Drain Backs Up in Heavy Rain | Is It Your Problem

If your gully backs up every time it rains hard, the most useful thing you can know is whether the problem sits on your side of the boundary or Severn Trent's. That single question decides whether you're calling a drainage company or logging a report with the water authority. And the answer isn't always obvious, because both failures can produce identical symptoms in your garden.

The short version: if every property on your street floods at once, the public sewer is surcharging and Severn Trent owns the problem. If it's only yours, or yours is noticeably worse than the neighbours', there's almost certainly something wrong with your private drainage as well. Often it's both, and that's where it gets worth understanding properly.

Two Systems, One Pipe, One Problem

Most Solihull homeowners treat drainage as a single thing. It isn't. There are two separate systems: foul water carrying sewage from toilets, sinks and baths, and surface water carrying rainwater from roofs, drives and gullies.

In newer estates like Dickens Heath, Monkspath and Hillfield, those two systems run to entirely separate outlets. Surface water goes to a soakaway or local watercourse; foul water goes to the treatment works. Under heavy rain, the surface water network might back up, but your foul drains stay clear.

In older streets, particularly Olton, Elmdon and the older sections of Shirley and Lode Heath, both types of water share a single combined sewer. That pipe was sized for ordinary use. A thunderstorm drops thousands of litres from an entire street of rooftops and paved drives into it in twenty minutes. It can't cope. Water backs up and comes out through the lowest point available, usually a garden gully, sometimes a ground-floor toilet. That's sewer surcharge, and on a combined system it's an inherent design limitation, not a fault you caused.

Why Some Streets in Solihull Get It Worse

Two things stack the odds against certain properties.

The clay. Most of the borough sits on Mercia Mudstone Group, a heavy Triassic clay that sheds water rather than absorbing it. Rain falling on a lawn in Knowle or Balsall Common doesn't soak away in any meaningful timeframe. It runs off into gullies and drains already carrying roof water from the same storm.

Impermeable surfaces. The cumulative effect of front garden paving across Bentley Heath, Hockley Heath or Castle Bromwich is significant. Every square metre of tarmac that replaced a lawn sends water directly into a drain rather than letting it soak in slowly. Streets that managed adequately thirty years ago are now running more surface water through the same pipe diameters.

Low-lying spots compound both: the older flat ground around Marston Green and Chelmsley Wood sees combined sewer infrastructure from the 1960s handling rainfall loads it was never designed for.

The Question That Matters: Whose Drain Is It?

Since 2011, most of what used to be called private shared sewers became the responsibility of Severn Trent. The practical rule is this: the pipe serving only your property, from your house to the boundary (and in most cases to the first manhole), is your responsibility. Once it serves more than one property, Severn Trent owns it.

If the public main is surcharging, water will push back up private laterals regardless of how clean your own pipework is. Report it to Severn Trent on their flooding line. They have a statutory duty to investigate sewer flooding and to keep records of repeat events. Document each occurrence with photos, dates and water levels. If it's happening more than once a year at your address, that record becomes your evidence when pushing for a permanent infrastructure fix.

But here's the part most people miss. Even when Severn Trent's sewer is the underlying cause, the state of your private drain determines how badly you're affected. A private lateral that's 40% constricted with root intrusion or grease means the surcharge water backs up faster and to a higher level than in a clear pipe next door. Keeping your own pipework in good shape doesn't fix the public sewer problem, but it can be the difference between a flooded gully and a flooded kitchen.

What a Storm Actually Reveals

The pattern we see repeatedly across Solihull: a drain performs perfectly for eleven months, a thunderstorm hits, everything backs up, and the homeowner concludes the storm caused the blockage. It didn't. The storm exposed a partial blockage that was already there.

Root intrusion is the most common culprit in tree-lined streets around Knowle, Dorridge and older Shirley. Roots narrow the pipe bore gradually. Normal flow gets through. Storm flow, three times the volume, can't, and everything backs up. Grease and fat build-up does the same thing over a slower timescale.

The diagnostic check is straightforward: lift your manhole cover during dry weather. Standing water in a dry spell means something is already restricted downstream. Don't wait for a storm to confirm it.

What You Can Do on Your Side

A few actions that genuinely shift the risk before the next storm season:

• Clear your gullies. A gully choked with leaves stops surface water entering the drain at all. It backs up on your path or patio rather than getting away. Ten minutes and a pair of gloves fixes it.
• Get a CCTV survey if you're in an older property that's flooded before. It tells you whether you have root intrusion, a pipe deformity or a collapsed section. That information decides whether jetting, root cutting or relining is the right response.
• If you've paved a front garden, check whether permeable paving or a soakaway could take some of that water out of the system. Planning rules have required it for new paving since 2008, but older installations predate that.
• Talk to a drainage engineer about whether a non-return valve is appropriate for your property. On a foul drain, a check valve needs careful specification: fitted without proper assessment, it can trap sewage inside the property during a surcharge event, which is worse than no valve at all, and installation must comply with Building Regulations and your sewer undertaker's requirements. Don't treat it as a DIY fix. Get it specified by someone who knows what's upstream.

Repair Now or Wait for the Next Flood

If a CCTV survey finds root intrusion or a cracked section, the question is repair now versus repair after the next flood causes water damage to a ground-floor room. The cost comparison isn't difficult. High-pressure water jetting clears most blockages in one visit. Root cutting removes intrusive growth and buys time. Where the pipe itself is cracked or has lost its shape, relining is generally faster and less disruptive than excavation. Drainsco's 20-year guarantee applies to remedial drain repairs, relining and pipe replacement, confirmed in writing on every qualifying job.

If the problem is the public sewer surcharging, a clean private system reduces your exposure while you push Severn Trent through the right channels. The two actions aren't mutually exclusive.

Drainsco covers Solihull and the B postcodes from B37 to B94, with engineers available around the clock and no call-out fee. If you're not sure whether what you're seeing is a private drain fault or a public sewer event, a call gets you a straight answer and a clear price before anyone starts work.