blogTL;DR: Drain smells are caused by trapped debris, dried water traps, sewer gas ingress, or faults deeper in your drainage system. Most are fixable with basic maintenance. Persistent or sewage-strength smells usually signal a blockage or structural fault that needs a professional diagnosis. This post explains the common causes, what they mean, and when to call in a drainage engineer.
Drain smells are one of the most common complaints from homeowners and landlords across Cornwall. If you’ve noticed that a drain smells unpleasant, you’re not alone, and you’re right not to ignore it. A bad smell coming from your drains can mean something as simple as a dry water trap, or something more serious like a cracked pipe, a partial blockage, or sewer gas venting into your home.
Left unaddressed, the underlying cause rarely fixes itself. In some cases it gets significantly worse.
This post covers why drains smell, what each type of smell tells you, which fixes you can do yourself, and when you need a professional inspection to get to the root cause.
What Causes Drains to Smell?
Drain smells come from one of four main sources: organic waste breaking down inside the pipe, a dried or damaged water trap, a partial blockage trapping gases, or a fault in the drainage system itself. Identifying which type you have determines the right fix.
The most common source is organic matter. Hair, grease, soap residue, and food particles accumulate inside pipework and begin to decompose. That decomposition produces hydrogen sulphide, which is the compound responsible for the characteristic rotten egg smell many people notice.
The second most common cause is a dry P-trap. A P-trap is the curved section of pipe beneath your sink or bath (shaped like the letter P when viewed from the side). It holds a small amount of water that acts as a barrier, blocking sewer gases from rising back up through your plughole. If a drain hasn’t been used for a while, that water evaporates. Once the seal is gone, gases travel freely back into the room.
Other causes include:
- Partial blockages that trap decomposing material and create stagnant pockets inside the pipe
- Cracked or misaligned pipe joints allowing sewer gas to escape into the fabric of your building
- Ventilation pipe faults that prevent gases from dispersing safely above roofline
- Septic tank issues in rural properties where the system needs emptying or servicing
The smell’s location, strength, and whether it’s constant or intermittent all give useful clues about which of these is actually happening.
Does a Smelly Drain Mean There’s a Blockage?
Not always, but frequently yes. A partial blockage is one of the most common reasons a drain starts to smell. Fully blocked drains stop water flowing; partial blockages trap organic material in the line, where it sits, decomposes, and generates gas.
The key distinction is whether the drain is still running. If water drains slowly rather than freely, that points strongly towards a partial obstruction. If the drain runs normally but still smells, the cause is more likely a dry water trap, poor ventilation, or a pipe fault further along the system.
A CCTV drain survey is the most reliable way to confirm which you’re dealing with. The camera travels the full length of the pipe and shows exactly what’s inside: build-up, root ingress, displaced joints, or structural damage. Without that visual inspection, you’re guessing.
Partial blockages are worth taking seriously. They don’t resolve on their own. They accumulate and eventually become full blockages, which carry a higher risk of backflow and property damage.
Why Does My Kitchen Sink Smell?
Kitchen sink smells are almost always caused by grease and food residue building up inside the trap and the pipe immediately downstream. Cooking fats cool and solidify inside pipework, forming a coating that organic matter sticks to. Over time, that coating decomposes.
This is a textbook drainage maintenance problem, and it’s extremely common in Cornwall properties with older clay pipe systems where pipe gradients have shifted over time. Slow-draining kitchen sinks in particular suggest grease accumulation is already narrowing the pipe bore.
You can often address a mild kitchen sink smell yourself:
- Boil a kettle and slowly pour it down the drain to loosen any grease
- Follow with a cup of bicarbonate of soda, then a cup of white vinegar
- Leave for 15-20 minutes, then flush with more hot water
- If the smell persists after this, or the drain is slow, the blockage is likely further into the pipework
One thing worth noting: chemical drain cleaners are not recommended. They can damage older clay and plastic pipe systems, and they treat the symptom rather than the cause. High-Pressure Jetting clears the full pipe bore and removes the debris rather than just breaking it up and pushing it further along the line.
For persistent kitchen sink smells or slow drainage, blocked drains in Cornwall is a useful starting point for understanding your options.
Why Does My Bathroom Drain Smell Like Sewage?
