Why does my bathroom smell of drains but nothing is wet?

A soil stack leak in a sealed boxing lets odour out through gaps in the casing long before enough water escapes to show on a surface. Dried-out traps on unused outlets and a failed WC pan connector produce the same smell and are cheaper to rule out first.

Should I replace the whole cast iron stack?

Rarely. Cast iron that is sound is very long-lived and quieter than plastic. What usually needs replacing is the adaptor joint between iron and plastic, or one perished section. Full replacement is a decision for when the iron is genuinely failing along its length.

Is the boxing in my bathroom likely to contain asbestos?

It might if it dates from before 2000. Asbestos insulating board was widely used for partitions, bulkheads and panelling, and textured coatings such as Artex are also common. Where the material is uncertain it should be tested before anyone cuts into it.

The stain is on my neighbour wall, not mine. Whose stack is it?

In a terrace the stack often sits in or against the party wall and can serve both houses or just one. Establishing which property it serves, and where the joint that failed sits, is the first job. It changes who instructs and who pays.

Can a soil stack leak be found without opening the boxing?

Partly. Thermal imaging and moisture mapping will show where the water has tracked, and a dye or smoke test through the system will confirm the stack is the source. Confirming the exact joint and repairing it means opening the casing.

Fulham

The boxing in the corner of a Victorian terrace bathroom almost always conceals the soil stack and the point where original cast iron meets later plastic, and that transition joint is the single most common source of a slow, smelly leak that stains a wall long before anything drips.

Leak Fix LondonLast updated 19 September 2026

A Leak Fix London engineer working on pipework in a London home

The box in the corner

Every Fulham terrace bathroom has one: a plasterboard or ply casing in a corner, sometimes tiled over, usually with no access panel. Behind it is the soil and vent pipe taking waste from the WC, the basin and the bath down through the house, and in most of these properties that pipe is not one thing.

The original stack was cast iron, either running externally up the rear elevation or internally through the house. At some point between the 1970s and the 2000s, part of it was replaced in plastic, because a bathroom was moved, an extension was built, or a length of iron rusted through. The two are joined with an adaptor and a rubber seal, and the boxing went up afterwards so nobody would have to look at it.

That joint is the thing that leaks. Not dramatically. It weeps when the WC is flushed, which is to say intermittently, in small volumes, into a sealed void with no ventilation.

Why you smell it before you see it

A soil stack leak behaves nothing like a supply pipe leak, and the difference is diagnostically useful.

It is intermittent, tied to use rather than constant. Nothing happens overnight.
The water is contaminated, so it produces odour, staining that is brown or grey rather than clear, and sometimes fungal growth on the back of the plasterboard.
It is not pressurised, so it does not show on a meter and it does not affect boiler pressure. Nothing on your bill changes.
It stains a wall in a line rather than producing a spreading patch, because it runs down the outside of the pipe and along a horizontal member before it drops.

The smell is the early warning and people explain it away for a remarkably long time. If a bathroom or the room next to it has a faint drain smell that gets worse in warm weather and nobody can find a source, the boxing is where to look. Soil pipe leaking covers the symptom in more detail.

The side return, where old meets new

The other reliable failure point in a Fulham terrace is the rear extension. Almost every one of these houses has had the side return filled in, which moved the kitchen and often added a shower room or a utility. That work threaded new pipework into a house whose original plumbing was never designed to be extended.

What you get is a sequence of junctions between eras and materials: original cast iron to 1970s plastic to modern push-fit, with the occasional length of copper on the supply side joined to plastic barrier pipe. Each transition is a potential failure and each one is now buried behind finishes.

The particular problems worth knowing about:

Inadequate fall. A waste run threaded through an existing structure often achieves less fall than it should, so solids settle and the run partially blocks. Blockage raises pressure at joints that were only ever designed to carry free-flowing waste.
Unsupported horizontal runs. Long plastic waste runs with too few clips sag between supports and the joints open at the low point.
Buried rodding access. Access points that the installer left, and the plasterer covered.
Party wall proximity. In a terrace the stack frequently sits close to or within the party wall structure, which means a leak can present in the neighbour's house rather than in yours.

