What Causes Bad Drain Smells? 7 Common Causes Fixed
Table of Contents
What Causes Bad Drain Smells? The 7 Common Culprits
Why Does My Drain Smell Like Sewage?
Smelly Shower Drain Causes: What's Growing in Your Trap
How to Get Rid of Drain Smells: Step-by-Step Fixes
When to Book a Drain Inspection for Bad Smells
How to Prevent Drain Smells Coming Back
Conclusion
Frequently Asked Questions
Last Updated: 18 October 2026
What Causes Bad Drain Smells? The 7 Common Culprits
A drain that smells is telling you something. It's rarely random, and it's almost never harmless. At BrightFlow Limited, we get called to homes and businesses across Essex and East London for exactly this problem, and the cause usually falls into one of a handful of categories.
So what causes bad drain smells? The short answer: trapped, rotting organic matter, a dried-out trap seal, a blocked or broken pipe, poor ventilation, or grease and fat build-up. Most cases trace back to one of these seven culprits:
Food and soap scum rotting inside the trap
A dried-out water seal in a rarely used drain
Grease and fat coating the pipe walls
Sewer gas escaping through a faulty trap
Ventilation or soil pipe faults
Root ingress or a cracked pipe
A build-up of hair and biofilm in shower drains
Below, we'll walk through each cause, how to spot it, and what to do about it. Some you can fix in ten minutes. Others need a camera down the pipe.
Why Does My Drain Smell Like Sewage?
A sewage smell means sewer gas is getting into your home, and that's a different problem from a general musty drain odour. Sewer gas contains hydrogen sulphide, which is why it smells of rotten eggs.
The most common reason is a broken water seal. Every trap under a sink, bath or toilet holds a small plug of water that blocks gas from rising. If that seal dries out, gets siphoned away, or the trap is faulty, gas flows straight up.
Other causes include:
A cracked or disconnected soil pipe
Blocked vent pipes on the roof
Loose push-fit joints behind a wall
If the smell is strong, constant, and worse near a downstairs toilet, get it looked at. Persistent sewer gas is a health concern and often points to a structural fault rather than a simple blockage.
Watch Out Never ignore a sewage smell that appears in more than one room. It usually means a fault in the soil or vent system, not a single blocked trap. Left alone, it can worsen and let more gas into the property.
Smelly Shower Drain Causes: What's Growing in Your Trap
Shower drains are the worst offenders in most homes. The reason is simple: hair, soap, skin cells and body oils all collect in the trap, and warm, damp conditions let bacteria break them down.
That breakdown produces the classic musty, sour smell. A few specific causes stand out:
Hair and biofilm: A mat of hair and slime coats the pipe walls and traps odour.
Soap scum: Fatty soap residue sticks to the inside of the trap and rots.
Slow drainage: Water sitting longer than it should gives bacteria more time to multiply.
A shallow trap: Some shower traps hold very little water and dry out fast.
The fix is usually mechanical, not chemical. Pull the trap cover, remove the hair by hand or with a hook, then scrub the trap with a brush and hot soapy water. A drain that still smells after that needs a closer look.
How to Get Rid of Drain Smells: Step-by-Step Fixes
Start with the simplest fix and work up. Most drain smells clear with the first two steps, so don't reach for harsh chemicals straight away.
Step 1: Flush with hot water. Boil a full kettle and pour it slowly down the drain in stages. This melts light grease and shifts loose debris. Give it ten minutes, then flush again.
Step 2: Clear the trap by hand. Place a bucket under the trap, unscrew it, and remove the contents. Scrub the trap with a brush and hot soapy water, then refit it. This clears the bulk of the rot.
Step 3: Use a drain snake for deeper blockages. Feed a manual snake down the pipe to break up hair and sludge further along. Twist as you go and pull the debris out slowly.
Step 4: Clean with a biological drain treatment. Enzyme-based treatments digest organic matter without damaging pipes. Follow the pack instructions and leave them overnight. Avoid caustic soda on older pipework.
Step 5: Check and refill every trap. Pour a jug of water down unused drains, including guest bathrooms and outside gullies. This restores the water seal.
Step 6: Sanitise the drain and surrounding area. A diluted bleach solution down the drain and around the plughole kills surface bacteria. Rinse well afterwards.
Step 7: Book a CCTV inspection if the smell returns. If it keeps coming back after all six steps, the cause is deeper in the pipe. A camera survey will find it.
