Plumbing systems are designed to remove wastewater while preventing air from the pipes entering the room. If you notice a sewer smell at home, it means this barrier has been compromised somewhere. There are three primary barriers: the water seal (trap), sealed connections, and the ventilation stack. An unpleasant odour is always a sign that at least one of these isn't working correctly.
The good news is that about half of these issues can be resolved by the homeowner in an evening – a dry water trap, a dirty overflow opening, or a disconnected washing machine hose are common and simple causes. The remaining cases require diagnostics, as the odour originates from a hidden source: pipe sediment, a crack under the floor, or the communal stack of an apartment association.
In this guide, we will examine all common causes, provide a logical inspection sequence (what to check first, then next), describe solutions, and conclude with prevention. The descriptions are based on ToruFix OÜ's daily work in flats, private houses, and apartment association plumbing systems in Tallinn and Harjumaa.
Quick Answer: Why Do Drains Smell?
An unpleasant odour from the drains occurs when sewer gas – primarily hydrogen sulphide, ammonia, and methane – escapes from the pipework into the room. Normally, a water seal (trap) prevents this gas: the standing water in the U-bend of the trap acts as a stopper. If this water evaporates, is siphoned out, or fails to reach the trap, the seal breaks, and the smell enters the room directly through the drain.
In practice, most cases boil down to five reasons: a dry or missing water trap, pipe sediment and an incipient blockage, a ventilation stack problem, a leaking or improperly installed connection, and a defect in hidden pipework. Each cause has a characteristic pattern – when the odour appears, where it is felt, and whether it is accompanied by slow drainage or gurgling.
The quickest first aid: run a couple of litres of water down every drain (including rarely used ones) and see if the smell dissipates within 30 minutes. If it does, it was a dry water trap. If not, a systematic inspection is required.
- Odour appears after a long absence → likely a dry water trap
- Odour with slow drainage → sediment or blockage in the pipe
- Odour appears when a neighbour or another flat uses water → ventilation stack problem
- Odour persists constantly in one spot, drainage works well → leaking connection or defect in hidden pipework
How Water Traps Work and Why They Empty
A water trap, or siphon, is the bent section of pipe under every sink, bath, shower, toilet, and floor drain, where a small amount of water remains after use. This layer of water – typically 50–60 mm high – prevents gas from moving from the pipe into the room. Toilets have an integrated trap, which is why water is always visible in the bowl.
A water trap loses its effectiveness when the water within it disappears. The most common reason is evaporation: if a drain isn't used for a couple of weeks, the water can completely evaporate, especially during a dry heating season. The same happens in guest WCs, garages, basements, saunas, washing machine drains, and summer cottages.
Another mechanism is siphoning. If a large volume of water moves through the pipework at once – for example, a toilet flush from an upper floor – a negative pressure is created in the pipe, which can suck the water from smaller traps. In such cases, you often hear a characteristic gurgling or slurping sound from the sink before the odour appears.
A third possibility is simply an incorrect trap or its absence. Sometimes, after 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 lacking a water seal. In such instances, the odour is constant and doesn't 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 into the drain and place your nose approximately 10 cm from the opening. If the odour disappears immediately and doesn't return within an hour, the trap was dry. If the odour returns within the same day, the water trap either leaks, is being siphoned empty, or the problem isn't related to that specific drain at all.
- For rarely used drains, pour two litres of water and add a tablespoon of cooking oil – the oil film slows down evaporation.
- If water disappears from the drain within a day, the drain has a crack or lacks a water seal.
- If gurgling recurs every time a neighbour uses water, it's a ventilation issue, not a trap fault.
Common Causes of Unpleasant Drain Odours
Below are the causes listed in order of their frequency observed in properties in Tallinn and Harjumaa. This sequence also serves as a good checklist.
1. Dry or Siphoned Water Trap
The most common and easiest problem to solve. Typical scenario: you return from holiday, and there's a noticeable sewer smell in the flat, 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 with food waste and detergent residues. This layer ferments and emits a sweet, putrid odour, strongest when hot water is run. In bathrooms, an analogous mix of soap, hair, and skin grease collects in the trap and the first metre of pipe.
Characteristic sign: drainage works but slower than before, and the odour intensifies after washing dishes or showering. This is a pre-blockage stage – still easy to resolve before the water completely stops draining.
3. Ventilation Stack Problem
Every plumbing 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, siphoning traps empty and forcing gas into rooms.
A typical sign is that the odour appears episodically, often in the evenings or mornings when water usage is highest in the building, and it's accompanied by gurgling. A faulty vacuum valve presents the same picture: the valve remains open, allowing air in the wrong direction.
4. Leaking or Improperly Installed Connection
Drainage pipes are connected with couplings and rubber seals. If a seal is displaced, dried out, or the pipe isn't fully inserted, a micro-leak can occur – water might not drip out, but gas can escape. Common locations are the toilet-to-floor connection (drying of the pan connector), the coupling behind a vanity unit, and the washing machine hose insertion point.
The same applies to a toilet installed with silicone, but where the pan connector underneath is torn: the toilet doesn't leak, but there's a persistent smell behind it that strengthens with flushing.
5. Defect in Hidden Pipework
The most complex case is a crack, a displaced coupling, or incorrect pipe gradient within the floor, behind a wall, or underground. Here, the odour persists even if all traps are full and drainage works. Often, dampness accompanies it: dark tile grout, peeling paint at the base of a wall, a musty smell behind a cabinet.
Such a problem cannot be solved based on assumptions. Precise diagnostics are required, as opening up the wrong location is expensive and yields no results.
