A bathroom often smells of drains because a trap has dried out or lost its water seal, a seal around the toilet has failed, or waste is building up in the pipes. Each trap holds a little water that stops smells rising from the drain, so anything that empties or bypasses that water can let odours into the room.
A drain smell does not always mean there is a problem with the sewer. Many causes are inside the bathroom itself.
Dry or Emptied Traps
Basins, baths and shower trays all have traps under them. If a fitting is rarely used, such as in a guest bathroom, the water in its trap can slowly evaporate and let smells through.
Traps can also be pulled empty. When a large amount of water drains from another fitting, it can suck the water out of a nearby trap. You may hear gurgling as this happens. Running each tap for a short time can refill traps that have dried out.
Shower trays with shallow traps can lose their water seal more easily than basins. If a smell seems to come from the shower, this is one of the first things worth checking.
Seals and Connections Around the Toilet
The toilet connects to the soil pipe with a pan connector. If this seal is loose, split or has moved, smells can escape around the back of the toilet, often along with damp after flushing.
A cracked pan or a poorly sealed joint on the soil pipe behind boxing can cause the same smell. Because these are hidden, the smell may be the first sign.
Build-Up in the Pipes and Ventilation
Hair, soap and toothpaste collect in traps and waste pipes, and this slime can start to smell over time. Showers and basins are common places for it.
Waste pipes also need air to drain properly. Many homes have an air admittance valve or vent pipe for this. If it sticks or is blocked, traps may be pulled empty more often.
If several fittings smell, drain slowly or gurgle at once, the cause may be further down the pipework or in the drain outside. That may require investigation beyond the bathroom.
Overflows on basins and baths can also smell. The channel behind the overflow slot collects grime and is rarely cleaned, so it can give off a musty or drain-like odour even when the traps are fine.
What to Check and When to Get Help
Run water into every fitting, including rarely used ones, and see if the smell fades. Check for damp around the base of the toilet. Avoid pouring strong chemicals into traps, and never mix cleaning products.
Keep the room well aired while you look into it.
If the smell keeps returning, our Oxford bathroom plumbing team can check the traps, toilet connection and waste pipework, and find where the odour is getting in, so the right part can be put right.