Cause | Quick Fix | When to Call a Professional |
Food and soap scum | Hot water flush, scrub trap | Smell returns within a week |
Dried-out trap | Refill with water | Trap won't hold water |
Grease build-up | Biological treatment | Repeated blockages |
Sewer gas | Check trap seals | Smell in multiple rooms |
Vent or soil pipe fault | None, needs inspection | Always |
Root ingress or cracked pipe | None | Always |
Shower biofilm | Clear hair, scrub trap | Slow drainage persists |
Pro Tip Before you buy any chemical drain cleaner, check the pipe material. Older properties often have pitch fibre or ageing plastic waste pipes, and caustic products can soften or damage them. A kettle of hot water and a manual snake solve most problems without the risk.
When to Book a Drain Inspection for Bad Smells
Book a drain inspection when the smell keeps returning, when more than one drain is affected, or when you can hear gurgling after flushing. Those three signs point to a fault in the pipework rather than a surface blockage.
A proper inspection means a CCTV survey. A camera travels down the drain and shows exactly what's happening: cracks, root ingress, misaligned joints, or a collapsed section. At BrightFlow Limited, we use high-definition CCTV.
Signs you need one now:
The smell returns within days of cleaning
Multiple drains smell at once
Water drains slowly across the property
There's a damp patch or subsidence near a drain
Your property has mature trees close to the drainage runs
Getting a survey early often saves money. A small crack found now is far cheaper to fix than a collapsed pipe under a patio later.
How to Prevent Drain Smells Coming Back
Prevention comes down to three habits: keep grease out, keep water in the traps, and clean on a schedule. Do those and most smells never start.
What works in practice:
Never pour cooking fat down the sink. Let it cool, then bin it.
Use a sink strainer to catch food scraps before they reach the trap.
Run hot water for 30 seconds after each kitchen clean-up.
Refill unused drains with a jug of water once a fortnight.
Clear shower hair weekly, before it becomes a mat.
Book a maintenance plan if you run a pub, restaurant, hotel or managed property. Grease traps and shared drainage need scheduled cleaning, not reactive fixes.
Key Takeaway The cheapest drain smell fix is a routine. A fortnightly hot-water flush and a weekly hair clear prevent most of the problems we're called out to. Businesses with grease traps should treat cleaning as scheduled maintenance, not an emergency.
For commercial premises, a customised maintenance plan pays for itself. We build these for schools, pubs and property managers across Essex and East London, with scheduled visits that catch problems before they become call-outs.
Conclusion
A smelly drain is a symptom, not the problem itself. Find the cause, fix it properly, and put a routine in place so it doesn't return.
If the smell keeps coming back, or you manage multiple properties and need a reliable plan, BrightFlow Limited can help. We offer 24/7 emergency response, upfront honest rates with no hidden surprises, and advanced hydro-jetting and high-definition CCTV to find the real cause. We're SafeContractor accredited and fully insured, so you know the job is in safe hands.
Request a quote and get your drainage back to normal.
Frequently Asked Questions
What causes a bad smell to come from a drain?
Most bad drain smells come from one of seven sources: a dry trap that lets sewer gas back in, food or grease trapped in the pipe, biofilm and bacteria coating the walls, a blocked or slow-draining section, a damaged pipe or seal, a missing or faulty non-return valve, or a build-up of hair and soap scum in a shower waste. The smell is usually a symptom of where waste and water are sitting rather than a single problem. Identifying which one applies to your drain is the first step to fixing it properly.
Why does my drain smell like sewage?
A sewage smell usually means sewer gas is entering your property. The most common cause is a dried-out trap under a floor drain, a bath or a sink that hasn't been used for weeks. Other causes include a cracked pipe, a loose push-fit joint, or a blocked soil vent pipe. If the smell is strong and constant, or you hear gurgling when you run taps, book a drain inspection for bad smells. A CCTV survey can confirm whether the issue is a simple trap problem or a defect in the underground pipework.
How do I get rid of a bad smell from a drain?
Start by pouring a bucket of hot, not boiling, water down the drain to clear loose debris. Follow with a mixture of bicarbonate of soda and white vinegar, leave it for 15 minutes, then flush again. If the smell persists, remove the trap or use a drain snake to clear hair and grease. For recurring smells, check that the trap holds water and that the waste pipe has the correct fall. Persistent odours after these steps usually point to a deeper blockage or a pipe defect that needs professional attention.
When should I call a drainage professional about a smelly drain?
Call a professional if the smell returns within a few days of cleaning, if multiple drains in the property smell at once, or if you notice slow drainage alongside the odour. These signs often indicate a blockage deeper in the system or a fault such as root ingress or a collapsed pipe. A professional drain inspection for bad smells uses high-definition CCTV to locate the cause without guesswork. For commercial kitchens and pubs, a maintenance plan that includes grease trap cleaning is usually more cost-effective than repeated emergency call-outs.
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