A sewage smell from a bathroom drain is almost always sewer gas venting back into the room through a compromised water trap. Sewer gas contains hydrogen sulphide, ammonia, and methane. Beyond the smell, it has health implications if it builds up in an enclosed space, so it’s worth resolving quickly.
The most likely causes, in order of frequency:
- A dry P-trap in a bath, shower, or basin that hasn’t been used recently
- A cracked or poorly fitted trap that’s lost its water seal
- A blocked or incorrectly installed soil vent pipe (SVP) causing pressure fluctuations that siphon the trap dry
- A crack or displaced joint downstream allowing gas to bypass the trap entirely
The fix for a dry trap is simple: run the water for 30-60 seconds to refill it. If the smell disappears and then returns after a few days without using that fitting, the trap may have a fault, or there may be a ventilation issue creating negative pressure in the waste system.
If the smell is strong and persistent regardless of trap condition, that points to a structural fault. At that point, a CCTV Drain Survey is the right diagnostic tool. It will locate cracks, displaced joints, and ventilation failures that can’t be detected any other way.
What Does It Mean If the Smell Is Coming from Outside?
A drain smell outside your property, specifically near gullies, manholes, or inspection covers, generally indicates one of three things: a partial blockage in the external line, a broken inspection chamber, or a sewer overflow event.
External drainage smells are more noticeable in warm weather. Heat accelerates the decomposition of organic material inside pipes and chambers, and air pressure differentials between warm above-ground air and cooler underground pipe air push gases upward through any gaps in the system.
If you’re noticing the smell near a specific manhole or gully, lift the cover (safely, using the correct tools or wearing gloves) and check for standing water. Water sitting above the bottom of the chamber that isn’t moving indicates a blockage downstream. If the chamber is clear and running freely, the smell may be coming from a cracked chamber wall or loose benching, both of which allow gas to escape from the system.
Cornwall’s ground conditions make external drainage faults more common here than in many other parts of the country. Granite bedrock causes pipe movement as ground settles differentially, and clay subsoils expand and contract with seasonal moisture changes. Both put stress on pipe joints over time. A routine CCTV survey as part of a drainage maintenance programme will catch these faults before they develop into structural failures.
How Clear Stream Diagnoses and Fixes Drain Smells
Drain smells are a symptom. The cause sits somewhere inside your pipework, and identifying it precisely is the first step to a permanent fix. Clear Stream’s engineers approach every smell complaint as a diagnostic job first.
The process typically runs as follows:
- An initial assessment of where the smell is strongest and whether it’s accompanied by slow drainage or visible symptoms
- A CCTV Drain Survey to inspect the full length of the affected pipe, identifying build-up, blockages, cracks, displaced joints, or ventilation faults
- High-Pressure Jetting to clear any blockage or accumulated debris, restoring the full bore of the pipe
- Where a structural fault is identified, a written report with footage is provided, along with options for repair including No-Dig Repair methods where applicable
- A fixed-price quote before any work begins, with no call-out fee
For properties on mains drainage, the process above covers most causes. For rural properties in Cornwall on private systems, there’s an additional consideration. Septic tank faults produce particularly strong sewage smells that can affect both interior and exterior areas of the property. If your property uses a septic tank and the smell is sewage-strength, it’s worth considering septic tank maintenance in Cornwall as part of the diagnosis.
All repair work carried out by Clear Stream comes with a 5-Year Guarantee. Engineers are on-site within 1-2 hours of your call, 24 hours a day, 365 days a year, covering the whole of Cornwall and Devon. There are no call-out fees, and every job begins with a fixed-price quote.
Can You Prevent Drain Smells from Coming Back?
Yes. Most drain smells caused by organic build-up are preventable with basic maintenance habits. The underlying cause is almost always material entering the drainage system that shouldn’t be there, or should be there in smaller quantities.
What to Avoid Putting Down Your Drains
- Cooking fat, oil, and grease: these cool and solidify inside pipework and are the single biggest cause of kitchen drain build-up
- Wet wipes: even those labelled “flushable” don’t break down the way toilet paper does; they catch on any rough surface in the pipe and accumulate
- Food debris: scrape plates into the bin before washing; even small particles accumulate over time
- Excess hair: fit a drain cover over your bath and shower plughole and clear it regularly
Maintenance Habits That Help
Running hot water down kitchen drains for 30 seconds after washing up helps keep grease in suspension rather than letting it settle. A monthly hot-water flush is a reasonable habit for any property.
