Non Return Valve for Drains

A non-return valve (NRV) on a drainage system is a valve fitted inside a drain that allows wastewater to flow out of your property but prevents water—and potentially rats—from travelling back into the property through the pipe.

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How it works

Think of it as a one-way door:

Your house → 🚽 → drain → sewer ✅ Wastewater can leave.

Sewer → 🐀 → drain → house 🚫 The valve closes and blocks the return route.

Most drainage non-return valves have a hinged flap or similar mechanism. When wastewater flows in the normal direction, the flap opens. If flow reverses, the flap is pushed closed.

Why it can help with rats

Rats can travel surprisingly long distances through sewer systems. If there's an unobstructed connection between your property's drainage system and the public sewer, a rat can potentially enter through the pipework and emerge via a toilet or another drainage connection.

A properly installed rat flap/non-return valve creates a physical barrier.

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Its benefits include:

• 🐀 Reduces the risk of rats entering the property through drains.

• 🚫 Prevents reverse movement through the protected pipe.

• 💧 Can also help prevent sewer surcharge/backflow entering the property.

• 🔧 Once installed correctly, it operates automatically without you having to do anything.

• 🏠 Can be particularly useful for properties with known rat activity or vulnerable drainage connections.

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But there's an important catch

A non-return valve isn't a guarantee that you'll never have rats.

For example, if there are other drainage connections, defective pipes, open inspection chambers, damaged gullies or gaps elsewhere, rats may have another route into the property.

And the valve itself needs to be maintained. Debris, fat, wipes, roots or other material can prevent the flap from closing properly.

There's also an important distinction between a normal non-return valve and a purpose-designed rat blocker/rat flap. If your primary concern is rats, I'd want the drainage contractor to specify that the device is suitable for rodent prevention, rather than simply installing a generic backflow valve.

If you're considering one for a house you're buying

I'd actually combine this with the CCTV drain survey we discussed earlier.

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The ideal sequence would be:

CCTV survey → identify how rats could potentially access the property → determine where a rat flap should be installed → install it at the appropriate point → CCTV/check that it operates correctly.

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How it would work

You'd typically have something like:

House → private drain → [INSPECTION CHAMBER + RAT FLAP] → sewer

The flap allows wastewater from your house to pass toward the sewer:

House → → → 🚽 → sewer ✅

But if a rat tries to travel back from the sewer:

Sewer → 🐀 → 🚫 [closed flap] → house

The inspection chamber gives the engineer access to install and subsequently inspect/maintain the valve.

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Why the location matters

Ideally, the valve should be positioned so that it protects the entire drainage system serving the property.

For example, if you have:

• Toilet/bathroom drains

• Kitchen waste

• A rear extension

• Garage/outbuilding drainage

all joining together before reaching the inspection chamber, putting the rat flap at that chamber can potentially protect all of those upstream drains.

But if there are other branches downstream or alternative connections, a single valve might not protect everything.

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One thing I'd definitely check

Ask the drainage company:

"Does this inspection chamber contain the single drainage route serving the property, or are there other connections that could allow rats to bypass the valve?"

That's an important question.

I'd also ask them to confirm:

"Is the proposed valve specifically designed as a rat blocker, and will it remain accessible for cleaning and maintenance?"

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There is a potential downside

A rat flap can be very useful, but you don't want it becoming blocked or stuck closed.

If the flap gets obstructed by wipes, grease, debris or other material, you could end up with wastewater backing up toward your property.

That's why I'd want the valve:

• Properly sized for the pipe.

• Installed in the correct direction.

• Easily accessible from the inspection chamber.

• Designed to minimise snagging/blockage.

• Checked periodically.

And this is where the CCTV survey becomes particularly useful: it can establish the condition and configuration of the drain before you install anything.