Sewer pipework is designed to carry wastewater away while preventing air from the pipes from entering the room. If you smell a drain odour at home, it means this barrier is compromised somewhere. There are three main barriers: the water trap (siphon), airtight connections, and the ventilation stack. An unpleasant smell is always a sign that at least one of these is not functioning correctly.
The good news is that approximately half of these issues can be resolved by the homeowner in one evening – a dried-out water trap, a dirty overflow opening, or a disconnected washing machine hose are common and simple causes. The remaining cases require diagnostics, as the smell originates from a place not visible to the naked eye: pipework sediment, a crack under the floor, or a communal stack in an apartment association.
In this guide, we will examine all common causes, provide a logical inspection order (what to check first, then next), describe solutions, and conclude with prevention. The descriptions are based on ToruFix OÜ's daily work in apartments, private houses, and apartment association pipework in Tallinn and Harjumaa.
A Quick Answer: Why Do Drains Smell Bad?
An unpleasant smell emanates from drains when sewer gas – primarily hydrogen sulphide, ammonia, and methane – escapes from the pipework into the room. Normally, a water trap prevents gas from entering: the water standing in the U-shaped part of the trap acts as a seal. If this water evaporates, is siphoned out, or fails to reach the trap, the seal is broken, and the smell comes directly into the room via the drain.
In practice, most cases boil down to five reasons: a dried-out or absent water trap, pipework sediment and an incipient blockage, a ventilation stack issue, a leaking or improperly installed connection, and a defect in hidden pipework. Each cause has a characteristic pattern – when the smell occurs, where it is felt, and whether it is accompanied by slow drainage or gurgling.
Immediate first aid: flush a couple of litres of water down every drain (even rarely used ones) and see if the smell disappears within 30 minutes. If it does, it was a dried-out water trap. If not, a systematic inspection is required.
- Smell appears after a long absence → likely a dried-out water trap
- Smell with slow drainage → sediment or blockage in the pipe
- Smell appears when a neighbour or another apartment uses water → ventilation stack problem
- Smell persists in one spot, drainage works well → leaking connection or defect in hidden pipework
How a Water Trap Works and Why It Drains
A water trap, or siphon, is the bent section of pipe under every sink, bath, shower, WC, and floor drain, where a small amount of water remains after use. This water barrier – typically 50–60 mm high – prevents gas from moving from the pipe into the room. On a WC, the water trap is built into the pan, which is why water is always visible in the bowl.
The water trap loses its effectiveness when the water within it disappears. The most common cause is evaporation: if a drain is not used for a couple of weeks, especially during dry heating periods, the water can evaporate completely. The same happens in guest WCs, garages, cellars, saunas, washing machine drain traps, and summer cottages.
Another mechanism is siphoning. If a large volume of water moves through the pipework simultaneously – for example, an upstairs WC flush – a negative pressure is created in the pipe, which pulls the water from a smaller trap along with it. In such cases, you often hear characteristic gurgling or slurping from the sink before the smell appears.
A third possibility is simply an incorrect trap or its absence. Sometimes, during construction or renovation, a washing machine or dishwasher drain is connected directly to the pipe without a trap, or a floor drain is replaced with a flat drain that lacks a water trap. In such cases, the smell is constant and does not disappear with any amount of water.
How to Check if a Water Trap is Empty
The check is simple. Pour about a litre of water down the drain and place your nose about 10 cm from the opening. If the smell disappears immediately and does not return within an hour, the trap was dry. If the smell returns within the same day, the water trap is either leaking, being siphoned empty, or the problem is not with that drain at all.
- Pour two litres of water into a rarely used trap and add a tablespoon of cooking oil – the oil film will slow down evaporation.
- If water disappears from the trap within a day, there's a crack in the trap or no water trap.
- If gurgling repeats every time a neighbour uses water, it's a ventilation issue, not a trap fault.
Common Causes of Unpleasant Drain Odour
Below, causes are listed in order of their frequency in properties across Tallinn and Harjumaa. This order also serves as a good checklist for inspection.
1. Dried-out or Siphoned Water Trap
The most common and easiest to resolve cause. A typical scenario: you return from holiday, and there's a noticeable drain smell in the apartment, strongest in the bathroom or kitchen. The solution is to add water to every drain, including those you don't use daily.
