Smell of drains in the house: causes and fixes

A drain smell in the house is unpleasant and hard to ignore. The good news is that the cause is usually simple. Every basin, bath, shower and toilet has a water trap, a U bend that holds water to stop sewer gas coming back up the pipe. When a smell gets into the house, something has let that gas past.

Sheffield’s older housing makes some causes more common. Big Victorian villas in Nether Edge and Broomhill, and terraces across the city, often have cellars with old floor drains, rooms that are rarely used and cast iron pipes that have been in place for over a century.

First: is it drains or gas?

A drain smell is like rotten eggs or sewage. A mains gas leak smells similar, because gas companies add a smell to it. If you are in any doubt, or the smell is near the boiler, hob, gas fire or meter, treat it as gas: leave the building, do not use switches and call the National Gas Emergency Service on 0800 111 999.

Cause 1: a dry trap

This is the most common cause by far. Water in a trap slowly evaporates if the outlet is not used. In a spare bathroom, a guest basin, a rarely used shower or a cellar floor drain, the trap can dry out in a few weeks, especially in warm weather or a heated room.

Fix: run water into every outlet in the house, including the cellar floor drain if you have one. If the smell goes, that was the cause. In a room that is not used often, run the taps every week or two.

Cause 2: cellar drains and old access points

Many Nether Edge villas and Sheffield terraces have cellars with:

• A floor drain or gully, sometimes with a trap that has dried, cracked or been removed.
• Old cleaning access points on the drain with loose or missing covers.
• The main drain running under or through the cellar.

A cracked joint or loose access cover in the cellar can let gas straight into the house. The smell then rises through the floorboards. Our Nether Edge area page explains more about cellars in those homes.

Fix: check the cellar for any open pipe ends, loose covers or dry gullies. Seal them, refill traps, or fit a new trapped gully.

Cause 3: a failed seal

• Toilet pan connector. The rubber or plastic connector between the toilet and the soil pipe can crack or slip, especially if the toilet rocks.
• Waste pipe joints. Push fit joints under a basin or bath can pull apart.
• Shower trays and baths. The waste fitting can work loose.

Signs: the smell is strongest in one room, near one fitting.

Fix: reseat or replace the seal or connector. A rocking toilet needs fixing down properly.

Cause 4: an old soil stack

The soil stack is the large pipe that carries toilet waste down to the drain and vents gas up above the roof. In Victorian and Edwardian houses it is often cast iron, sometimes inside the house. Cast iron cracks and its joints fail over time.

Signs: smell near where the stack runs, damp patches or rust stains on the pipe.

Fix: a plumber can inspect the stack, repair a section or replace it with plastic.

Cause 5: the vent is blocked or missing

The soil stack’s vent at roof level lets gas out above the house and lets air in so traps are not sucked dry when toilets flush. If it is blocked by a nest or leaves, or was cut off below the roof during an old loft conversion, traps can be pulled empty and gas has nowhere else to go.

Signs: gurgling from basins when a toilet flushes, smells after flushing.

Fix: clear the vent, or fit an air admittance valve where a full vent is not possible.

Cause 6: a partial blockage

A drain that is partly blocked can make waste back up slowly and push air through traps when other fittings drain. You may notice slow drains and gurgling as well as the smell. Our post on a blocked drain outside shows how to check the manhole. If neighbours are affected too, it may be a shared drain for Yorkshire Water to clear.

Quick checklist

Check	What it tells you
Run water into every outlet	Fixes dry traps
Sniff each room	Finds the strongest spot
Check the cellar for open pipes or dry gullies	Common in older homes
Rock each toilet	Loose pan can break the seal
Flush and listen at basins	Gurgling points to a venting problem
Lift the manhole	Full means a blockage downstream

When to call

Call a plumber or drain engineer if:

• The smell is still there after refilling traps.
• You can see a crack or damp on a soil stack.
• There are slow drains and gurgling as well.
• You cannot find the source.

A smoke test or CCTV camera survey can find hidden cracks and failed joints. Our blocked drains page explains what a visit involves. Tell us which room smells worst and whether you have a cellar, and we will tell you where we would look first.