Lower Ground Floor Flat Flooded by Sewer Surcharge: A Richmond Drainage Investigation

By Simon Crowther, FPS Environmental  | July 2026

A lower ground floor flat in Richmond, near London,  flooded twice during intense rainfall. On both occasions, water emerged internally through the bath and shower. After the first flood, non-return valves had been installed, but the property still flooded again.

FPS Environmental was instructed to find out why, and whether the risk could be reduced.

This was not a case where the client needed a generic flood risk document. They needed to understand why the property had flooded again despite previous drainage works, whether the existing non-return valves were in the right place, and what a more appropriate intervention would look like.

The key lesson is simple: diagnosis should come before installation.

A non-return valve may be the right type of product, but that does not mean it will work wherever it is fitted. In this case, the value of the investigation was not simply identifying that backflow protection was needed. It was establishing how the drainage system was arranged, where the surcharge pathway was likely to be, why the previous valve arrangement had not prevented recurrence, and where protection could realistically be installed. The report considered the types of intervention envisaged, the constraints around installation, and why the position of the valve mattered. The aim was not simply to say “fit a non-return valve”. It was to explain what needed to be protected, where the valve needed to go, how it might be installed, and what residual risks still needed to be managed, along with providing clear imagery to accompany the text.

Two floods, same mechanism

The first flood was in 2022, after around 15 minutes of torrential rain following a hot, dry spell. Water rose through the drains, entered via the bath, shower and rear door, and the flat needed a full reinstatement.

Afterwards, the management company did what many people would do. Quotes were obtained for non-return valves, a contractor installed them, and everyone reasonably assumed the problem was solved.

In June 2026, another heavy storm hit after a dry period. The owners were abroad when a neighbour sent them photos of the street underwater. The flat had flooded again, despite the valves.

The missing step was never the valves themselves. It was the investigation and design that should have come before them.

The flood maps said there was almost no risk

On paper, this property barely registers. The Environment Agency lists it at very low risk from rivers, the sea and surface water. It sits in Flood Zone 1. Thames Water holds no record of sewer flooding at the address.

None of that stopped it flooding twice.

National flood mapping is produced at a national scale. It may not have sufficient resolution to see a lightwell, a step down from the front door, or a flat whose floor sits below the surrounding ground. It cannot see private drainage at all. And a sewer flooding history search only shows incidents that were reported and recorded in a particular way, so an empty register does not mean surcharge has never happened. We covered a similar blind spot in our blog on what a CON29DW drainage and water search does and does not tell you.

For a lower ground floor flat, the real question is a simple one. When the drainage system is full, where does the water go? It goes to the lowest available outlets. Here, those outlets were the flat’s own shower and gullies.

Why the non-return valves didn’t stop it

During our site visit we lifted the inspection chambers and carried out water testing, running water from the internal fittings and gullies and watching where it arrived, so we could understand how the drainage was arranged rather.

The valves fitted in 2022 were genuine, flap valves. But they were not in the most suitable location for this situation. They had been installed on the inlets and on the property side of the chamber, protecting individual branches, but not on the final outlet to the public sewer. There also wasn’t a non-return valve in the outlet for the ensuite or bathroom connections.

That left an open route. When the sewer surcharged, water could still flow into the chamber, fill it, and overflow into the property’s drainage upstream of the valves.

The clearest evidence that positioning is everything – was next door. The neighbouring lower ground floor flat was understood to have had a valve fitted in the correct position around 25 years ago. It has not flooded through either event.

What we recommended

Once the mechanism was understood, the approach was more straightforward. Intercept the flow at the last point before the public sewer, with a full-port non-return valve at the outlet of the final inspection chamber, supported by lower-cost push-fit valves on the vulnerable branches as immediate measures.

But even here, you cannot just start digging. The gas service appeared to run through the obvious excavation area, which ruled out one installation method until its position could be safely confirmed. The drainage also serves the flats above, which introduces a known limitation of outlet valves on shared systems: during a prolonged surcharge, the building’s own flow can back up behind the closed valve. That risk has to be managed in the design, not discovered afterwards. This is exactly the work our backflow protection service exists for.

Isn’t a CCTV survey the answer?

When drainage flooding happens, the go-to is often a CCTV survey, and it can be a good option. It certainly seems logical. The difficulty is that a CCTV survey is only as good as its scope of works. We regularly find ourselves preparing the scope for the CCTV company, so that the data which comes back is usable and includes the necessary detail for us to answer the question being asked.

Then there is interpreting the data and the report. CCTV reports often make recommendations, but the camera only sees the pipe it is in. Without the fuller picture, the ground levels, the flood history, the sewer records and where the property sits within the wider network, those recommendations can be too isolated to be the correct thing. This is the same discipline we describe in what is forensic engineering: establish the mechanism first, then choose the tools.

Or as an associate of mine once succinctly described it:

“Prescription without diagnosis is malpractice”

I know it can seem frustrating to spend money on consultancy fees and not immediately on protection. When sewage has come up through your shower, you understandably want something physical, and you want it now. Having experienced my own family home flooding in 2007, I have full sympathy with that.

But in medicine, prescription without diagnosis is malpractice, and flood protection is no different. The valves at this property were a prescription written without a diagnosis. The product was likely fine. The design step was missing, and the property flooded again because of it.

Someone once told me during an enquiry, “I don’t want any of this, I want a team of people here with shovels digging trenches and building walls.” I understood the sentiment, but it is unlikely to be the correct approach.

This case study is written to help spread awareness, but the reality is we cannot force everyone down the correct path, and where the process isn’t followed, there is a limit to how much we can help. What I can say is that by getting professional advice first, you are best placed to ensure the relevant legislation is followed, and that any measures will be most effective and well received for future insurance requirements, including Flood Performance Certificates under the Flood Re reforms. DEFRA’s own property flood resilience evidence points the same way: measures work best when they are specified for the actual risk at the actual property.

Sometimes the outcome of an investigation can also be that further study is needed before anything is installed. That is not a consultant creating fees. That can be the difference between the right intervention, and the wrong one. Or even, the right intervention in the wrong location!

Example of Non-Return Valve installed onto Sewage Pipe

The Lesson

The key lesson is simple: diagnose the problem before paying for the solution.

If your lower ground floor or basement flat has flooded, FPS Environmental can help investigate what happened, review the drainage layout, test flow routes where needed, and advise on the most appropriate measures before further money is spent on works that may not solve the problem.