The 6 Most Common Causes
Drain smell in a house almost always has a specific, identifiable source. The smell of drains is primarily hydrogen sulphide (the rotten egg smell) and various other sewer gases produced by bacterial breakdown of organic matter in the drainage system. Here are the most common reasons it enters the living space:
1. Dry Water Trap (P-trap)
This is by far the most common cause. Every drain in your house — sinks, baths, showers, floor drains — has a U-shaped section of pipe (P-trap or U-bend) that holds a small amount of water. This water creates a seal that prevents sewer gas from entering the house.
If a drain is not used for 2–4 weeks (in warm, dry conditions even faster), this water evaporates and the seal breaks. Sewer gas passes straight through the empty trap into the room.
How to identify: The smell is associated with a specific drain in a room you do not use frequently. Running water in that drain temporarily resolves the smell.
Fix: Run water in every drain in the house for 30 seconds. For drains you rarely use, pour a small amount of cooking oil down them after filling — the oil sits on top of the water and slows evaporation significantly.
2. Drain Blockage
A partial blockage accumulates decomposing organic matter — food waste, grease, hair, soap scum — that produces hydrogen sulphide and other gases as it breaks down. Unlike a full blockage which causes obvious drainage problems, a partial blockage may allow water to drain while still producing significant smell.
How to identify: The smell is worst near a specific drain, the drain is slow, or there is a gurgling sound when water drains.
Fix: Clear the blockage with a drain snake or enzyme drain cleaner. Avoid caustic soda in older properties with clay or cast iron pipes. For deep or communal blockages, a plumber with drain jetting equipment.
3. Failed Toilet Seal (Wax Ring or Pan Connector)
The seal between the toilet pan and the soil pipe prevents sewer gas from escaping around the base of the toilet. Over time, particularly in older properties, this seal can fail — either because the wax ring has deteriorated or because the pan connector has cracked.
How to identify: Drain smell is concentrated near the toilet. The toilet pan may rock slightly or there may be a visible gap at the base. The smell does not improve when other drains are run.
Fix: A plumber can replace the wax ring or pan connector in a few hours. This is not a DIY job for most people as the toilet needs to be unbolted from the floor.
4. Cracked Soil Pipe or Vent Stack
The soil pipe carries waste from toilets and drains to the sewer. The vent stack allows sewer gas to escape from the top of the soil pipe into the atmosphere rather than into the house. If either is cracked or damaged, sewer gas can escape into wall cavities and enter the living space.
How to identify: Drain smell that appears to come from walls or ceilings rather than from any specific drain. The smell may be worst in one area of the house regardless of which drains are used. A CCTV drain survey can confirm a cracked soil pipe.
Fix: Repair or replacement of the affected pipe section by a plumber. This may involve opening walls or floors to access the pipe.
5. Tree Root Intrusion into Drains
In older properties with clay drainage pipes, tree roots can penetrate pipe joints and grow into the drainage system. This causes a partial blockage, allows soil gas to enter the drain, and can cause sewage to back up into the property over time.
How to identify: Slow drainage throughout the property rather than in one specific drain. Gurgling in multiple drains simultaneously. Garden trees or shrubs near the drainage run. A CCTV drain survey will confirm root intrusion.
Fix: Root cutting followed by pipe relining or replacement. This requires a drainage specialist.
6. HVAC, Extractor Fans and Pressure Issues
Modern homes with well-sealed construction can develop negative air pressure situations where the extraction of air by extractor fans, tumble dryers or HVAC systems creates a slight vacuum. This vacuum can pull water out of P-traps and draw sewer gas back through the drainage system.
Also: if bathroom or kitchen extractor fans connect to a communal ventilation duct, odour from one property can enter another through the shared duct system.
Fix: Ensure air admittance valves (AAVs) on the drainage system are functioning. Have extractor fans checked for correct operation. In communal buildings, report suspected ventilation issues to the managing agent.
How to Diagnose Your Situation
Run all drains for 30 seconds
Every sink, bath, shower and floor drain. If the smell disappears → dry trap. If it persists → continue diagnosis.
Localise the smell
Walk room by room. Is the smell strongest near a particular drain? Near the toilet? In a specific area of wall or floor? Localisation points to the source.
Check for slow or gurgling drains
Slow drainage or gurgling when neighbours use water indicates a blockage or communal drainage issue.
Inspect the toilet base
Any movement of the pan when sat on, or visible gap at the base, suggests a failed seal.
If none of the above is obvious
A CCTV drain survey by a drainage specialist will identify any structural issues including cracked pipes and root intrusion. This typically costs £80–200 and is money well spent for a persistent problem.
Quick Reference: Fix for Each Cause
- Dry trap: Run all drains regularly; cooking oil over infrequently used drains
- Blockage: Drain snake, enzyme cleaner, or plumber with jetting
- Failed toilet seal: Plumber to replace wax ring or pan connector
- Cracked soil pipe: CCTV survey then pipe repair or relining
- Root intrusion: Drainage specialist for root cutting and pipe relining
- Pressure / AAV: Check and replace air admittance valves; assess extractor fan setup