In short
A smell from your drains usually has one of three causes. A dry trap may have lost its water seal, which lets sewer gas rise up the pipe. Waste may be rotting in a partial blockage, while a venting fault may stop gas escaping safely up the soil vent pipe. Which one applies depends on where the smell is, so if the checks below do not clear it, request a quote online so an engineer can trace the cause.
Rotten-egg smell: rotting waste in the pipe
A rotten-egg smell comes from hydrogen sulphide gas. It forms when organic matter breaks down without oxygen. As a result, food scraps, grease, hair and soap scum are the usual causes.
In kitchens, the smell often builds up in the trap under the sink or near a dishwasher connection. Fat cools there, sets on the pipe wall and starts to rot. In bathrooms, hair and soap collecting in a shower or bath trap is more likely.
The smell means something is sitting and rotting rather than draining away. It is less serious than a full sewage smell. However, it is worth clearing before it becomes a blockage.
Sewage smell: usually a dry trap, sometimes a blockage
Every sink, bath, shower and outside gully has a U-shaped trap beneath it. The trap holds a little standing water. That water seal stops sewer gas rising into the home between uses, because the water blocks the pipe.
If a fixture is rarely used, the water in its trap can dry out over weeks. For example, a spare bathroom, a utility sink or a cloakroom basin may dry out. Once the seal breaks, sewer smell passes straight through. This is a common cause of a sewage smell, and it is also the easiest to fix yourself.
A home that has stood empty for a few weeks often has dry traps. If you have just moved in and it smells of sewage, run every tap, shower and toilet for a couple of minutes first. If that does not help, however, or the smell comes from a fixture you use daily, a partial blockage further down may be pushing gas back up. Where a drain serves several flats or homes, a blockage in the shared run can send a smell to other connected fixtures.
A smell that comes and goes: often a venting problem
A smell that is not constant often points to venting, since it may appear after a flush, after the washing machine empties or only on some days.
Most properties have a soil vent pipe, also called a soil stack. It carries waste down and lets sewer gas escape above head height. It also draws air in, which lets water drain smoothly instead of glugging.
Some properties, however, use an air admittance valve instead, fitted inside. A blocked stack head, a cracked joint or a failed valve stops the pressure evening out. Gas can then push back into the home, often with gurgling from the affected fixture.
Quick checks before you ask anyone
A few simple checks can resolve the most common causes, because most are easy to reach:
- Run water in every fixture, including ones you rarely use, to refill dry traps
- Clear hair and debris from shower and bath plugholes, then pour warm water through
- Check the outside gully, because leaves or debris there can cause a smell that seems to come from indoors
- Note when the smell happens, since a constant smell near one fixture points to a dry trap or local blockage, while one that comes and goes points to venting
When it is more than a smell
Some signs mean the cause is beyond a simple fix, since the problem lies further along the pipe:
- The smell is getting stronger rather than clearing after the checks
- More than one fixture is affected at the same time
- You notice slow drainage or gurgling alongside the smell, so see our guide to signs of a blocked drain
- Sewage is visibly backing up, which is a sewage emergency rather than a routine job
- The smell comes from a toilet that is also slow to flush
In any of these cases, a CCTV drain survey is a good way to find the cause rather than treat the symptom again and again. It is a common mistake to keep bleaching a smelly drain for months, because the real problem may be a partial blockage or a venting fault that bleach cannot reach.
Asking for help
This site is a request service for drainage work. You send your details through the form, and your request goes to an engineer who can respond. The engineer who takes your job runs a separate business. The engineer therefore sets the price and the arrival time, so agree both before work starts. Say which fixture smells and when, and whether the smell is constant.
A sewage smell with water coming back up can be urgent. Town pages cover this, such as emergency drain help in Forfar and emergency drain help in Perth. In Fife, see urgent drain problems in Newport-on-Tay, urgent drain problems in Tayport or urgent drain problems in Wormit.
Frequently Asked Questions
Why does my bathroom smell like sewage?
Most often a trap in the bathroom has dried out and lost its water seal. This can happen under the sink, the bath or a rarely used shower, because unused traps dry out. Running water in every fixture is the first thing to try.
Why does my shower drain smell?
Usually hair and soap scum are rotting in the shower trap. Clearing the plughole and flushing it with warm water solves most cases. If it persists, however, there may be a partial blockage further down the pipe.
Why does my toilet smell like the sewer?
The cause could be a dry trap in a nearby unused fixture, a venting issue or a partial blockage in the toilet's own drain run. If flushing is also slow, have it checked rather than masking the smell, since a slow flush suggests a blockage.
Why does it smell like rotten eggs?
That smell is hydrogen sulphide gas from organic waste rotting in a pipe. Grease, food debris and hair are the usual causes. It means something is sitting in the pipe, yet it is not always a full blockage.
Is a drain smell dangerous?
A faint whiff from a dry trap usually clears once the water seal returns. A strong or lasting sewage smell, however, is a real problem, so deal with it promptly. Sewage backing up into the home is an emergency, so stop using water and ask for help.
