Bathroom Smells of Sewage | Five Checks Before You Call

Sewage smell in a bathroom is one of those things that makes you assume the worst immediately. Blocked drain. Cracked pipe. Expensive. But half the call-outs we attend where a homeowner swears blind there's something wrong underground turn out to be a dry trap under a guest basin or a perished wax seal under a toilet. Two-minute fixes, not excavations.

Before you ring anyone, work through this. You're looking for the source, not the symptom.

Check 1: The Trap Under Any Rarely Used Sink or Shower

Every sink, basin, shower tray and bath has a U-bend trap beneath it. That curve holds a small plug of water that physically blocks sewer gas from travelling back up the pipe and into the room. If a fitting hasn't been used in a few weeks, the water evaporates. The plug disappears. The smell comes straight through.

This is the most common cause we find in spare bathrooms, holiday lets around South Cerney and Fairford, and any bathroom with a secondary basin nobody uses day-to-day.

What to do: Run the tap for thirty seconds. Do it for every basin, shower and bath in the room. Add a capful of vegetable oil to slow evaporation if the fitting won't be used regularly. Wait ten minutes and sniff again. If the smell drops noticeably, you've found it.

Check 2: The Toilet Wax Seal

Under every toilet, where the pan meets the soil pipe in the floor, there's a wax ring. It compresses when the toilet is bolted down and creates an airtight seal. Over time, that wax can crack, dry out or deform if the toilet has ever rocked slightly. A failed wax seal lets sewer gas leak directly into the room at floor level.

You won't always see this from above. The tell is a smell that's strongest right at the base of the toilet, sometimes with a faint staining or dried residue around the collar. Rock the toilet pan gently with both hands. Any movement at all, even a millimetre, and the seal is compromised.

What to do: A replacement wax ring costs a few pounds. The job involves isolating the water, removing the pan, pressing in the new ring and rebolting. A competent DIYer can do it in an hour. If you're not confident, a plumber can. It's not a drainage job.

Check 3: The Soil Vent Stack

The soil vent stack is the large-diameter pipe, usually 110mm, that runs vertically through your property and out through the roof. It carries waste from toilets, baths and sinks down to the sewer, and it also vents sewer gases safely upward and out. Two jobs, one pipe.

If the vent terminal on the roof gets blocked, by a bird's nest, debris, or a failed cage, the venting function stops. Gas that should exit upward has nowhere to go and finds the next available exit: your bathroom drain, your toilet bowl, or around any loose fitting. You'll often notice the smell is worse after heavy rain or when the wind changes direction.

A cracked stack is a different problem. A hairline crack in cast iron or older uPVC can bleed gas into a wall cavity or directly into a bathroom if the pipe runs internally. On Cirencester's older terraces in Watermoor or around the town centre, cast iron stacks are common and they do corrode.

What to do: If you can safely see the top of your stack from a window or roof hatch, check the terminal cap is in place and clear. Don't go on the roof yourself to investigate. If you suspect a crack in an internal section of stack, a CCTV survey can confirm it without anyone touching a spade.

Check 4: The Floor Gully or Access Cover in an Older Property

A lot of Cirencester's housing stock, particularly on Chesterton, Stratton and the older parts of Siddington Road, was built with open or semi-sealed floor gullies in bathrooms and utility rooms. These can dry out just like a sink trap, but they're often covered by a tile or a fitting and forgotten about.

Lift any small circular or rectangular cover in your bathroom floor. If there's one, pour a bucket of water down it. If it gurgles readily and the smell reduces, the gully trap was dry. Seal any covers that aren't needed.

Check 5: The Shared Manhole or Neighbour's Line

If you've worked through every fitting in the bathroom and still can't pin the smell down, check whether it's concentrated near an external wall or around a vent brick. In terraced and semi-detached properties, shared drainage runs close to party walls. A blockage or partial collapse in a neighbour's line can pressurise the shared system and push gas back through the nearest low point, which might be your bathroom.

Lift your external manhole cover if you have one. A healthy chamber smells faintly of drains but not overpoweringly. If it's backing up, gurgling or has an obvious blockage sitting in the benching, the problem is in the shared run.

What to do: Call us for a CCTV survey. If the fault is in a shared or adoption boundary, that changes who's responsible for the fix. Worth knowing before you start spending.

One Thing Worth Remembering

A persistent sewage smell that comes and goes with no obvious source, in a bathroom you use every day, with no dry traps and a solid toilet, should be taken seriously. Intermittent gas smells from a soil system can indicate a partial blockage creating back-pressure, a cracked pipe section, or a failed drain joint. None of those get better on their own.

Drainsco covers Cirencester and the GL7 postcodes, engineers on call 24 hours a day, no call-out fee, and a clear price before any work starts. If you've worked through this list and you're still no closer, give us a ring and we'll tell you honestly whether it needs a survey or whether there's something else worth trying first.