Sewer Smell in the House: Causes and How to Fix Each One

Sewer gas in a house is more than unpleasant — hydrogen sulphide (the primary culprit) is toxic at high concentrations and can cause headaches, nausea and, in extreme cases, loss of consciousness. The good news is that most causes of indoor sewage smell are straightforward to diagnose and fix. This guide covers all six common causes, how to identify which one you have, and what to do about it.

Understanding where the smell comes from

Sewer gas is produced by the anaerobic decomposition of organic waste in the drainage system. Under normal conditions, the design of the plumbing prevents it from entering the building in three ways:

1. Water traps (U-bends) under every drain outlet maintain a water seal that blocks gas
2. Soil vent pipes allow gas to escape upward through the roof rather than backward into the building
3. Inspection chamber covers seal the underground sewer from above-ground air

When any of these three fail, sewer gas finds its way into living spaces.

Cause 1: Dry trap (most common)

Every sink, bath, shower, floor drain, and dishwasher outlet has a U-bend trap. The water in this bend forms the gas seal. If a drain isn’t used for several weeks, the water evaporates and the seal is lost.

Where to look: Rarely-used sinks (guest bathrooms, utility room sinks), floor drains in garages or plant rooms, the dishwasher outlet if it hasn’t been run recently.

How to fix it: Run the tap or pour a litre of water down the drain. The smell should clear within a few hours once the trap is refilled. Pour a little cooking oil on top of the water — it floats and dramatically slows evaporation.

For floor drains that are genuinely never used, fit a dry-trap cover (a mechanical valve that opens under water flow and closes when dry) to avoid the problem recurring.

Cause 2: Cracked or poorly-fitted trap

A trap that has cracked, has a poor joint, or has been installed without a proper seal will leak gas continuously even when full of water. This often goes undetected for years.

Where to look: Under kitchen sinks (cheap plastic traps degrade, particularly if drain cleaning chemicals have been used repeatedly), under baths (accessible via the bath panel), and in cupboards where you can smell something when you open the door.

How to fix it: Inspect the trap visually. If the trap itself is cracked or the joints are loose, replace it — trap replacements are inexpensive (£5–£20 for standard plastic fittings) and are push-fit on most modern UK plumbing.

Cause 3: Venting problem (soil stack or vent pipe)

Every soil pipe (the large-diameter pipe that carries toilet waste to the underground drain) must be vented to atmosphere — typically terminating above roof level. This prevents siphoning of water traps when a large flush creates negative pressure in the system.

If the soil vent pipe is blocked (bird nests, leaves, frost-heaved cap, or an incorrectly fitted AAV — air admittance valve), negative pressure when flushing will siphon the water out of nearby traps, allowing gas in.

Signs of a venting problem: Smell appears or gets worse immediately after flushing. Gurgling sounds from sink traps when the toilet is flushed. Multiple traps losing their seals when only one drain is used.

How to fix it: Check that the soil vent pipe is clear and unobstructed at roof level. Check that any AAV (air admittance valve, a device that allows air in but not out) is operating correctly — these fail with age and should be replaced rather than serviced. A drainage engineer can test the system with a smoke test to identify exactly where venting is failing.

Cause 4: Blocked or partially blocked drain

A partial drain blockage — not enough to cause complete backing up, but enough to allow decomposing waste to accumulate — generates significant gas that can find its way back into the building through any available unsealed path.

Signs: Smell is strongest in the kitchen or bathroom where the partial blockage is located. Drains running slowly before the smell developed.

How to fix it: Clear the blockage. If the smell persists after clearing, have the drain jetted clean (rather than just rodded) to remove the residual organic deposits that continue to off-gas.

Cause 5: Inspection chamber or manhole cover

If an inspection chamber is near the property and its cover is cracked, broken, or unsealed, gas escapes at ground level and enters through air gaps in the floor (particularly floor voids in older buildings). This can be mistaken for a plumbing problem when the source is actually in the garden or under a path.

How to identify it: The smell is strongest at the ground level rather than at drain outlets. It may worsen in warm weather when bacterial activity increases gas production.

How to fix it: Inspect all manhole covers near the property. Cracked or broken covers should be replaced (available at builders’ merchants). Covers that aren’t sealed can be made temporarily gas-tight with a bead of mastic; a proper fitting cover is the long-term solution.

Cause 6: Drain or sewer structural failure

A cracked underground drain allows sewer gas to escape into the soil and then migrate into buildings through floor voids, cracks in the foundation, or gaps around services. This is the most serious cause and the hardest to diagnose without specialist equipment.

Signs: Smell persists despite fixing all visible traps and venting. Strong smell in rooms over the suspended floor void. Smell increases in wet weather (rain saturates the soil, concentrating the gas).

How to diagnose: A CCTV drain survey and a smoke test. Smoke is introduced into the drainage system at a sealed access point and pressurised. Any structural breach allows smoke to emerge from the ground surface or through building fabric — locating it precisely without excavation.

How to fix it: Depends on the defect found. Small cracks can often be sealed with drain lining (CIPP). Displaced joints may be repaired by re-lining or excavation depending on severity.

When to call a professional immediately

Call a drainage engineer (not just a plumber) urgently if:

• The smell is constant and you can’t find the source
• Multiple drains are affected simultaneously
• You can smell sewage in rooms that aren’t near any drainage
• You or family members are experiencing headaches or nausea
• There’s visible sewage or wet patches appearing from ground level