For landlords managing multiple properties, or commercial operators with high-volume drainage, a scheduled maintenance programme is the more practical approach. Periodic High-Pressure Jetting keeps pipe bores clear before blockages form, rather than responding to them after the fact.
Persistent smells after doing all of the above usually indicate a deeper problem. At that point, prevention isn’t enough: you need a professional drain inspection to find what’s actually causing it.
Conclusion
A drain that smells is telling you something. In most cases it’s one of a small number of identifiable causes, each with a specific fix. Dry water traps and minor build-up can often be addressed yourself. Persistent smells, sewage-strength odours, slow drainage, or smells coming from outside point to something more significant.
The right approach is to identify the cause accurately before spending time or money on a fix that might not address it. That’s what a CCTV Drain Survey does.
Key takeaways:
- Dry P-traps and organic build-up are the most common causes, and often the most straightforward to address
- Sewage smells inside your property suggest a trap or ventilation fault that needs professional diagnosis
- External smells near manholes or gullies indicate a blockage or structural issue in the external line
- Persistent smells rarely resolve without intervention
If the smell has been there for more than a few days, or if it’s strong enough to notice throughout your property, don’t wait. Call Clear Stream on 01872 222555 or visit clearstreamdrainage.co.uk to book an inspection. No call-out fee, fixed-price quote, and on-site within 1-2 hours anywhere in Cornwall or Devon.
Frequently Asked Questions
Why does my drain smell like rotten eggs?
A rotten egg smell from a drain is caused by hydrogen sulphide gas, produced when organic material decomposes inside your pipework or drainage system. It’s the same compound responsible for the characteristic sewage smell. It indicates organic build-up, a partial blockage, or sewer gas entering through a compromised water trap. If the smell is strong or persistent, get a professional inspection rather than trying to mask it.
Is a smelly drain a health risk?
Sewer gas contains hydrogen sulphide, ammonia, and methane. In normal domestic concentrations, it’s unpleasant rather than acutely dangerous. However, in poorly ventilated spaces or where a fault allows significant volumes of gas to accumulate, it can cause headaches, nausea, and in extreme cases more serious effects. A strong, persistent sewage smell inside your home is worth treating as a priority, not something to leave and monitor.
Why does my drain smell after it rains?
Rain increases ground water pressure, which can force gases up through cracks or gaps in external pipework and into inspection chambers. Heavy rainfall can also overwhelm partial blockages and stir up decomposing material that was previously sitting undisturbed. If your drain consistently smells after rain, that pattern points toward a fault in the external drainage line, often a cracked pipe or displaced joint. A CCTV survey will confirm it.
Why does the drain smell but drain normally?
If water is draining freely but there’s still a smell, the most likely cause is a dry or faulty water trap rather than a blockage. It could also indicate a ventilation fault in the soil vent pipe, allowing pressure changes to siphon the trap dry, or a crack in the pipe downstream allowing gas to escape. Run water in the affected fitting for 30-60 seconds. If the smell goes but returns within a few days, the trap isn’t holding water properly and needs inspection.
Can I fix a smelly drain myself?
For a dry trap or mild kitchen build-up, yes. Refilling a P-trap by running the tap is a straightforward fix. A hot-water-and-bicarbonate-of-soda flush can clear light kitchen residue. Beyond those two remedies, DIY chemical treatments carry a risk of pipe damage and rarely address the underlying cause. If the smell is coming from sewage, is present in multiple locations, or keeps returning, a professional inspection is the right next step.
How often should I have my drains inspected to prevent smells?
For a standard residential property on mains drainage with no previous history of problems, a CCTV drain survey every 3-5 years is a reasonable maintenance interval. Landlords with multiple properties, holiday lets with high occupancy turnover, and commercial premises with heavy drainage use benefit from shorter intervals, typically annually. Properties in rural Cornwall on private drainage systems including septic tanks should schedule an inspection more regularly given the higher maintenance requirement of private systems.
Why does my drain smell worse in summer?
Heat accelerates bacterial activity and decomposition inside pipework, which increases gas production. Warmer air also holds less moisture relative to cooler underground conditions, which can dry out P-traps faster in summer particularly in rooms that aren’t used daily. High summer temperatures also make any existing smell more detectable. If the smell intensifies significantly in warm weather, check that infrequently used fittings have their traps refilled, and consider having the drainage line inspected if the smell is strong.
