Bad smell in the bathroom: six typical causes

Six common causes

• the water trap of a rarely used floor drain or sink has dried out
• the trap is partly full of sediment and is fermenting
• the washing machine or dishwasher hose is connected without a water trap
• the ventilation pipe is blocked and air is moving in the wrong direction
• the floor drain's seal or connection is loose
• the sewer pipe is damaged inside a wall or floor

How to rule out the cause

Pour a couple of litres of water into every drain and floor drain. If the smell disappears for a few days, the water trap was dry. If the smell persists right away, it's likely sediment or a connection issue.

If the smell only appears when neighbours are using water, it points to ventilation or the stack. In that case you need to look beyond the flat itself.

When a more thorough check is needed

A persistent smell combined with a damp wall or ceiling means water is moving somewhere it shouldn't. Then the pipework is inspected with a camera, or a leak is searched for before removing any tiles.

Washing machine, dishwasher and floor drain

A frequent cause is connecting the appliance's drain hose directly to the sewer without a water trap. Then the smell travels through the appliance into the room and seems to come from the appliance itself. The solution is a connection with a water trap and the hose at the correct height.

It's also worth checking the floor drain's seal and the part under the grate. If a gap has formed around the drain during tiling work, gas can bypass the water trap.

How ventilation affects the smell

The sewer needs air. If the ventilation pipe leading to the roof is blocked or fitted with the wrong valve, negative pressure builds up in the pipework and water traps get sucked dry. A typical sign is a smell that appears exactly when neighbours are using water.

No cleaning product solves this kind of situation. The ventilation's patency and the stack's condition need to be checked, often together with the housing association.

When a smell points to a construction fault

If the smell has been there since a renovation, it's not sediment. Typical causes are a connection without a water trap, an incorrectly installed floor drain, or a ventilation pipe that ends inside the structure. Such faults don't go away with maintenance — the connection needs to be redone.

In an older building, a persistent smell that appears suddenly points more towards damaged pipework or a loose connection. Before opening the tiles, it's worth inspecting the pipework with a camera so that only the necessary spot is opened.