Drain Backs Up Only When It Rains | What That Tells You

I had a job in Dedworth last autumn where the customer had been bailing out her patio for three years. Every time it rained hard, the gully backed up. Between storms, everything looked fine and the drain cleared itself. So she assumed it wasn't that serious.

It was serious. The main run under her garden had partially collapsed and was holding silt. Every downpour pushed more water into a pipe that couldn't carry it. Bailing was never going to fix that.

Here's the thing most people miss: the intermittent pattern is actually your most useful clue. A drain that only misbehaves in heavy rain is telling you something specific about the cause. Understand the pattern and you've already narrowed it down.

Why 'just get it jetted' is often the wrong first move

When a customer calls me after a flood event, nine times out of ten they expect a jet wash. And occasionally that's right. A gully packed with autumn leaves and grit will back up in heavy rain and a high-pressure clean sorts it in an hour.

But if you've had the drain jetted twice already and it still floods every downpour, you're not dealing with a blockage. You're dealing with a structural or capacity problem. Those need different tools and a different conversation.

Jetting a collapsed pipe doesn't un-collapse it. Jetting a spent soakaway doesn't restore its percolation. Jetting your connection point doesn't fix a surcharging public sewer. Knowing which one you're facing is the whole game.

The four things rain-only flooding usually points to

1. Surcharging from the public sewer

This is the one people least want to hear, because it's often not their problem to fix, and not entirely their problem to solve.

The sewer running under your street was built to a certain capacity. Across a lot of Windsor, Eton, Old Windsor, Spital and parts of Clewer Village, that infrastructure is Victorian or Edwardian. It was calculated for a smaller population and a different rainfall pattern. When a heavy storm hits now, the sewer fills faster than it can drain. Water surcharges back up through private connections and surfaces at the lowest point on your property, usually a gully or external manhole.

Surcharging signs: the water is discoloured, there's an odour, and your neighbours are dealing with the same thing at the same time.

If Thames Water's public sewer is the cause, they have a duty under the Water Industry Act 1991 to maintain it to a standard that prevents flooding of properties. If their asset is causing internal flooding inside your home, that duty is enforceable and you may be entitled to compensation. Logging every incident in writing, with dates, photographs and your postcode, builds the evidence base. A CCTV survey from your side confirms exactly where your private drain ends and Thames Water's asset begins, which is what you need before that conversation gets serious.

2. A silted or failed soakaway

Surface water from roofs, drives and patios often runs to a soakaway rather than the foul sewer. A soakaway is a rubble-filled pit or a plastic crate system buried in the garden, designed to let water disperse slowly into the surrounding ground.

They work well when new. After fifteen or twenty years, silt washes in and clogs the void. The surrounding soil can become permanently saturated. In clay-heavy ground around Datchet, Wraysbury and parts of Clewer, this happens faster, because clay drains poorly to begin with. Once the soakaway is spent, surface water backs up to the nearest gully every time it rains hard.

A camera survey can't assess how well water is percolating into the ground, because that happens outside the pipe. What it can do is confirm the outfall point and check whether the entry to the soakaway chamber is silted. If the foul drains are clear, the gully floods only in rain, and the camera shows the drain discharging to a soakaway rather than the public sewer, you've got your answer without needing to dig.

3. A partially collapsed or deformed pipe

This is what the Dedworth job turned out to be.

Old clay pipes crack. Tree roots push through joints, the surrounding ground erodes, and the pipe sags. At that low point, silt accumulates. In dry weather, enough water gets through that you'd never notice. Hit the pipe with the peak flow from a heavy downpour and the restricted bore simply can't cope.

Plastic pipes crush. It happens under driveways where installation was shoddy, or where ground that was never designed for heavy loads has been crossed repeatedly by delivery lorries. A crushed section of 110mm pipe acts like a partial blockage every single time there's a heavy shower.

This is exactly what a CCTV drain survey is built for. The camera goes in, we record, and within an hour I can show you on a screen where the collapse or deformation is, how bad it is, and what the options are.

4. An undersized or misconnected drain

Sometimes the drain was never right for the job it's being asked to do now.

Extensions, garden rooms, extra hardstanding, a new section of roof draining into an old pipe sized for the original house. We see this in Cranbourne, around Ascot, and in infill developments near Bray and Maidenhead where permitted development has let properties expand significantly over the decades.

A 100mm clay pipe handling a Victorian terrace was adequate in 1930. Add a rear extension, a garden office and a paved-over lawn, and the peak flow during a downpour overwhelms it. The maths were never revised because nobody pulled a permit.

Misconnections are a separate headache. Surface water connected to the foul sewer, or foul water connected to the surface water system, creates flooding problems and is also an environmental offence. A camera survey with a dye test confirms whether the connections are correct.

What a CCTV survey actually shows for a flooding problem

For repeat rain-related flooding specifically, here's what I'm looking at on the footage:

• Pipe wall condition: cracks, root ingress, displaced joints
• Gradient: is water sitting in sections where it should flow freely?
• Bore: any restriction from a crush, calcium build-up or root mass?
• Connection point: does this drain run to the public sewer, a soakaway, or a surface water system?
• Previous repairs: have they held, or are they failing again?

That information tells me whether the fix is a jet clean, a patch repair, a full no-dig reline, soakaway replacement, or a documented complaint to Thames Water. You can't make that call without the footage. Guessing is just expensive.

What's actually fixable

Most of it. A collapsed clay section can be relined without digging up the garden. Root intrusion can be cut back and a structural reline bonded over the weakened joint. A failed soakaway can be excavated and replaced with a modern crate system sized properly for your plot. A misconnection can be corrected.

The one exception is a surcharging public sewer that Thames Water hasn't upgraded. You can't reline an asset you don't own. What you can do is build a documented case, push them formally, and in the case of internal flooding, understand your rights under the Water Industry Act. That's a better position than assuming it's your problem and spending money on your own drains when the fault lies in the street.

Anyone in Windsor who floods every winter: stop guessing. A camera survey gives you the cause in writing, with footage you can show to a loss adjuster, a surveyor, or Thames Water. Documented evidence beats 'it keeps flooding and I don't know why' every single time.

Drainsco covers Windsor and the surrounding SL4 area, with engineers on call around the clock. No call-out fee, and a clear price before any work starts. If you're getting the same flooding every time it rains, call us and we'll tell you straight what we're looking at.