Maintenance and repairs

Odour from the drain in the bathroom: how to determine the cause step by step

When there is a sewage smell in the bathroom, the cause is usually simple: a dried or clogged odour trap, a loose joint or negative pressure in the pipes. To avoid cleaning blindly, first determine where and when the smell occurs. A simple test one drain at a time will help. If the whole flat or house smells, the problem is more likely in the soil stack or in the ventilation of the sewage system.

Prepared by: TGB Kúpeľne team, bathroom showroom in Senec since 1991updated 9. 11. 20265 min read

Illustrative image: linear shower drain with removable grate and odour trap on a towel

An overview of the most common causes of odours and their solutions can be found in the article Blocked drain in the section Odour from the drain. Here we go a step further: how to find the cause when it is not clear, what types of traps exist, and what to do when the whole flat or house smells.

First, determine where and when the smell is coming from

Before you start dismantling waste traps, answer a few questions. Each answer narrows down the search.

Test one drain at a time

1. Pour water down all drains – sink, shower, bath, bidet, floor drain, and washing machine waste. Let the water run for a while to refill the traps.
2. Ventilate and wait. If the smell disappears, the cause was a dried trap.
3. If the smell remains, cover the drains one by one – with a plug or wet cloth. When the smell weakens after covering one of the drains, you have found the source.
4. Smell the overflows of the sink and bath – the overflow channel can become clogged and smell, even when the waste trap is clean.
5. Check the connections – waste trap nuts, the sleeve at the WC, and the connection of the floor drain with the tiles. Wipe them with a dry paper towel.

How to dismantle and clean the waste trap is described in the articles Waste trap and Drain cleaner and cleaning spiral.

Types of traps

The trap retains odours from the sewage. We distinguish three types, each describing a different part of the standard for floor drains.

For example, the standard water trap Alcadrain has a height of 51 mm, while the combined trap SMART is 30 mm. Geberit states that the shower element with a wall drain has a height of 30 mm, noting that this differs from the requirements of EN 1253. For a low trap in a shower that you do not use often, pour water into it more frequently or supplement it with a mechanical trap if offered by the manufacturer.

Odour after flushing: negative pressure and ventilation

When water from the WC or bath drains rapidly, negative pressure is created in the pipes, sucking water from the trap of another drain. A vent valve according to EN 12380 can help – it allows air into the pipe under negative pressure and seals under positive pressure. The design and installation are the work of a plumber; details are in the article Blocked drain.

In a family house, the sewage system should be ventilated according to the project, most often with a pipe above the roof. If the ventilation pipe gets blocked by leaves, a nest, or ice, traps may start to be sucked dry, and odours may appear even at the windows. Leave the inspection to a plumber – working on the roof requires safe access.

Odour throughout the flat or house

• Apartment building – odour in multiple places or simultaneously at neighbours indicates a problem in the soil stack or its ventilation. This is addressed by the building manager.
• Family house – check the inspection chamber and the ventilation of the sewage system. Drain cleaning companies perform camera inspections of the pipes, and some also conduct smoke tightness tests.
• Cottage with a septic tank or treatment plant – odour may come from their ventilation or a full septic tank. The rules for septic tanks are described in the article WC at the cottage and in the garden.

Odour not coming from the drain

• Odour from neighbours' ventilation – in a prefab block of flats with a shared shaft, air can flow into the bathroom from the ventilation pipe. A backdraft damper behind the fan can help – more in the article Bathroom fan.
• Mould in silicone, behind the bath or in corners – musty smell. See Mould in the bathroom.
• Leakage under the floor tiles or behind the wall tiles – a damp wall or floor smells even when the drains are fine.
• Hot water – if only the water from the hot tap smells, not the drain, the cause may be in the boiler. This will be assessed by service; more in the article Boiler maintenance.

How to prevent odours

Before a long holiday, pour water into all drains and floor drains, and let them run for a while upon your return. Clean traps and channel traps regularly to prevent build-up. During renovations, consider drains that will be used infrequently – laundry room, technical room, second shower. A dry or combined trap is suitable for these. It is cheaper to replace an old or leaky siphon than to search for a fault – you can find them in the e-shop in the category Washbasin siphons. More guides are in the section Maintenance and repairs of the bathroom.

Common mistakes

• Chemicals for odours – drain cleaner will not refill a dried trap; water is sufficient.
• Air freshener instead of finding the cause – the smell will return, and a leaky joint may leak.
• Forgotten trap insert after cleaning the linear shower drain.
• Low trap in an infrequently used shower without regular refilling.
• Powerful fan for a shared shaft without agreement with the manager – odours and moisture may push towards neighbours.

Sources: STN EN 1253-1 (floor drains with a water seal of at least 50 mm), EN 1253-6 (less than 50 mm), EN 1253-7 (mechanical seal), EN 1253-8 (water and mechanical seal), STN EN 12380 (vent valves), technical sheets from Alcadrain (51 mm, SMART 30 mm) and Geberit (shower element 30 mm differing from EN 1253); our articles Blocked drain, Siphon, and Bathroom fan.

Terms in the dictionary: Siphon · Soil stack · Floor drain (point drain) · Overflow · Construction height of the drain · Sanitary silicone