A drain smell inside the house almost always means the same thing: somewhere, sewer air has found a route past a water seal that is supposed to be blocking it.

The job is finding which seal has failed. There are only about six candidates, and they can be worked through methodically.

What a plumber actually checks, and in what order

1. Has a trap dried out?

Every waste fitting has a U-bend holding a plug of water. If an outlet is not used for a few weeks, that water evaporates and the seal is gone.

The usual suspects are a spare bathroom, a utility sink, a shower that never gets used, and the standpipe behind a washing machine. Running the tap for thirty seconds refills it. If the smell goes, that was it.

Worth doing before anything else because it costs nothing and it is genuinely the most common cause.

2. Is there an air admittance valve, and has it failed?

Many houses have an air admittance valve, a one-way valve that lets air into the waste system without letting smell out. They sit in a boxing, a loft, or under a sink.

They fail. When they do, they stay open and vent sewer air straight into the room. A failed one is cheap to replace once found.

3. Is the WC pan connector sound?

The flexible connector between the toilet and the soil pipe perishes and can pull partly out, particularly if the toilet has ever been rocked or refitted.

A leak there smells strongly and is often behind or under the pan where you cannot see it. Moving the toilet slightly and smelling behind it usually settles the question.

4. Is the drain itself partly blocked?

A partial blockage holds waste in the run where it sits and decomposes. The smell comes and goes, is worse in warm weather, and is often accompanied by slow draining somewhere.

Lifting the nearest inspection chamber cover tells you a lot in ten seconds. If it is holding water when nothing is running, there is a blockage downstream.

5. Is the soil pipe damaged inside the structure?

Internal soil pipes run in boxing, in walls, and through roof voids. A cracked joint or a poorly made connection inside that boxing vents into the cavity and the smell appears somewhere apparently unrelated.

This is where a camera earns its keep, because opening up boxing on a guess is destructive.

6. Is there a redundant or open connection?

Old properties often have capped-off branches from previous bathrooms. Caps come off, rodents get in, and you have an open route from the drain into the floor void.

What it usually turns out to be

Most of the time it is a dried out trap or a failed air admittance valve, and both are quick. When it is neither, a camera survey is the sensible next step rather than opening up boxing and floors speculatively.

When to stop and call someone

Frequently asked questions

Why does the smell only appear sometimes?

Warm weather makes it worse, and so does wind direction if a vent is involved. Intermittent smells often point at a partial blockage or a failing air admittance valve rather than a dry trap.

Can I just pour bleach down it?

It masks it for a day. If the seal has gone or there is a blockage, bleach does nothing about the cause.

Do I need a CCTV survey?

Not straight away. Work through the traps and the air admittance valve first. If those are clear, a survey stops you guessing.

Is a drain smell a health risk?

Persistent sewer gas indoors is worth taking seriously and worth resolving rather than living with.

Need a plumber in Herts, Beds or Cambs?

Vantage Plumbing and Heating covers Hertfordshire, Bedfordshire and Cambridgeshire. The line is answered by a person at any hour of any day, and you get a price before anyone starts. Call 01462 506374 or request a callback.

Related: drainage services · CCTV drain surveys · drain blockage · blocked sinks and drains.