TL;DR
A smell coming from your drains almost always points to one of three causes: a dry trap that's lost its water seal and is letting sewer gas rise up through the pipe, a partial blockage where trapped waste is slowly decomposing, or a venting fault stopping gas from escaping safely up the soil vent pipe the way it should. Which one applies usually depends on which fixture the smell is near and whether it's constant or comes and goes. If the checks below don't clear it, request a quote online and we'll trace the actual cause with a camera rather than guesswork.
Rotten-Egg Smell: Usually Decomposing Waste Trapped in the Pipe
A rotten-egg smell is hydrogen sulphide gas, produced when organic matter breaks down without oxygen — food scraps, grease, hair, and soap scum are the usual culprits. In kitchens, this typically builds up in the trap beneath the sink or around a dishwasher connection, where fat cools and solidifies against the pipe wall and starts to decompose. In bathrooms, it's more often hair and soap residue collecting in a shower or bath trap. The smell is a sign that something organic is sitting and rotting rather than draining away — less serious than a full sewage smell, but worth clearing before it develops into an actual blockage.
Sewage Smell: Usually a Dry Trap, Sometimes a Real Blockage
Every drain fixture — sink, bath, shower, and outside gully — has a U-shaped trap beneath it that holds a small amount of standing water. That water seal is what stops sewer gas from rising up into your home between uses. If a fixture isn't used regularly — a spare bathroom, a utility room sink, a rarely-used cloakroom basin — the water in the trap evaporates over weeks, breaking the seal and letting sewer smell straight through. This is by far the most common cause of a genuine sewage smell, and also the easiest to fix yourself (see the checks below).
Dry traps are especially common in Dundee's student rental stock. With the University of Dundee and Abertay University both sitting close to the city centre, a large share of the flats around the West End, Hawkhill, and Stobswell sit empty through the summer vacation, and a trap left unused from June to September will usually have evaporated dry by the time the tenants return. If you've just moved into a flat that's been standing empty over the summer and it smells of sewage, running every tap, shower, and toilet for a couple of minutes is the first thing to do.
If running water doesn't resolve it, or the smell is coming from a fixture used every day, it's more likely a real blockage further down the pipe pushing gas back up towards the nearest opening rather than letting it vent forward. This is common in Dundee's older tenement and terraced properties, where shared drainage stacks serving several flats can carry a smell from a blockage on one floor up through connected fittings on another.
Intermittent Smell That Comes and Goes: Often a Venting Problem
If the smell isn't constant — it appears after a flush, after the washing machine empties, or only on certain days — the cause is often the venting rather than the drain itself. Most properties have a soil vent pipe (SVP), also called a soil stack, which carries waste down and lets sewer gas escape safely well above head height, while drawing air into the system so water drains smoothly rather than glugging. On Dundee's tenement and terraced stock that's usually the cast-iron or plastic stack running up the external rear wall above the back green and terminating above the eaves; newer properties and internal conversions often use an air admittance valve (a Durgo valve) fitted inside instead. A blocked stack head — leaves, moss, or a bird's nest at the open top — a cracked joint, or a failed air admittance valve all stop that pressure equalising properly, and gas gets pushed back into the property instead, often with gurgling from the affected fixture.
Quick Checks Before You Book Anyone
A few simple checks can resolve the most common causes without any professional visit:
- **Run water in every fixture, including ones you rarely use.** This refills dry traps and is often all it takes to clear a sewage smell.
- **Clear hair and debris from shower and bath plugholes**, and pour warm water through afterwards.
- **Check the external gully** outside the property — a back green gully clogged with leaves or debris can cause a sewage smell that seems to come from indoors.
- **Note when the smell happens.** A constant smell near one fixture points to a dry trap or local blockage. A smell that comes and goes with flushing or draining elsewhere points more towards a venting issue.
When It's More Than a Smell
Some signs mean the cause is beyond a DIY fix and worth escalating straight away:
- The smell is getting stronger rather than clearing after the checks above
- More than one fixture is affected at the same time
- You notice slow drainage or gurgling alongside the smell — see our guide to signs of a blocked drain
- There's any visible sign of sewage backing up, which needs treating as a sewage emergency rather than a routine job
- The smell is coming from a toilet that's also slow to flush or refilling oddly
In any of these cases, a CCTV drain survey is the fastest way to find the actual cause rather than treating symptoms repeatedly. It's a common mistake to keep bleaching a smelly drain for months when the real issue is a partial blockage or a venting fault that bleach can't touch.
Frequently Asked Questions
**Why does my bathroom smell like sewage?** Most often because a trap somewhere in the bathroom — under the sink, bath, or a rarely used shower — has dried out and lost its water seal, letting sewer gas rise through. Running water in every fixture is the first thing to try.
**Why does my shower drain smell?** Usually hair and soap scum decomposing in the shower trap. Clearing the plughole and flushing with warm water resolves most cases; if it persists, there may be a partial blockage further down the pipe.
**Why does my toilet smell like the sewer?** This can indicate a dry trap in a nearby unused fixture, a venting issue, or in some cases a partial blockage in the toilet's own drain run. If flushing also seems slow, it's worth having it checked rather than just masking the smell.
**Why does it smell like rotten eggs?** That's hydrogen sulphide gas from decomposing organic waste trapped in a pipe — usually grease, food debris, or hair. It's a sign something is sitting and rotting rather than draining away properly.
**Is a drain smell dangerous?** An occasional whiff from a dry trap isn't a health risk once the water seal is restored. Persistent sewage gas indoors, or any sign of actual sewage backing up, should be treated as a genuine problem and dealt with promptly rather than lived with.
Still Smelling It After the Checks?
If you've tried the quick fixes and the smell hasn't shifted, it's telling you something's genuinely wrong further down the system rather than just a dry trap. Request a quote online and we'll get a camera down the pipe to find the actual cause, whether that's a blocked drain that needs clearing or a venting issue that needs a proper look.