Bathroom Smells of Sewage | Five Checks Before You Call

A sewage smell in your bathroom is one of those things that makes you want to call someone immediately. Understandable. But roughly half the time I visit a job like this in KT1 or KT2, the drain itself is fine. The smell has a different source entirely, and the fix takes five minutes and costs nothing.

Here's how to find it before you pay anyone to do anything.

Why the drain gets blamed first

Drains smell when they block. That's true. But your drainage system also has several points where gases can escape without there being any kind of blockage at all. Hydrogen sulphide, the compound that smells like rotten eggs, is produced naturally in sewage. Normally your pipework is designed to keep it out of your house. When any one of several components fails or dries out, it gets in.

The smell usually hits hardest after a shower or after you flush. Sometimes it's constant. The source determines the fix, and most sources aren't underground.

Check one: the trap under your least-used basin or shower

Every waste outlet has a U-shaped trap beneath it. That U holds a small plug of water that physically blocks sewer gas from coming back up. If you never use that basin or shower, the water evaporates. No water, no seal. Gas comes straight through.

This is extremely common in guest bathrooms, en-suites that don't get daily use, or the downstairs cloakroom that only gets opened when someone comes round.

What to do: Run the cold tap for 30 seconds in every basin, shower tray and bath in the property. Do the same with any floor drain. Then wait 20 minutes and check whether the smell has gone. If it has, you've found it. A small amount of plumber's grease smeared around the overflow cover slows evaporation. Alternatively, look at trap primers or bottle traps designed to hold water longer.

Check two: the toilet wax ring

The toilet pan sits on a wax ring (sometimes called a wax seal or closet gasket). It creates an airtight connection between the base of the pan and the soil pipe below. Over time, especially if the toilet rocks slightly when you sit on it, that wax compresses unevenly, cracks, or shifts.

The result is a slow leak of gas at floor level. You often can't see water, and the toilet flushes perfectly well. The seal can fail without any visible damp at all.

What to do: Sit on the toilet and shift your weight gently side to side. Any movement at all, even a couple of millimetres, means the wax ring is compromised. Check the bolts at the base of the pan (the ones covered by plastic caps near the floor). If they're loose, or if the pan itself can be rocked, that's your likely culprit. A plumber or our engineers can lift the pan and replace the ring in an hour or less.

Check three: the soil vent stack

The soil stack is the large vertical pipe, usually 100mm or 110mm diameter, that runs from your ground floor drainage up through the roof. It vents gas safely above the roofline and also provides the air pressure equalisation that stops your traps siphoning dry when you flush.

If the stack has a crack anywhere, gas leaks out into the void inside your walls or ceiling before it ever reaches the roof. In older Kingston properties, particularly Victorian and Edwardian terraces in Norbiton and Canbury, cast iron stacks corrode from the inside. Plastic stacks on extensions occasionally crack at push-fit joints, especially after a cold winter.

What to do: Go to your loft or any point where you can see the stack pipe. Run your hand along any accessible section and look for rust staining, damp streaks on adjacent timbers, or visible cracks. Press on push-fit joints to check they're fully seated. A strong sewage smell in the loft confirms gas is escaping before it clears the roof. This one does need professional attention, but a CCTV survey of the stack from above is usually enough to locate the damage without opening any walls.

Check four: the condensation vent cap or cowl

The top of your soil stack, above the roofline, has a cage or cowl over it to stop birds nesting. Occasionally these get blocked, sometimes by a dead pigeon, sometimes by debris. A blocked cowl stops the stack venting properly, which forces air back down and can push gas through weakened traps.

You'd usually notice this because multiple traps seem to be losing their seals at once, and the smell appears after heavy flushing.

What to do: This needs a roofer or a drainage engineer with a long enough camera rig. Don't go on the roof yourself. But it's worth knowing this cause exists before anyone starts pulling pipes apart inside the house.

Check five: the inspection chamber seal

If you have an old inspection chamber under a manhole cover outside, check whether it's in the extension footprint, under a decking area, or immediately adjacent to an air brick. Older brick chambers in properties across Ham, Coombe Hill and Kingston Vale often develop cracks at mortar joints. Gas can travel through gaps and find its way in under a suspended floor or through a cavity wall.

What to do: Lift the manhole cover safely (use a hook or a flathead screwdriver, not your fingers). The chamber should smell, obviously, but look at the condition of the brickwork. Any crumbling mortar, root intrusion or displaced benching is worth noting. If you've got a suspended timber ground floor and a cracked chamber close to the house, the connection is usually straightforward.

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Work through those five checks in order. They take maybe 20 minutes in total and cost nothing except the discomfort of looking inside a manhole.

If the smell persists after all five, or you find something at check three, four or five that you can't fix yourself, Drainsco covers the full KT postcode area with engineers available around the clock. No call-out fee, a clear price before we start, and any remedial work we carry out comes with a 20-year guarantee. Give us a call and describe what you found.