A drain smell inside the house is most often a trap that has dried out under a rarely used basin, shower or floor gully, or a blocked drain outside. Refill the traps and see if it clears. If the smell keeps returning, spreads between rooms or arrives with a damp patch, a cracked soil or waste pipe is likely and needs tracing.
You walk into the downstairs loo or the back bedroom and there it is: that flat, sour, sewage smell. You have cleaned, opened the windows, poured something down the plughole, and by the next morning it is back. The good news is that indoor drain smells come from a short list of causes, and the first two are things you can sort out yourself in ten minutes. The less good news is that the third cause is a leaking waste pipe, which does not fix itself and tends to leave damage behind.
Here is how to work through the list in order, and how to tell when the job has moved from a quick check to water leak detection.
How the smell is supposed to be kept out
Every sink, basin, bath, shower and floor drain has a bend under it that holds a plug of water. That water is the seal. Foul air in the drainage system cannot get past it into the room. Toilets have the same seal built into the pan. All of those fittings feed into a soil pipe that vents at roof level, or through an air admittance valve in the loft, so that a flush does not pull the water out of a nearby trap.
Anything that breaks that chain lets the air out. Your nose finds it before your eyes do.
Cause one: a trap that has dried out
This is the most common reason by a distance. If a fitting is not used, the water in its trap slowly evaporates and the seal is gone. It happens in guest bathrooms, utility room gullies, en-suites in a spare room and shower trays nobody uses. Warm weather speeds it along, which is why the smell often appears after a fortnight away.
In Cornwall the classic case is the holiday let or second home. A cottage in Padstow or St Ives sits empty from October to Easter, every trap in the place dries, and the first guests of the season open the door to a smell that has nothing to do with the cleaning. Owners who leave a property empty for long spells should have a changeover checklist that includes running every tap and flushing every toilet, and a splash of cooking oil on the water in a rarely used trap slows the evaporation. While you are there, glance at the water meter too: a meter moving when nothing is on is a different problem, but an empty house is where it goes unnoticed.
The test: run every tap for a minute, flush every toilet, pour a jug of water into any floor gully. If the smell has gone within an hour and stays gone, that was it.
Cause two: a blocked or dirty drain
Not every drain smell is inside the house. An outside gully full of leaves and fat, or an inspection chamber that has backed up, can send a smell in through a window, an air brick or a door. Inside, a plughole coated in soap, grease and hair gives off a sour, stale smell that sits right at the fitting rather than filling the room, and the overflow slot on a basin or bath has its own little channel that collects grime and rarely gets cleaned.
The test: lift the gully grid and the inspection chamber lid outside. Standing water or a slow gurgle means a blockage, which is a drainage job, not a leak. Clean plugholes and overflows with a bottle brush and hot water. If the smell was localised to one basin and it clears, you are done.
Cause three: a vent problem
If the smell comes and goes, especially just after a toilet is flushed or a bath drained, the vent may be at fault. A blocked soil stack vent or a failed air admittance valve lets a flush suck the water out of the nearest trap for a moment, and a puff of foul air follows. Gurgling from a basin when the toilet flushes is the giveaway. A plumber can replace a valve in minutes.
Cause four: a leaking soil or waste pipe
This is the one to take seriously. Push-fit joints perish, the rubber boot where a waste pipe enters the stack splits, cast iron stacks in older houses crack along a seam, and a soil pipe under a solid floor can fail where it was never bedded properly. Where foul air escapes, waste water usually escapes with it. Because these pipes run inside boxing, under baths and beneath suspended floors, the water has plenty of time to soak into timber and plaster before anything is visible.
Signs the smell is a leak rather than a dry trap:
- It returns within a day or two of refilling every trap and cleaning every plughole.
- Damp arrives with it. A stain on the ceiling below a bathroom, a soft patch of floor by the toilet, blistered paint on a boxed-in pipe run. Our article on a damp patch that will not dry describes what that looks like.
- It is strongest at pipework, not at a plughole. Under the bath panel, behind the toilet, in the airing cupboard, at floor level in a downstairs room beneath a bathroom.
- Mould keeps coming back in one place however often it is cleaned.
