Rat Flaps and Rat Valves: How to stop rats and influence people

The problem with drainage defects is they are underground and usually beneath your floor.

To get to them therefore you would need to excavate down causing considerable disruption and damage to your property as well as incurring very significant cost.

However man as ever has thought of an ingenious way of avoiding this and came up with the idea of the rat flap (or rat valve as some call them).

This is a metal or plastic device that has a flap – the key feature is the flap only operates in one direction (this requires important consideration when it comes to fitment but we’ll come to that later).

These flaps are fitted into the drains using the manholes (inspection chambers) for access.

They allow waste water to flow out towards the sewers but they stop any rat traffic travelling up from the sewers in the other direction (if fitted the right way round – reverse this scenario if they are not!).

The beauty of this is it stops rats from reaching the defect buried beneath your house so you don’t need to dig anything up.

Quite often these defects don’t affect the operation of the drainage system – water only usually flows along the bottom 10% of the pipe during normal conditions so unless the defect is within the area of water flow nothing really changes.

So bingo – valve in and job done right?

Well as with much in life, unfortunately it’s not that simple and there’s a bit of detail involved.

You have to consider the valves themselves and also where they are located….


There are lots of different valves out there – twin flap designs, single flap designs, serrated flap designs, convex flap designs, stainless steel designs, plastic designs, push in designs and gripping designs.

For a start, forget anything plastic – rats will rip these to bits in hours.

Forget anything galvanised – you want 306 stainless steel as a minimum.

Then consider flow rate – water from inlets typically has a high flow rate as its generally falling from height and has less pipe to travel along.

Then consider pipe diameter – has it been lined?Is it clay or PVC?

Then consider access – how deep is the manhole?Is it a modern PVC manhole or a brick period manhole?

Then consider whether the pipe section your aiming to fit the valve too has sufficient straight run or is swept.

Without giving away all our secrets to our competitors, all of these factors dictate what type of valve you fit.

If you fit the wrong valve to the wrong scenario then you risk rats lifting the valve, the valve sticking open or the valve affecting/clogging the drainage system (which will cause you an additional headache to the persisting rat problem).

Right valves fitted to right scenarios are incredibly long lasting and reliable – they don’t clog (unless you really start abusing your drainage system).

You also need to understand drainage law to ensure you don’t fit a valve to a section of drain that actually belongs to Thames Water or other authority – they won’t appreciate that and will remove it as soon as they become aware of it.


The valve will only work if it is fitted to the correct location – the correct location is only determined by a comprehensive inspection and understanding of the drainage system.

The correct location also involves an understanding of drainage law.

Again without giving away all the hard earnt intel, the degree of flow varies between inlets and main runs and from manhole to manhole due to a range of reasons.

The degree of flow in turn dictates what type of valve to be fitted.

If a valve intended for a high flow situation is fitted to a low flow situation then chances are the passage of solids through it will be impeded and rats are likely to learn how to lift it.

Finally this is not a job for the DIY’er.

Manholes are surprisingly dangerous places and cause thousands of injuries and deaths (yes really) every year.

Manhole lids can be incredibly heavy and cause back injuries of finger crushes – do you have the right equipment to lift them?

Manholes contain very high levels of viruses and bacteria – do you have the correct PPE and sanitisation equipment?

Manholes can contain very dangerous gases – do you have the necessary and calibrated gas analysers?

Manholes can be located in dangerous areas such as roads – are you able to protect yourself from being run over?

Even when you’ve got yourself prepared for the above, you’ll find fitting the valve not as straightforward as you may have thought – sometimes special installation tools are needed and some knack to get them into place.

If you fit them the wrong way round then they simply stop that pipe from flowing sewage – this means it slowly fills up and then empties back out into your house eventually.

Even after fitment, you’ll need a drainage camera to ensure the valve is sitting correctly and operates fully through the full range of motion.

So best left to the experts – and that’s us!

Nobody knows valves like we do and we always ensure the best valves are fitted to the correct locations.

We also go on to consider and ensure that there are no above ground entry points either otherwise valves will only remedy the problem in part, but that’s a story for another blog…