Sewer smell in a Korean apartment: why it happens and how to stop it

Cause 1: a dry trap

Every drain in your home is supposed to hold a small amount of standing water in a bend below the plughole. That water is the seal. It is the only thing between your room and the gases in the sewer pipe, and it works purely because it is there.

If a drain goes unused, that water evaporates. Once the bend is dry there is nothing sealing the pipe, and sewer gas rises straight into the room. It is most noticeable in a guest bathroom nobody uses, a utility-room floor drain, or a second sink — and it gets much worse in summer heat and in winter when heating is running, because both dry the water out faster.

Cause 2: the washing machine hose sitting in the drain

In most Korean apartments the washing machine drains into a floor drain on the balcony, and very often the hose has simply been pushed down into that drain. When the hose blocks the opening, the drain cannot hold a proper water seal — the hose is effectively acting as an open pipe from the sewer into your balcony, and the smell drifts into the flat from there.

There are floor-drain fittings designed to take a washing machine hose while still maintaining a seal. If your balcony smells and the hose is jammed straight down the hole, that is the thing to look at first.

Cause 3: a broken trap, a damaged vent, or pressure in the shared stack

If the water seal is present and the smell is still there, the problem is with the plumbing itself rather than with evaporation.

• A cracked or badly fitted trap cannot hold water, so it drains itself between uses and behaves exactly like a dry trap that you keep refilling.
• A building's drain stack needs a vent pipe to let air in and out. If that vent is damaged or obstructed, draining water pulls a vacuum and siphons the water out of your traps — you will often notice gurgling from another drain when a nearby fixture empties.
• Pressure changes in the shared stack, typically when several households drain at once, can push sewer gas back past a weak seal. This is the version where the smell comes in bursts rather than sitting there constantly.

Cause 4: unfinished sealing in a new building

In newly built or newly renovated units, the pipes pass through the slab inside sleeves, and the gap around the pipe has to be sealed. If that sealing was skipped or done badly, smell travels up through the gap without going through any drain at all — which is why the smell seems to come from a wall or the floor rather than from a plughole. New apartments are also where you most often find protective caps and construction debris still sitting inside the drain.

Why does it only smell at night?

A very common description: the bathroom is fine all day, and the smell arrives in the evening or is waiting for you first thing in the morning. There are three reasons, and they often overlap.

Siphoning when the building drains all at once

Evening is when a residential building uses the most water: everybody showers, runs the washing machine and empties the kitchen sink within the same couple of hours. A large slug of water falling down the shared stack drags air with it, and if the vent cannot supply that air fast enough, the stack pulls a partial vacuum. The easiest place to get air from is your trap — so the water seal gets siphoned out of a fixture you were not even using, and sewer gas walks straight in. That is the pattern behind gurgling from the floor drain when a neighbour empties a bath.

Nothing refills the trap overnight

The reverse case. Whatever the seal loses during the day gets topped up every time you run a tap. Between about midnight and morning nothing goes down any drain for seven or eight hours, so a trap that is slightly low or slightly leaky finishes emptying and the smell builds in a closed room. This is why the smell is often strongest the moment you open the bathroom door in the morning and fades an hour later — you have already used the basin.

Cooler air and a closed room

Ventilation fans are usually off at night, windows are shut, and the air in the bathroom is still. Any gas that does get in accumulates instead of being carried away. Temperature changes across the evening also shift the pressure balance inside the stack slightly, which is enough to push gas past a weak seal.

What to try, and what not to

When to call someone

• You poured water in and the smell is back within three days
• More than one room smells at the same time
• The drains are also running slowly, not just smelling
• You hear gurgling from another fixture when you drain a sink or bath

Any of those points away from simple evaporation and towards the trap, the vent or the shared stack. What we do is put an endoscope camera in to look at the condition of the trap and the venting, so the diagnosis is something you can see rather than a guess. If it turns out to be in the shared stack, that is usually a matter for the building — see our guide on who pays for what in a rental.