A drain smell inside a home is hard to ignore and oddly hard to trace. It comes and goes, it seems strongest in a different spot each time, and there is often no sign of water anywhere. That is because a smell and a leak are not the same thing. Gas from the drainage system can escape through a gap that never lets out a drop of water, and a waste pipe can leak only for the few seconds it is in use.
This guide explains the usual causes, the checks you can do, how professionals use smoke testing, CCTV and moisture mapping, and how to tell whether you need leak detection or a drainage contractor.
Why does my house smell of sewage when there is no visible leak?
Every basin, bath, shower, sink and toilet connects to a drainage system that contains foul air. Normally a small amount of water sitting in a trap under each fitting blocks that air from entering the room, and the soil stack vents the rest above the roof. A smell indoors means that barrier has failed somewhere. The usual reasons are:
- A trap that has dried out or been emptied, leaving an open path from the drain.
- A failed seal on the soil stack, a toilet pan connector or a waste pipe joint, often hidden behind boxing.
- A cracked or split pipe, particularly on older plastic or cast iron stacks.
- A faulty air admittance valve that stays open, or a blocked vent that lets the traps be sucked dry.
- A waste leak that only runs when a fitting is used, soaking into a floor or wall where it slowly goes stale.
Is it a dried-out trap?
This is the first thing to rule out because it is so simple. Traps in rooms that are rarely used lose their water to evaporation, especially in warm weather or with the heating on. Common culprits are guest bathroom basins, unused showers, floor gullies in wet rooms and utility rooms, and washing machine standpipes that are not used often.
Run water into every outlet in the house for a few seconds, including ones you never use, and flush any toilet that has sat idle. If the smell fades over the following day, a dry trap was the likely cause.
A trap can also lose its water while the fitting is in use. When a large volume of water runs down a pipe, it can pull the water out of a nearby trap, which is often heard as a gurgle. If you hear gurgling from a basin when the bath empties or a toilet flushes, mention it to the plumber, as it points to a venting or pipe design issue rather than a leak.
Where does the smell appear, and what does that suggest?
| What you notice | Likely cause | First check |
|---|---|---|
| Smell in a room that is rarely used | Dry trap | Run water into every outlet in that room |
| Smell around the base of a toilet or behind its boxing | Pan connector or soil pipe seal | Look and smell at the connector, check for damp around the pan |
| Smell strongest near a boxed-in pipe running floor to ceiling | Soil stack joint, crack or access cap | Remove any access panel and look at the joints |
| Smell after a bath or shower drains | Trap being sucked dry, or a leaking waste joint | Listen for gurgling, check for damp below the fitting |
| Smell with a damp patch or stained ceiling | Waste water leaking into the structure | Moisture readings around the patch and the fittings above |
| Smell outside as well as in, or near a manhole | Below-ground drainage problem | A drainage contractor |
Could a cracked pipe or failed seal behind boxing be the cause?
In most homes the soil stack and the branches to toilets and baths are hidden behind boxing, bath panels and kitchen units. The joints on these pipes rely on rubber rings, solvent-welded sockets or, on older stacks, jointing compound. Any of them can fail with age or movement.
- Toilet pan connectors can shift if the pan is disturbed, leaving a gap that lets out air and, on flushing, a little water.
- Ring seals on the stack can harden or be pushed out of place when a pipe is moved during other work.
- Solvent-welded joints that were not made properly can crack or pull apart.
- Old cast iron stacks can corrode or crack, often at the back where nobody looks.
- Access caps and rodding eyes can be left loose after a blockage has been cleared.
A seal can let out smell without leaking water, because the pipe only runs full for a moment. A cracked pipe on the underside may leak every time a fitting is used, but the water can soak into a timber floor or a wall and dry off between uses, leaving a smell and a slow stain instead of a puddle.
What checks can you do safely?
- Refill every trap and flush every toilet, then note whether the smell changes.
- Open any access panels you can remove without tools or damage, and look for wet joints, stains or white residue.
- Check around each toilet for damp flooring, a pan that moves, or a gap at the connector.
