Why Do My Drains Smell? Sewage Smell in the House Explained
Written by the Urgent Drainage Solutions teamPublished Updated 7 min read
Quick answer
Drains usually smell because a trap has lost its water seal and is letting sewer air into the room, or because grease, hair and soap are rotting in the waste pipe. A sewage smell that keeps returning, especially with gurgling or slow drains, usually means a blocked or cracked drain, a failed air admittance valve or a leaking joint.
- Every sink, bath, shower and toilet has a water-filled trap; if it dries out or is siphoned, you smell the drains.
- Running water through little-used fittings fixes most one-off smells in minutes.
- Gurgling means air is being dragged through a trap, usually because the drain is partly blocked or poorly vented.
- Severn Trent says a smell inside your home is, in nearly all cases, a private plumbing issue.
- A smell that won't go away after cleaning traps needs a drain check or CCTV survey.
Drains smell because sewer air is getting into the room, usually through a trap that has lost its water seal, or because grease, hair and soap are rotting inside a waste pipe. A sewage smell that keeps coming back, particularly with gurgling or slow drains, usually means a blocked or cracked drain, a failed air admittance valve or a leaking joint, and those need investigating. If you suspect the drain itself, our blocked drain and CCTV drain survey services find and fix the cause.
Here's how to work out which it is, what you can check yourself, and when to call someone.
Why do my drains smell?
Because a barrier that should keep sewer air out has failed, or because something is decaying in the pipe. Every sink, bath, shower and toilet has a trap: the U-shaped bend under the plughole, or the bend built into a toilet, that holds a small plug of water. Approved Document H says every discharge point should have a trap "to prevent foul air from the system entering the building", and that traps should keep a seal of at least 25mm of water under working conditions. Its Table 1 gives 75mm seals for sinks and washbasins and 50mm for baths, showers and WCs.
If that seal is lost, even briefly, you'll smell the drains. If it's intact but the trap or pipe beyond it is coated in grease, hair and soap, the smell comes from the gunk itself.
What causes a sewage smell in the house?
Most sewage smells indoors come from one of six causes. The table shows how to tell them apart.
| Cause | Typical signs | What to do |
|---|---|---|
| Dry trap | Smell from one fitting in a little-used room | Run the tap or flush to refill the trap |
| Siphoned trap | Smell after another fitting drains, often with gurgling | Check venting; call a plumber if it repeats |
| Dirty trap or waste pipe | Smell from one basin, sink or shower; slow draining | Clean the trap and waste |
| Blocked drain | Several fittings slow, gurgling, a full chamber outside | Drain unblocking or jetting |
| Failed air admittance valve | Smell near a boxed-in soil pipe, in a loft or cupboard | Replace the valve |
| Cracked or displaced drain, or leaking seal | Smell that never fully goes, damp patches, sometimes rats | CCTV survey, then repair |
An air admittance valve is a one-way valve fitted on top of some soil pipes inside the house instead of an open vent through the roof. It lets air in when water drains but should close to keep smells in. Approved Document H says these valves "should be accessible for maintenance", so they can be checked and replaced.
Severn Trent's sewage smells policy says that if the smell is inside your home or coming from a sink, bathroom or drain, "in nearly all cases" it will be an issue with your private plumbing.
Why does my bathroom smell of sewage?
Usually a dry or siphoned trap, or a dirty waste. Spare bathrooms, en suites, shower trays and utility floor drains that go unused for days lose their water to evaporation. Severn Trent's guide to sewerage smells says loss of trap seal on shower trays and sinks can happen overnight, which is why smells are often noticed first thing in the morning or last thing at night, when the gap between uses can be 14 hours or more.
Around the toilet, a smell can also come from a failed seal where the pan meets the soil pipe, or from a cracked pan connector. If the floor around the toilet is damp or stained, or the smell is strongest at the base, that's the likely cause.
Why does my drain gurgle?
Gurgling is air being dragged through a trap as water flows past. When a bath or washing machine empties into a partly blocked or poorly vented pipe, the falling water creates suction, and the nearest trap gives up some of its water with a gurgle. Approved Document H explains that pressures in pipework can break trap seals, which is why branch pipes are designed and ventilated to limit them.
