The 6 Most Common Causes of Drain Smell in a Flat
Drain and sewage smell in a flat almost always has a specific source. Identifying which one is the key to finding the right fix:
- 1. Dry water trap (P-trap or U-bend): The most common cause, and the easiest to fix. Every drain has a U-shaped section that holds a small amount of water to create a gas seal. If a drain is not used for a few weeks, this water evaporates and the seal is broken, allowing sewer gas to enter the flat.
- 2. Blocked drain: A partial blockage accumulates organic matter that decomposes and produces hydrogen sulphide and other sewage gases. The smell is typically worse near the blocked drain and when water is run.
- 3. Broken, cracked or missing P-trap: Physical damage to the P-trap under a sink, bath or shower tray, or a missing trap on a floor drain. Common in older properties that have had DIY plumbing work.
- 4. Failed toilet seal (wax ring or pan connector): The seal between the toilet pan and the soil pipe. If this fails, sewer gas leaks around the base of the toilet. Often accompanied by a slight movement of the toilet pan.
- 5. Shared drainage issues: In a purpose-built flat, you share drainage stacks with neighbours. Blockages or issues in the communal stack can push sewer gas back into individual flats, even when your own drains are functioning normally.
- 6. Negative air pressure (air admittance valve failure): When water drains rapidly, it can create a partial vacuum that sucks the water out of P-traps in nearby drains. Air admittance valves (AAVs) prevent this by admitting air from the outside. A failed AAV can allow sewer gases back in.
Dry Trap: The Most Common Cause
Dry traps are responsible for the majority of drain smell complaints in flats. They are particularly common in:
- Rarely used bathrooms or en-suites
- Guest bathrooms that go weeks without use
- Floor drains in utility rooms or bathrooms
- Properties that have been vacant
Fix: Run water in every drain in the flat for 30 seconds — sink, bath, shower, floor drains, even the overflow. This refills the traps and restores the gas seal. If the smell disappears after doing this and returns after a few weeks, you have a dry trap problem. The solution is either to use those drains regularly, or to pour a small amount of cooking oil down the rarely-used drain after filling it with water — the oil sits on top of the water and significantly slows evaporation.
Blocked Drains
A partially blocked drain accumulates decomposing organic matter that produces sewer gas. Signs of a blockage:
- Slow draining water
- Gurgling sounds when water drains
- Smell that is worst immediately after water is drained
- Multiple drains affected at once (suggests communal stack blockage)
Fix: Clear the blockage. For sink and shower drains, a drain snake or plunger is often effective for accessible blockages. Enzyme drain cleaners (not caustic soda) applied regularly help prevent organic accumulation. For stubborn or communal blockages, a plumber with drain jetting equipment is needed. In a flat, communal drainage blockages are the landlord’s or managing agent’s responsibility to resolve.
Broken or Missing Seals
Broken P-traps and failed toilet seals require physical repair by a plumber. Signs of a failed toilet seal: a slight movement or rocking of the toilet pan; visible gap between the toilet base and the floor; smell concentrated near the toilet that does not improve when other drains are run. A plumber can replace the wax ring seal or pan connector in a few hours.
Shared Drainage in Purpose-Built Flats
In a purpose-built flat, all drains connect to a communal soil stack. A blockage anywhere in the stack affects all flats connected to it. Symptoms of a communal stack problem:
- Multiple flats in the building reporting drain smell at the same time
- Gurgling in your toilet or drains when a neighbour runs water
- Smell that appears in your flat despite all your drains working normally
Communal drainage issues are the responsibility of the freeholder or managing agent. Report to them in writing and request a drain survey if the problem persists.
Ventilation and HVAC
In some flats, bathroom extractor fans connect to a communal ventilation duct. If the ducting is damaged or there is a blockage, odour from one flat can enter another through the shared system. Also, if the extractor fan is not functioning correctly, it can create negative pressure that draws sewer gas from the drains.
How to Diagnose the Source
Run all drains for 30 seconds
Every sink, bath, shower and floor drain. If the smell disappears → dry trap. If it does not improve → something else.
Check for slow drainage or gurgling
Slow draining or gurgling sounds point to a blockage. Note which specific drain is affected.
Smell test by location
Follow your nose. If the smell is strongest in a specific room or near a specific drain, the source is likely there. If it seems to come from the whole flat equally, a communal stack issue or HVAC problem is more likely.
Check the toilet base
Flush the toilet and watch for any movement of the pan. Sniff around the base. A failed toilet seal typically produces smell near the toilet that is not dependent on running other drains.
Ask neighbours
If neighbours in adjacent flats are also experiencing the smell, the issue is in the communal drainage system — report to the managing agent.
DIY Fixes
- Dry trap: Run water in all drains regularly. Pour cooking oil over infrequently used drains after running water to slow evaporation.
- Minor blockage: Drain snake, plunger or enzyme drain cleaner for accessible sink and shower blockages.
- Limescale or grease buildup in kitchen drain: Enzyme drain cleaner applied weekly helps break down organic accumulation before it becomes a blockage.
When to Call a Plumber
- The smell persists after running all drains (dry trap fix has not worked)
- Drains are slow or gurgling (indicates a blockage beyond the accessible section)
- The toilet pan rocks or has a gap at the base
- Multiple drains are affected simultaneously
A plumber or drainage specialist can perform a CCTV drain survey to identify blockages, collapsed pipes or structural defects in the drainage system. This is particularly useful in older London properties where drainage systems have not been updated for decades.
When Odour Treatment Is Needed
Once the drainage problem is fixed, the sewer gas smell should dissipate within a few days as ventilation clears the affected area. However, in some cases — particularly where a drain has been leaking for a long time, or where there has been a sewage overflow or flood — the organic compounds from sewage contaminate the surfaces of the flat beyond what simply airing out will address.
In these situations, professional fogging and ozone treatment neutralises the residual MVOC and hydrogen sulphide compounds that have absorbed into walls, flooring and soft furnishings. We always recommend fixing the drainage problem first and confirming it is resolved before treating the odour.