Everywhere, or one room? Answer that first
People search the same words whether the problem is one basin or the whole ground floor, and the two have almost nothing in common.
One room. A dried-out trap, a shower waste packed with biofilm, a washing machine standpipe, a wet-room gully with a shallow seal. Fixable by hand in an afternoon, and covered in why a bathroom smells of drains.
Several rooms, or the hall and the stairs. Foul air is not coming out of a fitting; it is getting into the building and then moving around it. Traps do not fail in six places at once.
Walk the property with the windows shut and note where the smell is strongest and where it is absent. A smell in the hall but not in either bathroom is a very different fault from one that is only bad in the en-suite.
Four questions that narrow a whole-house smell before anything is opened
These four answers do more than any amount of sniffing at plugholes.
- Is it worse in certain weather, or after heavy rain? Wind and rain change the pressure in the drainage system and in the ground around the building. A smell that arrives with a south-westerly and disappears on a still day is almost always coming from outside — a vent, a chamber, a gully, or a defect below ground.
- Is it worse when the heating is on? Warm air rises and leaves the building at the top, which pulls replacement air in at the bottom. If the bottom of your house connects to a void that connects to a drain, the heating season is when you will smell it. It also explains a smell that appears in October and vanishes in April.
- Did it start after building work? A loft conversion, a new bathroom, a kitchen moved to the other side of the room, a boiler swapped. Any of those can leave a soil branch capped badly, a stack cut and rejoined inside a duct, or an air admittance valve fitted in a sealed boxing where it cannot draw air.
- Is it constant, or does it come in waves? A background smell that never lifts, in the same place, points at a permanent opening inside the fabric. Waves following a shower or a WC point at a fault that only discharges when something runs.
When it tracks the weather, look outside the building
Outside causes share one tell: they are not constant. Wind direction, rainfall and the water table move them.
- An unsealed or damaged inspection chamber. Cracked covers, missing bolts, perished seals and covers relaid by a paving contractor with no seal at all are all common. A chamber outside a back door will scent a whole ground floor whenever the wind is in the wrong quarter.
- A soil vent pipe terminating too low, or too close to a window. A vent shortened during a roof or render job, or an extension built so a bedroom window now sits beside an existing vent, puts the outlet where you breathe.
- A defect in the buried drain run. A cracked or displaced length vents into the soil, and the smell comes up through a suspended floor void, an airbrick or a service entry. Heavy rain makes it worse because the run surcharges.
- A dry or silted gully. An external gully has a trap too. Blocked with leaves, and with nothing discharging into it for weeks, the seal evaporates exactly like an indoor one.
Cover seals, gully clearing and the vent position are worth checking yourself. A defect in the buried run needs a drainage contractor with a camera, which is not us.
When it never lifts, look inside the fabric
A constant smell, strongest at a particular wall, cupboard, stairwell or ceiling rather than at any drain, means foul air has a permanent path into the building. In a London terrace or a converted flat the usual candidates are short:
- A cracked or open soil stack inside a duct or boxing. A hairline split, a push-fit socket that has crept apart, or a branch never properly solvent-welded discharges foul air into the boxing, and the boxing connects to the rest of the flat through every service penetration. Worth reading alongside a leaking soil pipe.
- A failed air admittance valve. Where a stack terminates inside the building, an AAV lets air in when something discharges and shuts to stop foul air coming back. Stuck open, it vents the stack into a cupboard or a loft continuously. Some are fitted in sealed boxing with no air supply, which makes them fail early.
- A disconnected or collapsed drain under a solid floor. In older properties the run under the ground floor is often clay. A joint pulled apart by settlement discharges under the slab. There is no puddle and no stain; the smell is the only symptom, and it is everywhere because the subfloor is everywhere.
- A dry trap somewhere nobody visits. The utility basin, the garage WC, the cellar gully, a spare shower room. Unused for a month, the seal evaporates and the whole stack vents into the house through it. Run every outlet in the property, including the ones you forgot about.
The branch nobody remembers capping
This gets its own heading because it catches people repeatedly, and the culprit is often a job done properly in every other respect.
When a washing machine, a dishwasher, a bidet or an old sink comes out, the waste branch is often left in the wall or floor with a push-fit cap, a bag and tape, or nothing. It then sits behind a new kitchen unit or under a new floor and nobody sees it again. The result is a permanent open connection from the drainage system into a void, and a smell in a room with no drainage in it.
If the smell started within a year of a refit, work out what was removed and where that branch went. A redundant branch should be cut back and capped at the stack, not stopped at the far end.
Why living with it is not a sensible plan
Sewer gas is not just unpleasant. UK law treats the inside of a sewer as a place with foreseeable serious risk, and the definitions are instructive even though they were written for people at work rather than for your hallway. The Confined Spaces Regulations 1997 define a confined space to include a chamber, pit, pipe or sewer where a specified risk arises, and define specified risk to include serious injury arising from a fire or explosion, and loss of consciousness or asphyxiation arising from gas, fume, vapour or lack of oxygen.
Household concentrations are nowhere near those inside a sewer, and nobody is calling your kitchen a confined space. Two practical points follow anyway.
- Do not put your head into an inspection chamber and do not send anyone down one. That is the situation those regulations exist for.
- Ventilate while you investigate. A persistent smell means air is moving from the drainage system into your living space. Find the opening rather than masking it with a plug-in air freshener.
If anyone is getting headaches or nausea that clear when they leave the house and return when they come back, treat it as a reason to move faster, and tell whoever attends.
The part of this that is not our job, and we will say so
We find and repair water leaks. Clean water, under pressure, escaping from supply, heating or waste pipework. That is what the equipment is for and what the hourly rate buys.
A cracked soil stack behind plasterboard, a collapsed drain under the floor, a defective run below ground, a chamber that needs rebuilding — those are drainage jobs. They want a CCTV survey and, depending on what the camera finds, a patch liner, a re-line or an excavation. A drainage contractor does that work properly and we do not pretend otherwise.
Foul drainage above and below ground is covered by Building Regulations Approved Document H, which sets out pipe sizes, protection of pipes, manholes and inspection chambers. Any drainage contractor rebuilding a chamber or re-laying a run should be working to it.
What we can settle, and what it costs
There is a real overlap, and it is why people call us about a smell at all: many drainage faults are also leaking water, and some smells sit on top of a clean-water leak that has been running for months. What a visit establishes:
- Whether the property is losing water at all. The meter and a stop-tap isolation answer that in minutes, and it is worth knowing before anyone digs. See a meter turning with everything off.
- Where the building is wet, as opposed to where it smells. Moisture mapping across floors, walls and the ceiling below finds the wettest point rather than the most visible one.
- Whether a supply, heating or waste pipe in the void is discharging — which is water leak repair, ours, and repairable the same visit where access allows.
- A written report of what was found, prepared to support a trace-and-access claim if the damage turns out to be insurable. The insurer decides what the policy covers.
Detection is £150 per hour, agreed before we attend, with no call-out fee, and if we do not find a leak you do not pay for the time spent looking. See pricing. If the answer is that there is no water leak and you need a drainage contractor, that is a finding too, and you get it in writing.
