A bathroom may smell of drains because a trap has dried out, waste has built up in a basin, bath or shower waste, or a seal around the toilet or a pipe joint is letting smells escape. It can also be caused by a problem with how the waste pipes are vented, or by an issue in the drain outside.
Because a bathroom has several fixtures connected to the same system, the smell may not come from where it seems strongest. Noticing when the smell is worst, such as after a shower or a flush, can give useful clues.
Dry or Disturbed Traps
Each basin, bath and shower has a trap that holds a small amount of water to stop drain smells coming up. If a fixture is not used for a while, such as in a guest bathroom, this water can dry out.
Running water into each fixture can refill the traps. If the smell goes away, a dry trap may have been the cause. If water is pulled out of the traps when other fixtures drain, often with a gurgling sound, there may be a venting or drainage issue that needs checking.
Build-Up in Wastes
Shower trays and baths that are used every day can still build up residue in the trap and waste, even if they drain normally.
Hair, soap and toiletries can collect in basin, bath and shower wastes. This can start to smell, especially in warm, damp conditions. A slow-draining fixture may be a sign of this kind of build-up.
The overflow on a basin or bath can also collect grime and start to smell, even when the waste itself is clear.
Toilets and Pipe Joints
A toilet is sealed where it connects to the soil pipe. If this seal is damaged or the toilet has moved, smells may escape around the base. A loose joint on a waste pipe or soil pipe can also let smells into the room, sometimes from behind boxing or panels.
Water or staining around the base of the toilet, or a smell that is strongest after flushing, may point in this direction. It does not prove where the smell starts, though.
Problems Beyond the Bathroom
If the smell is also noticeable outside, or other rooms are affected, the cause may be in the soil pipe, its vent or the drain. A partly blocked drain can lead to smells as well as slow drainage.
Keeping the room well ventilated can help, but it will not fix the source. Avoid mixing cleaning or drain products, as some combinations can give off harmful fumes.
Our page on bathroom plumbing repairs explains how problems affecting several bathroom fixtures and their pipework are assessed.
If your bathroom keeps smelling of drains, a plumber can check the traps, wastes and connections and explain the likely cause.