Why does my bathroom smell of sewage?

The dry trap that causes most of it, the other four causes in order of likelihood, and what needs a professional.

Difficulty: beginner
Time: 45 min
Read: 4 min
Safety: warning

Short answer

Almost always a dried-out trap — the water seal in a waste pipe that stops drain gas coming back. Run every tap, the shower and flush the toilet, and pour water down any unused drain or floor gully. If it persists, the causes are a failed seal, a blocked vent or a damaged pipe.

Every waste fitting in a house has a U-bend holding a small amount of water, and that water is the only thing between you and the sewer. When it goes, the smell arrives.

Safety

A persistent sewer smell can mean drain gases entering the house, which in quantity are harmful and in rare cases flammable. Ventilate the room and do not ignore it. Never mix drain cleaners, and never mix bleach with anything acidic. If the smell is accompanied by sewage backing up into the house, treat the water as a biohazard and call a drainage contractor.

Step by step

Run everything first.Every tap, the shower, the bath, and flush the toilet. Traps in an unused bathroom or a spare shower evaporate dry in a few weeks, and refilling them solves a large share of cases in two minutes.
Pour water into unused drains.Floor gullies, a utility room drain, an unused shower. These are the ones nobody thinks of and the ones most likely to have dried out.
Check the toilet is sealed to the floor.A perished wax or rubber seal at the base lets gas escape around the pan. A slight rocking movement when you sit is the tell.
Look at the shower waste and trap.Shower traps are shallow, dry out fastest, and collect hair and soap that decomposes. Lift the grille and clean it out — that residue smells even when the trap is full.
Check the washing machine and basin connections.An appliance waste hose pushed too far into a standpipe siphons the trap dry every cycle. It should sit loosely in the top with an air gap.
Consider the soil vent pipe.The pipe that lets air into the drainage system so water can flow without pulling traps empty. If it is blocked — commonly by a bird's nest — traps siphon dry every time anything drains.
Watch for the siphoning signature.A gurgle from one fitting when another drains, and a smell that comes back within a day of refilling traps. That combination points at the vent rather than the traps themselves.
Check any air admittance valve.Modern installations often use an internal valve instead of a vent pipe, commonly under a basin or in a loft. They fail, and a failed one lets gas straight into the room.
Look for leaks under and behind.A cracked waste pipe or a loose compression joint behind a panel or under the floor lets both water and gas out. Damp patches, staining or a soft floor near the toilet.
Check outside as well.An open or damaged inspection chamber, a broken gully, or a blocked drain in the garden can push smell back towards the house. Lift the cover and look.
Clean rather than mask.Hot water with washing-up liquid, or an enzyme drain treatment. Air fresheners hide the symptom, and a sewer smell is information you want.
Get a drainage contractor for the persistent cases.If refilling traps and cleaning does not resolve it, a camera survey finds cracks, root ingress and collapsed sections. That is not diagnosable from inside the house.

Tips

Run every outlet in the house first. A dried trap in a spare bathroom is the single most common cause.
A gurgle elsewhere when something drains points at the vent pipe, not the trap.
Do not mask it. A sewer smell is a fault report, and air freshener deletes the message.

Common mistakes

Pushing an appliance waste hose deep into the standpipe — It siphons the trap dry on every cycle, so the smell returns constantly no matter how often you refill it. The hose should sit loosely in the top.
Pouring bleach down to fix the smell — It does not restore a water seal, it damages some pipework and seals, and mixed with other cleaners it produces toxic gas.

If it doesn't work

Smell in a rarely used bathroom

Cause: Trap evaporated dry — Fix: Run the taps and shower and flush the toilet. Pour water into any floor gully.

Smell returns within a day of refilling traps

Cause: Siphoning — usually a blocked vent or failed air admittance valve — Fix: Check the soil vent pipe terminal for a nest, and check any internal valve. Both are common and fixable.

Gurgling from a basin when the toilet flushes

Cause: Restricted venting — Fix: Clear the vent pipe or replace the air admittance valve.

Smell strongest at floor level near the toilet

Cause: Failed seal at the pan base — Fix: Have the seal replaced. A rocking pan confirms it.

Sewage backing up into the house

Cause: Blocked or collapsed drain — Fix: Stop using water fittings and call a drainage contractor immediately. Treat the water as a biohazard.

Questions people ask

Why does my bathroom smell of drains?

Almost always a dried-out trap — the water seal in a waste pipe that blocks sewer gas. Run every tap and shower and flush the toilet. If it comes back within a day, the traps are being siphoned dry by a blocked vent.

Is a sewage smell in the house dangerous?

Drain gases are harmful in quantity and should not be ignored or masked. Ventilate the room, find the cause, and get a drainage contractor if refilling traps and cleaning does not resolve it.