Drain Biofilm: The Most Common Hidden Cause
The inside of bathroom waste pipes — beneath the plughole and extending down into the drain — accumulates a sticky biofilm of bacteria, soap scum, hair and skin cells. This biofilm produces hydrogen sulphide and other gases that smell distinctly unpleasant. Crucially, it sits inside the pipe where cleaning products poured down the drain do not effectively reach it.
Signs of drain biofilm: a musty or sulphur smell that is strongest near the plughole; the smell worsens after hot water is run (which disturbs the biofilm and releases gases); the plughole area has a slimy feel around the edges.
The biofilm forms on the pipe walls, particularly in the curved U-bend section and the first section of horizontal pipe. Standard drain cleaners have limited contact with it because the liquid drains past rather than dwelling on the pipe walls.
The effective approach: remove the plughole cover and use a drain snake or flexible brush to physically scrub the inside of the pipe, then apply an enzyme drain cleaner and allow it to dwell for several hours. Enzyme drain cleaners break down the organic biofilm; bleach kills the bacteria on the surface but does not dissolve the biofilm itself.
Dry or Failed P-Trap
Every drain has a U-shaped section (P-trap) that holds water to create a gas seal. If a bathroom drain is not used for 2–3 weeks (a guest bathroom, for example) or if the trap has physically failed, this water seal is lost and sewer gas enters the bathroom directly through the drain.
Signs of a dry or failed trap: a sewage-like or sulphur smell; the smell is constant rather than linked to water use; running water in the drain temporarily reduces the smell (refilling the trap) but it returns within days.
Fix: run water in all drains for 30 seconds each. For infrequently used drains, pour a small amount of cooking oil down after filling the trap — it floats on the water and significantly slows evaporation. For a physically cracked or failed P-trap, a plumber needs to replace it.
Toilet Seal and Wax Ring
The seal between the toilet pan and the soil pipe prevents sewer gas from escaping around the base. Over time, particularly where the toilet has been disturbed or the floor has moved, this seal can fail. Sewer gas then enters the bathroom at floor level around the toilet base.
Signs: a persistent sewage smell concentrated near the toilet; the toilet pan rocks slightly or has a visible gap at the base on one side; the smell does not improve when you run other drains and does not improve with cleaning.
Fix: a plumber replaces the wax ring or pan connector. This typically takes 2–3 hours. It is not a DIY job for most people as the toilet needs to be unbolted from the floor.
Mould Behind Tiles and the Bath Panel
Failed grout and silicone sealant allow water to penetrate behind tiles and around the bath surround. The cavity behind tiles and beneath the bath tray is dark, unventilated and persistently damp — ideal for mould growth. The mould produces MVOCs that pass through grout lines and sealant joints into the bathroom.
Signs: a musty smell rather than a sewage smell; the smell is consistent and does not vary much with water use; grout lines appear dark or discoloured; silicone sealant has mould growth visible on or in it; the bath panel feels slightly damp when pressed.
Fix: remove and replace failed silicone sealant throughout the bath surround. For tiles where water has penetrated, re-grout affected areas after allowing the wall to dry thoroughly. Remove the bath panel and inspect the area beneath — if mould is present, treat with biocide, allow to dry, and improve ventilation to the space before replacing the panel.
Blocked or Failed Extractor Fan
A bathroom extractor fan that is not working correctly — or not working at all — allows humidity to build up, creating the conditions for mould and persistent damp smell. If the fan duct is blocked (common in older properties), warm moist air cannot escape and may even be drawn back into the bathroom in some wind conditions.
Signs: condensation on cold surfaces even with the fan running; the fan runs but you cannot feel air movement at the grille; the bathroom smells musty consistently; mould appears on ceiling corners and near windows.
Fix: clean the fan grille and blades (can be done with the fan cover removed). Check the duct is unobstructed — a mirror held at the external vent should show air movement when the fan runs. If the fan has failed, replace it. Ensure the fan runs for at least 15 minutes after bathing or showering, not just during.
Grout and Silicone Sealant
Old or poorly maintained grout and silicone sealant absorbs moisture and becomes a substrate for mould. Even when the mould is not yet visible, the grout is producing MVOCs that contribute to bathroom smell. Cleaning the surface of discoloured grout does not remove the mould from inside the grout material.
The most durable fix for significantly deteriorated grout: regrout rather than clean. Fresh grout is impermeable to water and does not support mould. For silicone, replace it entirely rather than applying new silicone over old — the new layer will peel away within months and the old mould underneath will continue to grow.
How to Identify Your Cause
Identify the smell type
Sewage or sulphur smell → drain, P-trap or toilet seal. Musty or earthy smell → mould (behind tiles, bath panel, grout). A combination of both is common in bathrooms with multiple issues.
Does the smell change with water use?
Smell worse immediately after running water → drain biofilm. Smell improves after running water then returns → dry trap. Smell constant regardless of water use → toilet seal, mould or extractor issue.
Check the toilet base
Flush and watch for any movement of the pan. Smell around the base. Any rock or gap → failing toilet seal.
Remove the bath panel
Inspect the area under the bath with a torch. Any visible mould, damp or standing water indicates a problem in this area.
Check extractor fan operation
Hold a piece of tissue at the grille when the fan is running. It should be held in place by suction. If not, the fan is not extracting effectively.
Quick Reference: Fix for Each Cause
- Drain biofilm: Physical scrubbing of pipe interior + enzyme drain cleaner with long dwell time
- Dry P-trap: Run all drains regularly; cooking oil for infrequently used drains
- Failed P-trap: Plumber to replace
- Toilet seal: Plumber to replace wax ring or pan connector
- Mould behind tiles: Replace failed grout and silicone; treat mould with biocide; improve ventilation
- Bath panel mould: Treat mould, improve ventilation behind panel, allow to dry
- Extractor fan: Clean or replace fan; check duct is clear