Asbestos: the thing that stops the job

This is a real constraint in these houses and it is regularly ignored. Asbestos can be present in buildings constructed or refurbished before 2000, and using it in construction or refurbishment has been illegal since 1999. In a Victorian terrace it is not the Victorian fabric you have to worry about; it is what was done to it in the 1960s, 70s and 80s.

The materials that turn up around bathroom boxing and soil stacks specifically:

Textured decorative coatings used to produce decorative finishes on ceilings and walls, known by trade names such as Artex.
Vinyl and thermoplastic floor tiles, along with associated sheet materials and screed materials.
Asbestos insulating board used for partition walls, bulkheads above ceilings and panelling, which is exactly what some 1970s boxing is made of.
Asbestos cement products including downpipes, gutters and flues.
Lagging on pipework and around boilers.

The practical rule: if the boxing is old, do not put a multitool through it to have a look. Textured ceiling coatings in particular should not be sanded, scraped or drilled on spec. Where the material is in doubt it gets tested before it gets opened, and that adds a few days rather than a few hours.

Opening the boxing without wrecking the room

Done properly, exposing a stack is a controlled operation rather than a demolition.

Identify the stack route through the house from outside and from the floors above and below, so the opening goes where the joint is rather than where the stain is.
Establish the material of the casing, and test it if pre-2000 and uncertain.
Cut a panel on the tile joints or along the framing rather than through the middle of a field of tiles.
Expose the joint, run water through the system, and watch. A soil leak has to be reproduced to be confirmed, because it is not continuous.
Replace the failed section or the failed seal, not the whole stack, unless the iron is genuinely perished.
Put the boxing back with an access panel this time. Every stack should have one, and almost none do.

That last point is the only piece of advice on this page that saves money on the next occasion rather than this one. A screwed or magnetic access panel at the joint costs very little when the boxing is already open and turns the next investigation into a ten-minute look.

What is not worth calling us for

If the smell is in the bathroom but the boxing is dry and the stack is sound, the likely answer is a dried-out trap on a seldom-used basin or shower, or a failed WC pan connector seal, both of which are cheap and quick. Run water into every outlet in the room and leave it a day before booking anyone.

If the staining is on an external wall behind the stack, check the stack's own joints from outside first. Cast iron socket joints that have lost their caulking weep externally and stain brickwork, and that is a repair you can watch happen from the garden rather than a detection job.

And if the water is clean and constant rather than foul and intermittent, it is not the stack. Go to bathroom and shower leak repair instead, because you are looking for a supply or a tray seal.

Where it is the stack, we replace the failed section and put the boxing, tiling and decoration back, which is the half of the job most firms hand to someone else.

Frequently asked

Why does my bathroom smell of drains but nothing is wet?: A soil stack leak in a sealed boxing lets odour out through gaps in the casing long before enough water escapes to show on a surface. Dried-out traps on unused outlets and a failed WC pan connector produce the same smell and are cheaper to rule out first.
Should I replace the whole cast iron stack?: Rarely. Cast iron that is sound is very long-lived and quieter than plastic. What usually needs replacing is the adaptor joint between iron and plastic, or one perished section. Full replacement is a decision for when the iron is genuinely failing along its length.
Is the boxing in my bathroom likely to contain asbestos?: It might if it dates from before 2000. Asbestos insulating board was widely used for partitions, bulkheads and panelling, and textured coatings such as Artex are also common. Where the material is uncertain it should be tested before anyone cuts into it.
The stain is on my neighbour wall, not mine. Whose stack is it?: In a terrace the stack often sits in or against the party wall and can serve both houses or just one. Establishing which property it serves, and where the joint that failed sits, is the first job. It changes who instructs and who pays.
Can a soil stack leak be found without opening the boxing?: Partly. Thermal imaging and moisture mapping will show where the water has tracked, and a dye or smoke test through the system will confirm the stack is the source. Confirming the exact joint and repairing it means opening the casing.

Sources

Other parts of London

No find, no fee · £150 per hour. The rate is fixed before we attend and does not rise for a difficult property, an awkward access or an evening visit. The total depends on how long the job takes; the rate cannot change.