Why Does My Drain Flood in Heavy Rain | Real Causes

I had a job in Motspur Park last autumn where the homeowner had been bailing out her back yard after every storm for three years. Three years. She'd had two other companies out, both of whom rodded the gully, said it was clear, and left. The flooding came back with the next heavy shower, same as always.

She wasn't imagining it. The drain wasn't blocked, not in the way most people picture a blockage. Something else entirely was going on underground, and nobody had bothered to look.

If your gully, manhole or garden dip floods every time the rain hammers down, and clears once the sun comes out, this post is for you.

The common belief: a blockage is a blockage

Most homeowners assume that if water isn't draining, there must be something physically stuck in the pipe. A wad of leaves. Fat. Roots. Rod it out, sorted.

That belief isn't always wrong. But with rain-related flooding that keeps coming back in exactly the same spot, it's usually incomplete. Sometimes it's completely wrong.

The drainage trade hasn't helped. Too many engineers rock up, run a jet down the nearest gully, collect a fee and drive away without asking why the flooding is happening in the first place. If you keep calling out for the same recurring problem, you deserve a proper answer, not another temporary fix.

What's really going on: four likely causes

1. Surcharging sewers

This is the one nobody wants to hear, because it's often not your problem to fix.

When heavy rain hits, the shared sewer network takes an enormous volume of water very quickly. In older parts of KT3 and the surrounding postcodes, you've often got combined sewers that carry both foul waste and surface water in the same pipe. Design them for a typical day and they cope. Design them for a cloudburst hitting a modern impermeable suburb and they absolutely don't.

When the sewer surcharges, meaning it fills beyond its capacity, water has nowhere to go but backwards. It backs up through the nearest low point, which could be a manhole in your garden, a gully at the bottom of your drive, or in the worst cases, a ground-floor toilet.

The giveaway is timing. If your flooding appears within minutes of heavy rain starting and clears quickly once it stops, without any visible debris, surcharging is high on the list. Your local drain may be completely clear. The problem is upstream or in the shared network.

Thames Water own and maintain the shared sewers. If surcharging is confirmed, they need to know. A logged history of incidents strengthens any report you make to them.

2. A silted or failed soakaway

Loads of properties in the Coombe, Beverley Park and Malden Manor areas rely on soakaways for surface water disposal, particularly detached and semi-detached houses with large driveways or gardens. A soakaway is essentially a rubble-filled pit that lets water percolate slowly into the ground.

They have a finite life. Silt washes in over years and fills the voids. Clay-heavy ground, common across much of south-west London, makes percolation slow even when the soakaway is new, and effectively impossible once it silts up.

What you see: surface water floods during rain, drains away slowly over hours or days once the rain stops. It's not instant like surcharging. The water is genuinely trying to go somewhere but the ground won't take it.

A CCTV survey can confirm the condition of the pipe leading to the soakaway. Checking the soakaway itself often needs a small excavation or a CCTV camera dropped in from above. Old soakaways also collapse, leaving a depression in the lawn as a hint.

3. A collapsed, fractured or undersized run

I pulled a camera through a 100mm surface water pipe in Green Lane a few months back. A section had bellied, meaning it had sagged in the middle and formed a low point collecting sediment. During light rain it mostly kept going. During heavy rain, the volume overwhelmed it and the low point backed up like a sink with a half-blocked plughole.

Collapsed clay pipes are genuinely common in KT3 and KT4. The housing stock in Old Malden, Norbiton, West Barnes and Malden Manor has a lot of original clay drainage from the 1930s and 1950s. It's had a long life, but clay cracks under ground movement and root invasion.

An undersized run is a different problem, often found on extensions or outbuildings added without proper drainage design. Someone connected a large roof area to a 68mm pipe that was never going to cope with a serious downpour.

4. Partial root intrusion

Roots don't always block a pipe completely. They can partially reduce the bore, or create a debris trap that lets normal flow pass but backs up the moment rainfall increases volume.

A root-infested pipe might drain perfectly well for eleven months of the year and flood your patio every time it rains hard in winter. It passes a visual inspection from the surface. It might even pass a basic rod. You won't know it's there without a camera.

Why a camera survey is the only honest answer

Everything above produces similar symptoms at the surface: flooding in the same spot, every time it rains hard.

The only way to know which cause you're actually dealing with is to look inside the pipe. A CCTV drain survey sends a camera through the run and shows, in real time, the condition of the pipe: bellies, cracks, root ingress, debris accumulation, the lot. You get footage you can watch. No guessing.

It also tells you clearly whether the problem is on your land and your responsibility, or whether it's in the shared sewer beyond your boundary. That distinction matters enormously before you spend money on repairs.

A survey that clears you of any private problem gives you documented evidence to push Thames Water or your local authority. One that shows a collapsed section on your land at least tells you exactly what you're fixing and where.

What's fixable and what isn't

If it's your pipe: collapsed sections can be relined without digging in most cases. Roots can be cut and the pipe descaled. A failed soakaway can be replaced or a new surface water connection made. Bellied pipes can sometimes be relined; in bad cases, excavation and relay.

If it's the shared sewer surcharging: you can't fix Thames Water's network yourself, and you shouldn't try. What you can do is fit non-return valves on your drainage to prevent sewer water backing up into your property during surcharge events. It won't stop surface flooding in the garden, but it will protect your house.

Some flooding scenarios involve both. A silted soakaway that's failing will make surcharging worse, because water that should be going into the ground is instead trying to get into the sewer. Fix your side of the problem first and you may well reduce the severity of the flooding even if the network itself isn't ideal.

What to do instead of calling for a rod

• Keep a log. Dates, duration, how quickly water clears. This builds a picture for any claim or referral to Thames Water.
• Don't just book a clearance. If this is recurring, book a CCTV survey. You need a diagnosis, not another temporary fix.
• Check where your surface water actually goes. Your deeds or a drainage search will show it, and it affects which solution makes sense.
• If a company can't explain what they found and why it's causing your specific problem, get someone else.

Recurring flooding in the same spot is information. The drain is telling you something. A camera tells you what.

Drainsco covers New Malden and the KT postcodes, with engineers available 24 hours a day. We'll give you a clear price before we start, there's no call-out fee, and if a repair is needed, our remedial work carries a 20-year guarantee. If you've got a spot that floods every winter and you've never had a proper look inside the pipe, call us and we'll sort that out.