Why Does My Drain Smell? Causes and Fixes for Every Room

Drain Smells Are Diagnostic Clues

A bad smell from a drain isn't just unpleasant — it's a signal. Different smells coming from different locations in the house point to different causes, and treating the wrong one wastes time and money. This guide takes you through the most common drain odour scenarios room by room, explains what's causing each one, and tells you what to do about it.

Kitchen Drain: Grease, Bacteria, and Food Build-Up

The most common source of kitchen drain odour is an accumulation of fat, grease, and food particles on the inside walls of the drain pipe and in the P-trap beneath the sink. Grease poured down the drain cools and solidifies on the pipe wall. Food debris caught in the trap decomposes. The result is a persistent sulphurous or rotting smell that's worst after the sink has been used.

What to try yourself: Pour a kettle of boiling water slowly down the drain, followed by a cup of bicarbonate of soda, then a cup of white vinegar. Leave for 30 minutes, then flush with more hot water. For persistent build-up, a proprietary enzyme-based drain cleaner (not caustic) applied overnight is more effective than chemical drain unblockers, which can damage older pipes and rarely penetrate grease fully.

If the smell persists despite cleaning, the trap may need removing and cleaning manually, or the issue may be further down the waste run — a build-up inside the wall pipe or at a poorly-graded section where grease collects. Our drainage team can clear blocked kitchen waste runs and inspect trap condition on a call-out.

Bathroom Drain: Hair, Soap, and Dry Traps

Bathroom basin and shower waste drains accumulate hair and soap residue over time, creating a blockage that harbours bacteria. The smell is distinctly organic — a damp, musty odour rather than the sulphurous smell of sewer gas.

What to try: Remove and clean the waste strainer. Pull out any hair build-up using a drain snake or a flexible cleaning brush. Clean the trap accessible under the basin. Pour an enzyme drain cleaner down the waste and leave overnight.

A different problem altogether is a dry trap. Every basin, bath, shower, and floor drain has a water-filled P-trap that acts as a barrier preventing sewer gases from entering the room. A drain that's rarely used — a guest bathroom basin, a shower in a spare room — can have its trap water evaporate, removing the barrier. The result is a strong sewer smell even with no blockage. The fix is simple: run the tap or pour a litre of water down the drain to refill the trap. Add a tablespoon of cooking oil afterwards to slow evaporation.

Sewer Smell Throughout the House

A sewer smell that appears throughout the property — or is strongest at floor level — often indicates a more serious drainage issue rather than a simple build-up:

• Cracked or displaced drain pipe — a crack in the drain below floor level allows sewer gas to enter the building fabric and permeate upwards. Requires a CCTV drain survey to confirm and locate.
• Blocked or inadequately sized vent pipe — the soil stack vent on the roof allows air into the drain system to prevent siphoning of traps. If it's blocked (birds' nest, debris) or absent, traps throughout the house are siphoned empty and sewer gas enters. A plumber can check and clear vent pipe blockages.
• Failed toilet pan seal — the flexible seal between the toilet pan and the soil pipe can harden and crack over years, allowing gas to escape at floor level. Usually detectable by the smell being strongest in the bathroom. Seal replacement is a simple job.

If a sewer smell is persistent and affecting multiple rooms, arrange a drainage inspection rather than continuing to clean surface drains. The source is likely underground or structural.

Hot Water or Boiler Smell

A sulphurous, rotten-egg smell from hot water taps specifically — but not cold taps — usually indicates bacterial activity in a hot water cylinder. Legionella and sulphate-reducing bacteria can produce hydrogen sulphide gas in cylinders set at too low a temperature, particularly after a period of disuse. The fix is setting the cylinder to maintain 60°C to pasteurise the water and eliminate the bacteria. If the smell persists after temperature correction, the cylinder may need chemical treatment and flushing. An engineer job — not a drainage issue.

When to Call a Plumber

Call our drainage team or plumbing repairs service if:

• DIY cleaning hasn't resolved the smell after 2–3 attempts
• The smell is strongest at floor level or throughout the property (possible drain crack or vent issue)
• A gurgling sound accompanies the smell (possible vent or blockage issue)
• The smell returns quickly after cleaning (possible blockage further down the run)

Book online or call 02039514510.

Frequently Asked Questions

Is a drain smell dangerous?

Sewer gas contains hydrogen sulphide and methane. At low concentrations — the typical levels in a domestic property with a drain odour — it's unpleasant but not acutely dangerous. At high concentrations in a confined space it can be a health risk. A persistent sewer smell in an enclosed room (particularly a basement or ground floor with concrete slab) should be investigated and resolved promptly rather than managed with air freshener.

Can drain smells cause health problems?

Prolonged exposure to low-level sewer gas can cause headaches, nausea, and dizziness. Mould associated with damp drain areas can worsen respiratory conditions. Resolving the source of the smell is the correct approach — masking it with fragrance does nothing about the underlying cause or any associated damp.

Why does my drain smell only sometimes?

Intermittent drain smells are often linked to specific conditions: after hot weather (traps evaporate faster), after heavy rainfall (surcharging of the drain system can force gas backwards), or when the heating is on (warmth accelerates bacterial activity in partially blocked drains). An intermittent smell from the same location that persists over time warrants investigation.

Will bleach fix a smelly drain?

Bleach kills surface bacteria but doesn't dissolve the grease and hair build-up that provides the food source for bacteria. It's a short-term fix at best. Enzyme-based cleaners are more effective because they biologically break down organic matter rather than just surface-sterilising it. For persistent odours, mechanical cleaning of the trap and waste run is more effective than any chemical treatment.