2. Grease and Sediment Layer on Pipe Walls
Over time, grease accumulates in kitchen drain pipes, binding food waste and detergent residues. This layer ferments, producing a sweet-rotten smell that is strongest when hot water is run. In the bathroom, the analogue is a mixture of soap, hair, and skin grease that collects in the trap and the first metre of pipe.
Characteristic sign: drainage works, but slower than before, and the smell intensifies after washing dishes or showering. This is a pre-blockage stage – it's still easy to solve before the water stops flowing completely.
3. Ventilation Stack Problem
Every drainage system requires air access. In an apartment building, the stack terminates on the roof with a vent pipe; in a private house, also on the roof or with a vacuum valve. If this opening is blocked – by leaves, ice, a bird's nest, or sealed during construction – pressure fluctuations occur in the pipework, which siphon traps dry and push gas into the rooms.
A typical sign is that the smell occurs episodically, often in the evenings or mornings when water usage is highest in the building, and is accompanied by gurgling. A faulty vacuum valve presents the same picture: the valve remains open and allows air to enter in the wrong direction.
4. Leaking or Improperly Installed Connection
Sewer pipes are connected using couplings and rubber seals. If a seal is dislodged, dried out, or the pipe is not fully inserted, a micro-leak can occur – water may not drip out, but gas can escape. Common locations are the connection between the WC pan and the floor (drying of the pan connector), the coupling behind a sink cabinet, and the insertion point of a washing machine drain hose.
The same applies to a WC pan installed with silicone but with a torn pan connector underneath: the pan doesn't leak, but there's a persistent smell behind the pan that intensifies with flushing.
5. Defect in Hidden Pipework
The most complex case is a crack, a dislodged coupling, or an incorrect pipe gradient in pipework located inside the floor, behind a wall, or underground. Here, the smell persists even though all traps are full and drainage works. Often, moisture also appears: darkened tile grout, peeling paint at the base of a wall, a musty smell behind a cabinet.
Such a problem cannot be resolved based on assumptions. Precise diagnostics are necessary, as opening up the wrong location is costly and fruitless.
6. Private House Septic Tank, Holding Tank, or Drainage System
In a private house, the source of the smell might be outdoors. An overflowing holding tank, a poorly ventilated septic tank, a blocked soakaway, or a vent pipe installed too close to the house can bring odours outdoors and through open windows into the house. Characteristically, the smell is stronger outside and in calm weather.
Diagnostics: How to Step-by-Step Locate the Source of the Smell
The aim of diagnostics is to move from simple to complex and rule out one cause at a time. Conduct the inspection within one day and note when the smell intensifies – time and location are the most valuable pieces of information.
- Step 1. Pour water into all drains: kitchen, sink, bath, shower, floor drain, guest WC, cellar or sauna drain. Wait an hour.
- Step 2. If the smell disappeared – it was due to the water trap. Mark your calendar to add water to rarely used drains once a month.
- Step 3. If the smell persists, localise it. Separately check inside the kitchen cabinet, the bathroom floor behind the WC pan, the corner behind the washing machine, and around the floor drain.
- Step 4. Check the drainage speed. Slow flow + smell indicates sediment or an incipient blockage.
- Step 5. Listen for gurgling. If the trap gurgles when someone else uses water, suspect the ventilation stack.
- Step 6. Check connections. A dry paper towel around a coupling or washing machine hose will show within half an hour if moisture is coming from there.
- Step 7. If all the above are in order, but the smell persists, it is a hidden defect – only a technical survey will help further.
When Professional Diagnostics are Needed
If the smell has persisted for over a week, returns after cleaning, or occurs in multiple apartments simultaneously, there is no point in trying more remedies. In such a situation, we use pipe camera inspection, which shows the inside of the pipe, and if necessary, smoke or pressure tests, which reveal where gas is escaping.
In an apartment building, it is crucial to distinguish between internal apartment pipework and the communal stack belonging to the apartment association, as they have different owners and different cost allocations. A camera inspection recording is also a good basis for communicating with the apartment association.
- Smell in multiple apartments or stairwell – check the communal stack, not just your own traps.
- Smell with moisture or mould – leak detection is needed, not just cleaning.
- Smell returns within a few weeks – the cause is the condition of the pipework, not the trap.
Solutions by Cause
Once the cause is identified, the solution is usually straightforward. Always start with the simplest approach and do not blindly use strong chemical agents – they will not remove the source of the gas and can damage older pipes and seals.
