Why Does My Drain Flood in Heavy Rain?

I had a job last month in Olton, a semi off Warwick Road, where the rear garden turned into a paddling pool every time it rained hard. The owners had lived with it for three years. They'd been told by someone, don't ask me who, that it was just the clay soil and there was nothing to do about it. So they bought wellies and got on with it.

Three years.

We put a camera down, found a partially collapsed clay pipe under the garden path, sorted it in a day. The flooding stopped. That's the version of this story I tell when someone rings me saying 'it only happens in heavy rain so we've been ignoring it.'

Don't ignore it. There's always a reason. Let's go through the real ones.

The common belief: it must just be the rain

Most people, when their gully backs up or their manhole lid starts bubbling every time there's a downpour, assume it's a surface water problem. Too much rain, overwhelmed system, nothing to be done. They chalk it up to the weather and wait for it to drain away.

Sometimes they're half right. But 'nothing to be done' is almost never true. And in a lot of the jobs I go to across Solihull, from Shirley to Marston Green, from Knowle right up to Chelmsley Wood, what looks like a rain problem is actually an underground problem that the rain just exposes.

Here's what's genuinely going on.

Surcharging: when the sewer runs out of room

The most common cause I see, especially on the older estates in Hillfield, Lode Heath, and parts of Castle Bromwich, is surcharged sewers. That's the technical term for what happens when the pipe fills completely and water has nowhere to go except back up.

Sewers are designed to cope with a certain flow rate. When you get a sustained downpour, the volume of surface water hitting combined or surface water sewers can exceed what they can carry. The sewer fills, pressure builds, and the water finds the nearest exit point. Usually that's a low manhole cover in someone's garden, or a gully that connects near the bottom of a run.

Here's the thing though. If the pipe is in good condition and correctly sized, the system should just about cope with normal heavy rain. If it's surcharging at every moderate shower, one of two things is usually happening: the pipe is narrowed by a blockage, root intrusion or silt build-up, or there's already a structural defect causing partial restriction.

A surcharged sewer that belongs to Severn Trent is their problem. One that's caused by your own lateral running into their main in poor condition is your problem. The distinction matters a lot when it comes to who pays. I wrote about that responsibility split in a separate post, so I won't repeat it here, but get the camera in before you call anyone.

A silted or failed soakaway

A lot of houses in the B90, B91, B93 and B94 postcodes, so your Dorridges, your Knowles, your Balsall Commons and Hockley Heaths, were built with soakaways for surface water. A soakaway is basically a rubble-filled pit that lets rainwater disperse slowly into the ground. They work well when they're young and the surrounding soil is taking water freely.

They do not last forever.

Over time, silt washes in, the surrounding soil becomes saturated, and the soakaway basically gives up. It fills faster than it empties, and the water backs up into the drainage run and out at the nearest gully or gulley pot. On a dry day? No sign of a problem. In heavy rain? Water everywhere.

A silted soakaway is fixable. We can jet it clear, or in worst cases excavate and replace it. A soakaway that's failed because the ground around it has changed, maybe a new extension altered the drainage catchment, maybe the water table in that area has risen, that's a different conversation and sometimes a new connection to the sewer is the right answer.

Undersized or undersized-over-time pipes

When houses in places like Dickens Heath, Monkspath, and Elmdon were built, the drainage engineer of the day calculated pipe sizes based on expected rainfall. The problem is that rainfall patterns have shifted, and some older systems simply can't keep up.

A 100mm surface water pipe serving a large driveway and a conservatory roof that was added in 2008 is probably undersized. Nobody upgraded the drainage when the roof area doubled. The pipe fills in ten minutes during a downpour and backs up.

This one tends to need a proper survey and occasionally a re-run, but it's not always as expensive as people fear.

Collapsed, cracked or root-damaged pipes

This is the one I find most satisfying to fix, because it's so clear-cut on camera. A partially collapsed run restricts flow. In dry weather, the reduced pipe diameter handles the low volumes of waste fine. When it rains heavily, the volume exceeds what the damaged section can pass, and water backs up.

Roots are a big contributor to this across Solihull. The mature street trees in Hampton-in-Arden, Meriden and Berkswell, and the older garden trees on the established estates, send roots into any joint they can find. A root mass inside a 150mm pipe cuts effective bore to maybe 60mm. That's enough to cause surcharging in a sharp downpour.

I had a job in Bentley Heath last autumn where an oak root had worked its way about four metres into the surface water drain under the patio. Dry weather, no sign of a problem. An hour of steady rain and the patio flooded to an inch deep. Cut the root out, relined the section where it had cracked the pipe. Job done.

What a CCTV survey actually tells you

I'll be straight: a CCTV drain survey is how you get a definitive answer. Not guesswork, not prodding a rodding eye with a cane, not waiting to see if it floods again.

We push a camera through every accessible section of your drainage run. We record it. We can tell you:

• Whether there's a blockage or root mass causing restriction
• The condition of the pipe walls, cracks, fractures, joint displacement
• Whether the pipe is the right gradient or has bellied (sagged) causing a silt trap
• Where any defects are, to the centimetre, so a repair is targeted not speculative

For most residential properties in Solihull, a survey runs around £150 to £250 depending on the length and complexity of the system. It tells you whether the problem is yours to fix, your neighbour's, or Severn Trent's. It's the cheapest thing you'll spend money on if you've been flooding repeatedly.

What's fixable and what isn't

Honest answer, because some of this trade is not honest:

Fixable by you or your drainage contractor: - Blockages and root ingress in your lateral drains - A failed or silted soakaway - Cracked or collapsed sections on private runs, often by relining without digging - An undersized connection that can be upgraded

Severn Trent's responsibility: - Capacity issues in the public sewer main - Surcharging caused by the condition of their asset - Collapsed sections on the adopted sewer

Severn Trent have a reporting line and they take flooding records seriously, partly because of regulatory pressure. If your survey shows the issue is on their side, you have every right to report it and push for a fix. A CCTV report from a contractor is useful evidence.

The grey area: - Shared private sewers between neighbours in older terraced or semi-detached streets in Hall Green, Acocks Green, or Shirley. Responsibility can get complicated and you may need a drainage solicitor if it escalates.

If you're in Solihull and this flooding happens to you every time the rain gets above a drizzle, give Drainsco a call. We cover all the B postcodes, we don't charge a call-out fee, and we'll give you a clear price before we start anything. Engineers available 24 hours a day. We'll tell you what we find, what it'll cost to fix, and if it's not our job to fix it, we'll tell you that too.