Common causes are a partial blockage further down the drain, a stuck air admittance valve, or a long or badly laid waste pipe. A gurgling toilet when the bath empties is a strong sign that the drain they share is partly blocked. A gurgle on its own is a warning; a gurgle plus a smell plus slow draining means the drain needs clearing.
Is a sewage smell in the house dangerous?
Usually it's unpleasant rather than harmful, but don't ignore it. Sewer gas contains hydrogen sulphide, which UKHSA describes as having "a characteristic odour of rotten eggs". UKHSA also says it is very unlikely the general population will be exposed to a level high enough to cause adverse health effects.
The real risks are what the smell points to: a blocked drain that could overflow, or a broken pipe leaking under the floor. If you think you can smell gas rather than drains, especially near a boiler, hob or meter, treat it as a gas leak and call the National Gas Emergency number on 0800 111 999.
What can you check yourself?
Work through this in order. Most smells are found within the first three steps.
- Run water in every fitting for a minute, including spare bathrooms, showers and floor drains, and flush every toilet.
- Find where the smell is strongest. One basin, the shower, the toilet, a cupboard, the loft, or outside?
- Clean the trap under a smelly basin or sink, with a bucket underneath. Our guide to unblocking a sink, shower or bath shows how.
- Check boxed-in soil pipes for an air admittance valve. If the smell is strongest there, the valve may be stuck open.
- Check the outside inspection chambers. If you can safely lift a light cover and the chamber is full, the drain is blocked further down. See how to unblock an outside drain.
- Try the glass test. Severn Trent suggests filling a glass from the tap near the smell and taking it into another room. If the water smells fine there, the smell is coming from the sink or drain, not your water supply.
If you use a drain cleaner, Which? warns that bleach can react with acid-based products such as limescale removers and vinegar, risking toxic gas. Never mix products.
Why do my drains smell outside?
An outside smell usually comes from a gully with no water in its trap, a blocked or overflowing chamber, or a cracked pipe. Gullies that take kitchen waste collect grease and food, and they smell as it rots; clean them out and see our guide to fat, oil and grease. A smell from a manhole in the street, a public sewer or a pumping station should be reported to your water company: Severn Trent on 0800 783 4444 across most of Nottinghamshire, Derbyshire and Leicestershire, or Anglian Water across much of Lincolnshire. Don't lift covers on public land.
Who is responsible for a smelly drain?
Inside your home, you are. Outside, the usual rules apply: Citizens Advice says you are responsible for drains inside your boundary that serve only your property, while the water company looks after lateral drains and public sewers. See who is responsible for blocked drains.
When does a drain smell need a CCTV survey?
When the simple fixes don't work. A camera survey shows cracks, displaced joints, root ingress and pipes that were never properly connected, and pinpoints where they are. Approved Document H notes that smoke testing "may be used to identify defects where a water test has failed", and it's sometimes used alongside a camera to find where smells escape. A drain that smells and attracts rats is a warning sign that the pipe is broken; our guide to rats in drains explains the link, and signs of a blocked or collapsed drain covers other clues.
When to call a professional
Call a drainage engineer if the smell keeps coming back after you've refilled and cleaned the traps, if more than one fitting is slow or gurgling, if an inspection chamber is full, or if you've noticed damp patches, sinking paving or rats. Our blocked drain service starts from £120 + VAT, and a CCTV drain survey starts from £149 + VAT. We confirm a fixed price before any work starts. For leaking waste pipes, soil stacks, air admittance valves and toilet connections inside the house, see our drainage plumbing service.
We cover Nottingham, Nottinghamshire, Derbyshire, Leicestershire and Lincolnshire, 7 days a week from 6am to 8pm, with no call-out fee. Call 07300 600 950 or book online.
Sources
Frequently Asked Questions
Cite this guide: “Why Do My Drains Smell? Sewage Smell in the House Explained”, Urgent Drainage Solutions, 6 October 2026. https://urgentdrainagesolutions.co.uk/blog/why-do-my-drains-smell
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