Dried-out Water Trap
Add water. Pour 2–3 litres of water into a rarely used drain, then a tablespoon of cooking oil, which will slow down evaporation for several weeks. If the home will be empty for a longer period, seal the drain with a plug or with film and tape.
Trap Sediment
A flexible trap can be unscrewed, emptied over a bucket, and cleaned with a brush. Before disassembling, place a bucket and a cloth underneath – there will always be water inside. When reassembling, ensure the seal sits straight and the nut is hand-tightened firmly, not overtightened with tools.
- Kitchen traps need cleaning approximately twice a year.
- Hot water + washing-up liquid dissolves grease better than caustic gel.
- Bicarbonate of soda and vinegar are suitable for light odours and thin layers, not for actual blockages.
Pipework Sediment
If the layer has extended beyond the trap, cleaning the trap will not help. The internal surface of the pipe must be cleaned mechanically or with high-pressure jetting to restore its full diameter. Only then will the source of the smell disappear, as the fermented layer is removed, not just flushed further downstream.
Ventilation Stack
Checking and cleaning the vent pipe on the roof falls under the apartment association's responsibility in apartment buildings. In a private house, the owner can check if the pipe end is clear and if the vacuum valve moves freely. Replacing the valve is an inexpensive job that solves a surprisingly large number of recurring odour problems.
Leaking Connection or WC Pan Connector
A displaced coupling is moved into the correct position, and the seal is replaced. For a WC pan, the pan must be removed, the old pan connector taken off, and a new one installed – this is an hour's work that will eliminate the persistent smell behind the pan. Temporarily sealing with silicone will not solve the problem, as gas will still find a way out.
Hidden Defect
A cracked pipe or one with an incorrect gradient is replaced. The extent of the repair is determined by the survey: sometimes, one metre of pipe under the bathroom floor is sufficient; sometimes, the renewal of the entire section or stack is necessary. Decisions are always made based on camera inspection footage, not assumptions.
What Not to Do
Some common practices make the situation worse or temporarily mask the problem, meaning the actual cause continues to develop.
- Do not repeatedly pour caustic drain cleaner – it damages seals and old cast iron pipes and is ineffective against sediment in the further parts of the pipe.
- Do not mask the smell with air fresheners or scented candles – sewer gas is a sign of a leak, not a hygiene problem.
- Do not permanently block the drain with a cloth or plug – pressure will find another route, often into a neighbouring apartment.
- Do not pour grease and food waste down the drain – this is the primary nutrient for the sediment layer.
- Do not demolish tiles or flooring before the defect's location is confirmed.
How to Prevent Unpleasant Odours
Prevention is exceptionally effective here, as almost all causes develop slowly. A small routine can prevent most incidents.
- Once a month, run water through all rarely used drains.
- Clean the kitchen trap and sink overflow opening twice a year.
- Use a grease-catching sieve in the kitchen sink and wipe pans clean with paper before washing.
- Once a month, pour a couple of litres of hot (not boiling) water with washing-up liquid down the drain.
- Before a long trip, check that traps are filled or covered.
- Have the apartment association check the vent pipe on the roof at least once a year, preferably in autumn.
- React immediately to slow-draining water – this is the first sign of a pre-blockage.
Apartment Building and Association Perspective
For an apartment association, an odour complaint is a useful early warning signal. If several apartments report the same problem, it usually concerns communal stack sediment or ventilation, where planned cleaning costs significantly less than a later emergency with flooding. A reasonable interval for apartment building general pipework maintenance is every 2–3 years, more frequently for properties with high grease usage.
Private House Perspective
In a private house, the outdoor system is an additional factor. Monitor the filling level and emptying schedule of the holding tank, keep the septic tank's vent pipe clear, and ensure that stormwater does not drain into the sewage system. In winter, it is worth checking that the vent pipe on the roof is not frozen – this is a frequent and seasonal cause in Estonia, bringing odours into the house especially during cold spells.
When to Call a Specialist
Some situations are not worth trying to resolve yourself, as the risk or cost is greater than the saving. Call a specialist if the smell is persistent and its source remains unclear, if it is accompanied by moisture or mould, if the problem affects multiple apartments, or if water starts backing up.
ToruFix OÜ operates in Tallinn and Harjumaa and handles pipework for apartments, private houses, and apartment associations. In practice, we always start with diagnostics: we determine whether the issue is a water trap, sediment, ventilation, or a pipework defect, and only then do we propose a solution along with the cost. This ensures the client doesn't pay for work that doesn't address the source of the smell.