Guide
Sewer smell in a Korean apartment: why it happens and how to stop it
A drain smell that appears in a bathroom or a utility room, especially one you barely use, is one of the most common complaints we hear from foreign residents — and the most common cause has a free fix you can do yourself in thirty seconds.
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.
What to try, and what not to
Do
- Pour water down every unused drain weekly to refill the seal
- Lift the drain cover and check what is underneath — hair mats, construction debris and missing trap inserts are all visible without tools
- Check whether the washing machine hose is blocking a floor drain
- Note which rooms smell and at what times; bursts versus constant tells us a lot
Do not
- Keep pouring bleach down the drain — it damages pipes and trap components over time and does nothing about the actual cause, which is usually a missing water seal rather than bacteria
- Seal a drain shut with tape or a plug as a permanent solution — you are hiding a plumbing fault and blocking a drain you may need
- Ignore gurgling from a second drain, which is a venting signal rather than a cleaning problem
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.
Frequently asked
I poured water down the drain and the smell came back. Now what?
If the smell returns within a few days, the trap is not holding water — either it is cracked or damaged, or a venting problem is siphoning the seal out every time something nearby drains. That needs a look inside with a camera; refilling it again will only buy you another few days.
My apartment is brand new. Why does it smell already?
New buildings have two typical causes. The pipe sleeves through the slab may not have been sealed properly, so gas travels up through the gap rather than through any drain. And traps in a new unit have often never been filled, or still contain construction debris and protective caps left in during the fit-out.
Should I tell the building management office?
Yes, if the smell is in more than one room, appears in bursts, or comes with slow drains or gurgling — those all point to the shared stack or the venting, which is the building's equipment rather than yours. If it is one unused drain in one room, refill the trap first; there is nothing for the management office to do about evaporation.