- Run one fitting at a time, then smell and look below it. A bath emptied in full, a basin filled and released, and a toilet flushed several times are all fair tests.
- Keep a diary of when the smell is worst: after certain fittings are used, in warm weather, or when the heating is on.
- Ventilate the room while the cause is found, and avoid mixing cleaning chemicals in drains to mask the smell.
Do not cut into boxing or lift tiled floors to look. A survey can usually narrow the search first. If the smell is strong and persistent, deal with it promptly rather than living with it.
How do smoke testing and CCTV find the source?
Both are drainage techniques, and they answer different questions.
- Smoke testing fills the drainage pipework with harmless visible smoke. Wherever the system is not sealed, smoke appears. It is useful for above-ground soil and waste pipes behind boxing, because a thin wisp at a panel edge or a skirting board shows where the gap is without opening anything.
- CCTV surveys send a small camera through the pipes, usually from a manhole or access point. They show cracks, displaced joints, root intrusion and blockages inside below-ground drains, and some cameras can inspect larger internal stacks.
Drainage contractors carry out this work. Where the smell comes with damp, the leak detection side is finding where water from a faulty waste pipe has spread.
How is moisture mapping used below a waste pipe leak?
Waste water leaks are intermittent, so they do not show on a meter test or a pressure test, and they are cold or only briefly warm, so thermal imaging alone can miss them. Moisture mapping takes readings on a grid across the ceiling, wall or floor below and around the suspect fittings, and plots where the readings are highest. The wettest area usually sits just below or downslope of the leak.
The engineer then uses each fitting in turn and repeats readings or thermal images, looking for the area that changes. A bath waste that leaks only when the bath empties, for example, shows as a fresh cool spread on the ceiling below shortly afterwards. Our guides to leaks under a bath and a bathroom ceiling leaking below the shower go through those fitting tests in more detail.
When is it a drainage job rather than leak detection?
If the problem is below ground, the right contractor is a drainage specialist. Signs include a smell outside, slow drains throughout the house, a manhole that fills or smells, and blockages that keep returning.
Responsibility for drains changed on 1 October 2011, when most private sewers and lateral drains in England and Wales were transferred to water and sewerage companies. Since then, a homeowner is generally responsible for a drain that is inside their boundary and serves only their property. A drain that serves more than one property, or runs beyond the boundary, is generally the sewerage company's. If a CCTV survey finds a fault on a shared section, your sewerage company is the first call.
Leak detection is the better fit when there is damp, staining or a ceiling mark alongside the smell, and the question is which pipe or fitting is letting water into the building.
When should you call a leak detection specialist?
- the smell comes with a damp patch, stain or swollen flooring;
- refilling traps has not helped and the pipework is hidden;
- you need to know which fitting is leaking before boxing or tiles are opened;
- you want a documented cause for a landlord or insurer.
Our damp and moisture detection survey is £250 to £450, and a thermal imaging survey is £150 to £250. Our guide to telling damp from a leak may also help. Call 020 3912 0200.
Frequently asked questions
Why does my bathroom smell of sewage but there is no leak?
Most often a trap has dried out or been siphoned, or a seal on the toilet connector or soil pipe has failed behind boxing. Air can escape through a gap that never leaks water.
Can a leaking waste pipe cause a smell?
Yes. Waste water that soaks into a floor or wall goes stale and smells, and the gap it leaks from can also let drain air into the room.
How do I find where a sewage smell is coming from?
Refill every trap, check around toilets and boxed-in pipes, and note when the smell is worst. If it persists, a drainage contractor can smoke test the pipework, and a moisture survey can find any water that has escaped.
Is a sewage smell in the house dangerous?
It should not be ignored. Ventilate the room, avoid masking it with mixed chemicals, and get the cause found promptly, especially if the smell is strong or persistent.
Who is responsible for a broken drain?
In England and Wales, a homeowner is generally responsible for a drain inside their boundary that serves only their property. Shared drains and those beyond the boundary are generally the sewerage company's responsibility.
