Bathroom Smells of Sewage | Five Checks Before You Call

A sewage smell in the bathroom is one of those things you can't ignore. It's unpleasant, it lingers, and it immediately suggests something expensive. But here's the thing: a blocked drain is usually the last cause, not the first. The majority of sewage smells I get called out to in New Malden, across streets in Coombe, Beverley Park, Malden Manor, West Barnes, all of them, turn out to be something a homeowner could have identified in five minutes with their nose and a torch.

So before you book anyone in, work through these checks. If you find the cause, you might fix it yourself or at least know exactly what to ask for when you do call.

Check 1: A Dry Trap Under a Rarely Used Sink or Basin

Every sink, bath, shower tray and floor drain has a trap: a U-shaped section of pipe that holds a small plug of water. That water plug is the only thing standing between your bathroom and the sewer gas below. If the fitting hasn't been used in a while, the water evaporates. The trap goes dry. The smell comes straight up.

Where to look: the basin in the guest bathroom, the shower in the en suite nobody uses, the floor drain in a downstairs WC. If the room sees very little use, this is almost certainly your answer.

What to do: Run the tap or pour a litre of water down the drain. Wait a few minutes. If the smell fades, you had a dry trap. Pour a small amount of cooking oil down each rarely used drain every few months. The oil sits on top of the water and slows evaporation.

That's it. No callout, no charge, done.

Check 2: A Cracked or Poorly Seated Toilet Pan

The toilet pan connects to the soil pipe via a flexible rubber connector or a wax ring seal (older installations). Both can fail. A cracked pan is less common but not rare, especially on older ceramics where someone's over-tightened the fixing bolts. A failed seal is far more common and often invisible.

Kneel down and run your fingers around the base of the pan where it meets the floor. Any movement when you press down is a bad sign. Any staining, soft grout, or a slight rocking when you sit means the seal is suspect. Even a hairline crack in the ceramic can vent gas into the room continuously.

What to do: If the pan rocks, the wax seal or pan connector needs replacing. That's a plumber job, not a drain job. If you can see a crack in the ceramic, the pan needs replacing. Don't let anyone sell you a full drainage excavation before this has been ruled out.

Check 3: A Cracked or Blocked Soil Vent Stack

The soil vent stack is the vertical pipe that carries waste from your WC and basin down to the sewer, and vents sewer gases up through the roof. It usually runs inside an internal void or boxed in next to the bathroom. In older properties across Coombe Vale and Norbiton, stacks are often cast iron and can develop hairline cracks at the joints.

A cracked stack vents gas directly into the wall cavity or the room. A blocked stack cap (birds' nests are the most common cause) stops the gas venting upward, so it finds another way out, often through the water traps it's supposed to push past.

What to do: If you can access the stack, look for staining, moisture, or the faint smell of drains near any exposed section. If the stack goes through a boxing, put your hand near any gaps and feel for a faint warm air movement. A CCTV survey won't find a stack crack inside a wall. That's a job for a drain camera on a flexible rig, or in some cases a smoke test, where we pump harmless smoke into the system and watch where it exits.

Check 4: A Failed or Missing Inspection Chamber Seal

Some properties, particularly semi-detached and terraced houses in Malden Park and Old Malden, have inspection chambers inside the building or just under a floor. The lid or frame seal can dry out, crack, or simply not be replaced properly after someone's had a look at it.

What to do: Find your nearest inspection chamber, typically in the kitchen or utility area but sometimes under a bathroom floor panel. Lift the lid and check whether the rubber seal or frame gasket is intact. Press the lid back firmly. If there's a smell around it, that's your source. Replacement lids and seals are available from any merchant. A cracked concrete chamber is a bigger job, but at least you'll know what you're dealing with.

Check 5: Root Ingress or Partial Blockage Causing Slow Drainage

This is the one most people jump to first. I'd put it last, because you should have ruled out everything above before spending money on a survey.

That said: if your bathroom drains are slow, if you hear gurgling from the basin when you flush the toilet, or if multiple fittings drain sluggishly at once, a partial blockage or root ingress is plausible. Partial blockages in the line allow waste to build up, and decomposing organic matter produces hydrogen sulphide. That's the rotten-egg, sewage smell.

In KT3 and the surrounding postcodes, mature gardens with willows and sycamores are common, and root ingress into clay drains is something we see regularly on CCTV surveys.

What to do: Don't pour chemical drain cleaner down first. It damages clay pipe joints and does nothing for root ingress. Run each fitting in the bathroom in turn and time how long it takes to clear. If everything is slow, and especially if it gets worse in wet weather, get a CCTV drain survey done before assuming you need a blockage clearance.

A Few Things Worth Knowing Before You Call Anyone

• Sewage smell that's strong first thing in the morning and then fades often points to dry traps or a poorly vented stack, not a blockage.
• A smell only present when the heating is on can suggest gas is being drawn out of traps by warm air convection. Dry trap, again.
• Don't let a general builder diagnose a drain problem. The two trades overlap but they don't share the diagnostic kit.
• If you've had building work done recently, check whether an inspection chamber lid or a drain cap was left off or poorly replaced. It happens more than you'd think.

If you've worked through these checks and you're still not sure where the smell is coming from, or if you're finding signs of a cracked stack or root ingress, that's the point to call a drainage specialist.

Drainsco covers New Malden and all the KT postcodes, with engineers available 24 hours a day. There's no call-out fee, you'll get a clear price before any work starts, and all remedial work, drain repairs, relining, pipe replacement, carries a 20-year guarantee. Give us a ring and describe what you've found. We'll tell you honestly whether it warrants a visit.