic matters practically: connecting a downpipe into a foul drain on a separate system sends a roof's worth of rainwater to a treatment works during every storm, and it is the sort of thing that overwhelms a system built for the steady trickle of a few households.
Which System Do You Have?
Four checks, none of which need anyone to come out.
- Count the covers. A separate system generally means two inspection chambers or two lines of them, one for foul and one for surface water. A single line serving everything suggests combined.
- Lift a cover and look after rain. A surface water chamber runs clear and is dry between downpours. A foul chamber has a steady flow and does not run clear. If your downpipe leads to a chamber that also shows foul flow, you are on a combined system or something has been wrongly connected.
- Follow the downpipes. On a separate system a downpipe usually goes into its own gully or straight into a surface water drain, quite often on the opposite side of the property from the soil stack. On a combined system it may join the same run.
- Check the age of the property. Broadly, older housing tends to be combined and newer development separate, with a great deal of variation and a lot of properties where later work has muddled the two.
- If it genuinely is not clear, the water authority holds sewer records for the area and they are the definitive answer rather than a guess from a cover.
Channels, Gullies and ACO Drainage
The hardware end of surface water on a domestic property, and worth knowing what the names mean because they get used interchangeably by people selling them.
A gully is a point drain: a single opening with a grid and a trap below, taking water from one spot. The thing at the bottom of a downpipe or outside a back door.
A channel drain is a linear drain: a long slot with a grating over it, taking water along its whole length. ACO is a manufacturer name that has become the generic term for these in the way Hoover did for vacuum cleaners, so ACO drainage and channel drainage usually mean the same thing in conversation.
Channel drains are what you use across the width of a driveway, at the threshold of a garage, or along the edge of a patio, where water arrives across a line rather than at a point. They are considerably better at that job than a single gully, which relies on the surface being laid to fall precisely toward it.
Installing one is a groundwork job rather than a plumbing one: a trench, a concrete surround, the channel bedded to a consistent fall, and a connection at one end to wherever the water is going. The connection is the part worth planning first, for the same reason as everything else on this page.
The Rules Worth Knowing
Three, and they cause most of the problems we see on domestic drainage.
Do not connect surface water into a foul drain on a separate system. It sends rainwater to treatment, it can surcharge the foul system in heavy rain, and it is not permitted. Where it has already been done, it is a genuine defect rather than an oddity.
Do not connect foul into a surface water drain. This is the more serious direction, because it discharges untreated waste to a watercourse or a soakaway. It happens, usually where somebody has added a downstairs toilet or a utility room and connected to the nearest available pipe without checking which it was.
New hard surfacing has rules of its own. Paving over a large area changes how much water runs off and where it goes, and there are planning requirements around permeable surfacing and drainage for driveways in many circumstances. Worth checking before laying a drive rather than after.
The practical version of all three: before connecting anything to anything, establish which system the pipe belongs to. A cover lifted and a bucket of water is usually enough to settle it.
When Surface Water Is the Problem
Symptoms that point at surface water rather than at a blocked foul drain, which is a distinction worth making before anyone starts rodding.
Flooding that only happens during or shortly after heavy rain, and clears on its own afterwards. That is a capacity or a blockage on the surface water side rather than anything to do with foul drainage.
A gully overflowing at the base of a downpipe while everything indoors runs away normally. The problem is between that gully and wherever it discharges.
Water pooling against a wall or across a drive after rain, with no drainage failure indoors at all.
Where all of the fittings inside are also slow, that is foul drainage and a different problem, covered in the blocked drains guides.
Combined Systems and a Lot of Rain
Glasgow gets a great deal of rain and a large part of the older city drains on a combined system, which is a combination worth understanding because it explains a specific and recognisable pattern of problem.
On a combined system, one pipe carries both foul waste and everything off the roofs. In ordinary weather that is fine. In a downpour the same pipe is suddenly taking the roof area of an entire tenement as well as the household waste, and a run that copes perfectly all week can surcharge within minutes.
So the diagnostic that matters here is timing rather than severity. A drain that backs up only during heavy rain and clears an hour later is behaving like a combined system under load, and rodding it on a dry Tuesday will very often find nothing wrong at all. That is not a wasted visit if it is expected; it is a wasted visit if nobody mentioned the rain.
The other local pattern is what happens when a back court gets surfaced. Replacing a soft back court with slabs or monoblock removes a large area that was previously absorbing rain and sends all of it to a drainage system that was never sized for it. Where a previously fine back court started flooding after resurfacing, that is usually the reason rather than a blockage, and the answer is drainage added to the new surface rather than clearing a pipe that is not blocked.
And on a shared back court, both the drainage and the surface are commonly common property, so any change is a factor conversation rather than an individual decision.
When to Call Someone Out
Worth calling when flooding happens only in heavy rain, because that is a surface water or capacity question rather than a blockage and it wants diagnosing differently. Also worth it before connecting a new downpipe, channel drain or driveway drainage into anything, since the wrong connection is a genuine defect that shows up later on a survey. Paul leads the drain team and Kyle runs the jetting, and a camera survey settles which pipe is which where the records are unclear. If the ground is common to a flatted building, the factor will need to be involved before work starts.
Drain Repairs in GlasgowCommon Questions
Surface water is clean rainwater from roofs and hard surfaces and needs no treatment. Foul water is waste from toilets, sinks and appliances and does. On a separate system they are two independent pipe networks; on an older combined system one pipe carries both. Which you have decides what may legally be connected where.
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“Our radiator pipe was bent and leaking. They answered straight away… Guy was here an hour later and he was wearing ID, very professional, patient and kind. We just got a new carpet in this room and he done an amazing job on getting little to no water on it. Couldn't recommend enough, price was reasonable.”
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“As a small business owner, I needed a drainage company that could respond quickly. The team attended first thing in the morning, cleared the blocked drain and carried out a full CCTV survey. All documentation emailed across promptly. Professional, efficient and great value for money.”
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Read Next
Full guide
What Is a French Drain?
A trench, a perforated pipe at the bottom, a permeable membrane lining it, and clean stone filling the rest.
What Is Foul Water Drainage?
Foul water is waste from inside the property, and it goes to treatment rather than to a watercourse.
How to Install a French Drain
Establish the discharge point first, then dig a trench falling consistently toward it at roughly one in a hundred.
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