- It spreads through more than one room instead of sitting over one basin.
- The smell is worse when the bath or shower drains, which means the leak is on the waste run and only wet when water passes.
| What you notice | Most likely cause | What to do |
|---|---|---|
| One unused basin or shower smells, no damp | Dried-out trap | Run water, add a little oil, done |
| Smell near a window or door, gurgling outside | Blocked gully or chamber | Clear it or call a drainage firm |
| Smell after a flush, basin gurgles | Vent or air admittance valve | Plumber to check the valve |
| Smell returns, damp or mould nearby | Leaking soil or waste pipe | Non-invasive trace before opening up |
| Smell only when bath or shower drains | Split waste joint under the floor | Trace the waste run |
How a hidden waste pipe leak is found
Pulling off bath panels and lifting floorboards on a guess is expensive and often misses the spot, so the aim is to find the point of failure first and open up only there. A waste pipe leak is different from a pressurised supply leak: it only carries water when a fitting is used, so the methods are different too.
- Dye and flow testing. Coloured dye is run through each fitting in turn while the suspected area is watched with a moisture meter. The fitting that produces a rise identifies which run is leaking.
- Smoke testing. Harmless visible vapour is introduced into the drainage system. Where it escapes into a room, there is a gap, whether that is a failed joint behind a wall or a hairline crack in a stack.
- Drain camera. A camera fed along the soil pipe shows cracks, displaced joints, root ingress and standing water from the inside.
- Thermal imaging and moisture mapping. These show how far the wet area extends behind a surface, so the repair and the drying can be planned properly.
Together they narrow the problem to a small, defined spot. Read what to expect from a leak detection survey if you want to know how a visit runs.
Older Cornish houses and their drains
Granite cottages and Victorian terraces around Penzance and Falmouth often have drainage that has been extended, re-routed and patched over a century. Cast iron stacks buried in render, clay pipes under slate floors, and a waste run that nobody can quite trace. A smell in these houses is worth investigating sooner rather than later, because the damage from a slow foul-water leak into lime plaster and old timber is slow to show and slow to dry. It is also the kind of damage that becomes a discussion with an insurer about whether it was sudden or gradual, so having the source identified early and in writing protects you.
When to stop investigating yourself
Refilling traps, cleaning plugholes and checking the outside gully are sensible first steps and cost nothing. The point to call someone in is when the smell keeps returning after those checks, when there is any sign of damp or mould alongside it, or when it is tied to pipework you cannot reach. A foul smell with a soft floor or a stained ceiling is not a cleaning problem. It is a leak, and the sooner it is located the less there is to put right.
Frequently asked questions
Why does my house smell of drains after I have been away?
The water seals in traps under unused basins, showers and floor gullies evaporate while the house is empty, and foul air comes up through them. Run every tap and flush every toilet when you return. If the smell clears and stays away, that was the cause.
Can a drain smell mean there is a leak?
Yes. A cracked soil pipe or a failed waste joint lets air out, and usually water too. If the smell returns after you refill the traps, spreads through more than one room, or comes with a damp patch, soft floor or recurring mould, treat it as a leak and have the pipe run traced.
Is a sewage smell in the house dangerous?
An occasional whiff from a dry trap is unpleasant rather than harmful. A persistent smell suggests foul air is getting in continuously, and if it comes with water escaping there is a hygiene issue as well as a damage one. Fix the source rather than masking it with air fresheners.
Why does the basin gurgle when I flush the toilet?
The flush is pulling air through the basin trap, which means the drainage vent is not doing its job. The stack vent may be blocked or an air admittance valve has failed. A plumber can check and replace the valve. Gurgling followed by a smell is the classic sign.
Does home insurance cover a leaking soil pipe?
Damage caused by water escaping from a fixed pipe is usually covered under escape of water, and finding the pipe can fall under trace and access. Insurers look at whether the damage was sudden or gradual, so having the source identified and documented early helps the claim.
Trace the smell to its source
If you have refilled the traps and the smell keeps coming back, we can test the waste and soil runs without pulling the bathroom apart and give you a report your insurer will accept. We provide leak detection across Cornwall, from Bude to Penzance.
