Sewer Smell in the House? How a Smoke Test Finds the Source

A sewer smell in the house almost always means sewer gas is getting past a seal it should not get past: a trap that has dried out, a joint on the soil pipe that has opened, a WC connector that has failed, or a vent that is not doing its job. When the drains flow normally and the smell still comes back, a smoke test is usually the quickest way to find exactly where the gas is escaping.

What causes a sewer smell in a house?

The smell is gas from the drain and sewer, mostly hydrogen sulphide, which is what gives it the rotten egg note. Every appliance in the house is protected by a water seal in a trap, and every joint in the soil and waste pipework is meant to be gas-tight. A smell indoors means one of those barriers has failed.

In London houses and flats the usual causes, roughly in order of how often they turn up, are:

• A dry trap. A shower in a spare room, a floor gully in a utility room or a basement drain that is rarely used loses its water seal through evaporation, and gas comes straight up.
• A failed WC pan connector. The rubber or plastic sleeve between the toilet outlet and the soil pipe perishes or gets knocked out of line, often after a new floor or a toilet replacement.
• An open joint on the soil stack. Internal stacks boxed in behind plasterboard are common in converted flats, and a joint that has pulled apart can leak gas for years without leaking water.
• Trap siphonage. Poor venting pulls the water out of a trap when another appliance discharges. The trap refills, then empties again, so the smell comes and goes.
• A faulty air admittance valve. These one-way valves let air into the pipework but should never let gas out. One that sticks open lets smell into the loft or boxing.
• A leaking or defective underground drain under or next to the building, where gas travels through the ground or a void into the house.

Check these first before paying for a test

Start by running water into every trap in the property, including ones you never use. A dry trap is the single most common cause, and refilling it costs nothing.

Then look for a pattern. A smell that appears after a bath empties or a washing machine pumps out points to siphonage and venting. A smell that is strongest in one room and constant points to a physical leak near that room. A smell that follows heavy rain can mean a surcharged drain or sewer pushing gas back up. A smell outside near a chamber is a different problem, and our guide to what a persistent drain smell means covers the drain side in more detail.

If refilling traps and checking the toilet connection has not fixed it, stop guessing. Sealing random joints with silicone, or pouring chemicals down the drains, rarely touches the actual leak point.

How a drain smoke test works

A smoke test fills the drainage system with harmless, visible smoke under very light pressure and watches where it comes out. Anywhere smoke appears inside the building is a place where sewer gas can get in.

On site, the engineer plugs the drain downstream, usually at the last chamber before the sewer, and introduces smoke into the system through a chamber or the top of the soil stack. The vent terminals on the roof are watched for smoke to confirm the system is filled. The engineer then goes room by room checking around WC connections, boxed-in stacks, behind bath panels, under kitchen units, in lofts near air admittance valves, and at floor and wall penetrations. Smoke escaping from a joint or a gap is photographed and recorded.

The smoke used for this is made for the job and does not stain, but it can drift through gaps, so neighbours in a shared building may see it and should be told beforehand. The test does not prove anything about pipes that are buried under the garden, because smoke leaking into soil is rarely visible at the surface.

Smoke test, CCTV survey or drain test: which one finds the smell?

They answer different questions. A smoke test finds leak points above ground and inside the building. A CCTV survey shows the condition of the pipes below ground. A pressure test proves whether a run is watertight or not, but does not say where the leak is.

The difference between the test types is set out in our guide to air and water drain testing. Where the evidence points below ground, a CCTV drain survey with a written report records the defect, its position and its severity, which is what a repair quote and, if it comes to it, an insurer will need.

Smells in flats and shared buildings

In a block, the leak is often not in the flat that can smell it. Sewer gas travels along a shared stack or through ceiling voids, so a smell in one flat can come from a failed connection in the flat above or below.

That changes who has to act. The stack and the shared drains are usually the responsibility of the freeholder or managing agent, while the pipework serving one flat usually belongs to that flat, but the lease decides. A smoke test is useful here because it produces photographs of exactly where the gas is escaping, which settles most arguments about whose pipe it is. Access to the other flats needs arranging in advance, or the test only shows half the picture.

When a smoke test is the wrong answer

A smoke test is wasted money if the drains are blocked, if the smell is plainly coming from an outside chamber, or if nobody has yet checked the traps. Clear the blockage and refill the traps first.

It is also the wrong tool if what you actually smell is not sewage. A musty smell can be damp or rot. A smell of gas, rather than drains, is a different emergency: leave the property and call the National Gas Emergency number on 0800 111 999. And if sewage is visibly backing up, the job is clearing the drain, not testing it.

If you are not sure which of these you are dealing with, tell us what you are smelling, where and when, and we will say which check makes sense first.