6. Private House Septic Tank, Holding Tank, or Drainage
In a private house, the source of the odour could be outside. An overflowing holding tank, a poorly ventilated septic tank, a clogged soakaway, or a vent pipe installed too close can bring the smell outdoors and into the house through open windows. Characteristically, the odour is stronger outside and on calm days.
Diagnostics: Step-by-Step to Locate the Odour Source
The purpose of diagnostics is to progress from simple to complex, eliminating one cause at a time. Conduct the inspection over a single day and note when the odour intensifies – time and location are the most valuable information.
- Step 1. Pour water into all drains: kitchen, sink, bath, shower, floor drain, guest WC, basement or sauna drain. Wait an hour.
- Step 2. If the odour disappeared – it was a water trap issue. Mark your calendar to add water to rarely used drains once a month.
- Step 3. If the odour persists, localize it. Separately check inside the kitchen cabinet, the bathroom floor behind the toilet, the corner behind the washing machine, and around the floor drain.
- Step 4. Check drainage speed. Slow flow + odour 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 escaping.
- Step 7. If everything above is in order, but the odour persists, it's a hidden defect – only a technical survey will help further.
When Professional Diagnostics Are Needed
If the odour has persisted for over a week, returns after cleaning, or appears in several flats simultaneously, it's no longer wise to experiment with remedies. In such a situation, we use a pipe video inspection, which shows the inside of the pipe with a camera, and if necessary, a smoke or pressure test to pinpoint where the gas is escaping.
In an apartment building, it's important to distinguish between internal flat pipework and the communal stack of the apartment association, as they have different owners and cost allocations. A video inspection recording also serves as good evidence when communicating with the association.
- Odour in several flats or stairwells – check the communal stack, not just your own traps.
- Odour with dampness or mould – leak detection is needed, not just cleaning.
- Odour 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 avoid blindly using harsh chemical agents – they won't remove the source of the gas and can damage older pipes and seals.
Dry Water Trap
Add water. For rarely used drains, pour 2–3 litres of water, then a tablespoon of cooking oil, which will slow evaporation for several weeks. If the home will be empty for an extended period, close the drain with a plug, or cover it 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 securely, not overtightened with a tool.
- Kitchen traps generally need cleaning twice a year.
- Hot water + dish soap dissolves grease better than corrosive gel.
- Baking soda and vinegar are suitable for mild odours and thin layers, not actual blockages.
Pipe Sediment
If the layer has extended beyond the trap, cleaning the trap won't help. The internal pipe surface must be cleaned mechanically or with high-pressure jetting to restore full bore. Only then will the source of the odour disappear, as the fermenting layer is removed, not just flushed further downstream.
Ventilation Stack
Inspection and cleaning of the vent pipe on the roof in an apartment building fall under the responsibility of the apartment association. 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 surprisingly solves many recurring odour problems.
Leaking Connection or Toilet Pan Connector
A displaced coupling is pushed into the correct position, and the seal is replaced. For a toilet, the pan needs to be removed, the old pan connector taken off, and a new one installed – this is an hour's work that ends the persistent smell behind the toilet. Temporarily covering with silicone won't solve the problem, as the gas will still find a way out.
Hidden Defect
A cracked pipe or one with an incorrect gradient is replaced. The extent is determined by investigation: sometimes, one metre of pipe under the bathroom floor is sufficient; other times, renewal of the entire section or stack is necessary. Decisions are always based on camera footage, not assumptions.
What Not to Do
Some common practices worsen the situation or temporarily mask the problem, meaning the actual cause continues to develop.
- Do not repeatedly pour corrosive drain cleaner – it damages seals and old cast-iron pipes and is ineffective against sediment further down the pipe.
- Do not mask the odour with air fresheners or scented candles – sewer gas indicates a leak, not a hygiene problem.
- Do not stuff a rag or plug into the drain as a permanent solution – pressure will find another way, often into a neighbouring flat.
- Do not pour grease and food waste down the drain – this is the main nutrient for the sediment layer.
- Do not demolish tiles or flooring before the defect location has been 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 strainer 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 dish soap down the drain.
- Before a long trip, ensure traps are filled or covered.
- Have the apartment association inspect the roof vent pipe at least once a year, preferably in autumn.
- React immediately to slow-draining water – this is the first sign of an incipient blockage.
Apartment Building and Association Perspective
For an apartment association, an odour complaint is a valuable early warning signal. If several flats report the same problem, it's usually due to sediment in the communal stack or ventilation issues, for which scheduled cleaning costs significantly less than a later emergency with flooding. A reasonable interval for apartment building main pipework maintenance is every 2–3 years, more frequently for properties with high grease discharge.
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 vent pipe clear, and ensure stormwater does not flow into the sewerage system. In winter, it's worth checking that the roof vent pipe isn't frozen – this is a common and seasonal cause in Estonia, bringing odours into the house during cold spells.
When to Call a Professional
Some situations are not worth trying to solve yourself, as the risk or cost outweighs the savings. Call a specialist if the odour is persistent and the source remains unclear, if it is accompanied by dampness or mould, if the problem affects multiple flats, or if water starts backing up.
ToruFix OÜ operates in Tallinn and Harjumaa, dealing with plumbing systems in flats, private houses, and apartment associations. In practice, we always start with diagnostics: we determine whether it's a trap, sediment, ventilation, or pipe defect, and only then do we propose a solution along with the cost. This way, the client doesn't pay for work that doesn't address the source of the odour.