Understanding where the smell comes from
Drain smells come from gas in the sewer or from waste in a pipe, so the source matters. Traps hold water that blocks the gas from entering the room. If that seal breaks, the smell can enter.
Start by finding where the smell is strongest. Check bathrooms, kitchens and utility rooms. This helps you narrow down the cause, because each room has different drains.
Cause 1: Dry trap (most common)
A trap can dry out if a fitting is not used for weeks, since the water slowly evaporates. The water seal evaporates and gas passes through. Rooms with a spare bathroom or floor drain are often affected.
Run water down the drain for a minute to refill the trap. If the smell goes, you have found the cause. Repeat now and then for rarely used fittings.
Cause 2: Cracked or poorly-fitted trap
A trap may leak or sit loose, and then the water seal may drain away. Then the water seal may drain away. Look for damp or stains under the fitting.
Tighten loose parts and replace cracked ones. If you are not confident, ask a plumber, because a poor fit can leak.
Cause 3: Venting problem (soil stack or vent pipe)
Drain pipes need air to flow well, so a blocked vent causes trouble. A blocked vent can pull water out of traps. You may hear gurgling too.
A blocked vent pipe on the roof may need a professional to clear. Do not climb onto a roof yourself. Our guide on noisy pipes explains related sounds.
Cause 4: Blocked or partially blocked drain
A blockage can hold waste in the pipe, so it rots and smells, while slow drains give a clue. Slow drains are a clue. Water may also sit in a gully.
Try cleaning the fitting. If the smell stays, the blockage may be deeper. Ask a drainage engineer to look at it.
Cause 5: Inspection chamber or manhole cover
A damaged cover can let smells out, because the seal is broken. A chamber may also be dirty or full. Look at covers near the house.
Do not open a chamber if you are unsure. The air can be harmful. A professional can check it safely.
Cause 6: Drain or sewer structural failure
A cracked or collapsed pipe can leak waste and smell, since the pipe no longer seals. Damp ground and sunken patches may point to it. A camera survey shows what is wrong.
Our page on CCTV drain survey explains how a survey works. This fault needs a repair, since cleaning will not help.
When to call a professional immediately
Call at once if the smell is strong and does not go away, because it may be a health risk. Call if sewage comes up from a drain, or if people feel ill. Headaches or dizziness can be a sign of poor air.
If you smell gas, which smells like rotten eggs, leave the house and call 0800 111 999. Sewer gas and mains gas are different, but you should treat any doubt as an emergency.
Related guides and local help
If you want to know more, read Shower Not Draining: What You Need to Know to Fix It. You can also read 10 Plumbing Myths That Cost UK Homeowners Serious Money and 10 Signs Your Drain Needs a Professional, Not Just a Plunger, because they cover closely related problems.
These guides help too: Outside Drain Blocked: What to Do and Who Is Responsible, 7 Things You Should Never Flush Down a Toilet in the UK, Kitchen Drain Problems: Causes, Fixes and Easy Prevention, Tree Roots in Drains: How They Get There and What to Do and How to Unblock a Toilet: Safe Methods and When to Stop. Each one explains a related topic in plain English.
Need help near you? Read our blocked drain guide first, or see Blocked Drains in Reading and Blocked Drains in York. You can also see Blocked Toilets in Bognor Regis, since every service page has a free quote form. We pass each request to a local drainage engineer.
