Why does my sink smell bad?
19 June 2026
Why Does My Sink Smell Bad?
That unpleasant odour rising from your kitchen or bathroom sink is one of the most common — and most complained-about — household plumbing problems. The smell can range from a mild mustiness to a full-blown rotten egg or sewage stench that fills the room. The good news is that most smelly sink causes are entirely fixable, often without calling a plumber. But some are signs of a more serious plumbing issue that should not be ignored.
This guide covers every common cause of a bad-smelling sink, practical solutions you can try yourself, and when the problem requires professional attention.
Is a Smelly Sink Dangerous?
In most cases, a smelly sink is unpleasant rather than dangerous. However, some causes — particularly sewer gas — can pose genuine health and safety risks.
Sewer gas is a mixture of gases produced by the decomposition of organic waste in the sewer system. It contains:
- Hydrogen sulphide (H₂S): The gas responsible for the characteristic "rotten egg" smell. At low concentrations it is merely unpleasant; at high concentrations it is toxic and potentially lethal.
- Methane: Odourless and colourless, but highly flammable and explosive in enclosed spaces.
- Ammonia, carbon dioxide, and other compounds: Collectively irritating to the respiratory system with prolonged exposure.
If the smell in your bathroom or kitchen is strong, persistent, and sulphurous — or if you feel nauseous, have headaches, or experience dizziness near the sink — ventilate the space immediately and call a licensed plumber. Do not use open flames near drains until the source has been identified and resolved.
The Most Common Causes of a Smelly Sink — and How to Fix Them
Cause 1: Build-Up of Food Waste, Grease, and Organic Matter in the Drain
What causes it:
This is the single most common cause of a smelly kitchen sink. Every time you wash dishes, food particles, cooking grease, oils, and organic residue rinse down the drain. Over time — particularly in the P-trap (the curved pipe section below the sink) and along the walls of the drain pipe — this organic matter accumulates. As bacteria break it down, the decomposition produces sulphur compounds and other foul-smelling gases that rise back up through the drain opening.
The smell is typically described as musty, rotting, or generally "drain-like." It tends to be worse in warm weather, when bacterial activity accelerates.
How to fix it:
The good news is this is one of the easiest fixes in home plumbing.
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Boiling water flush: Carefully pour a full kettle of boiling water slowly down the drain. This melts and dislodges grease deposits. Repeat two to three times. Note: only do this with metal pipes — if you have PVC or plastic drain pipes, use very hot (but not boiling) water to avoid softening the pipe.
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Baking soda and white vinegar: Pour half a cup of baking soda (bicarbonate of soda) directly into the drain, followed immediately by half a cup of white vinegar. The fizzing reaction helps lift and break down organic deposits on the pipe walls. Leave it for 15–30 minutes, then flush with hot water.
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Dish soap and hot water: Squirt a generous amount of dish soap into the drain and follow with a full kettle of hot water. The degreasing agents in dish soap are effective at cutting through accumulated grease.
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Clean the P-trap: Place a bucket under the P-trap (the curved pipe section under the sink), unscrew the slip-joint nuts on either side, remove the P-trap, and clean it out manually. Rinse it thoroughly before reinstalling.
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Regular maintenance: Prevention is better than cure. Run hot water for 30 seconds after every use of the sink. Scrape plates before washing and avoid pouring cooking grease or oils down the drain — ever.
Cause 2: A Dry or Empty P-Trap
What causes it:
The P-trap — the U-shaped or S-shaped curved pipe directly below your sink — is one of the most important yet least understood components of your home's plumbing system. It is specifically designed to hold a small amount of water at all times, creating a water seal that physically blocks sewer gases from rising up through the drain and into your home.
When the P-trap dries out — most commonly in a sink, bath, or basin that hasn't been used for an extended period — the water evaporates. Without the water seal, sewer gases rise freely up through the drain pipe and into the room. The smell is typically described as strongly sulphurous, like rotten eggs, or as a distinct sewage odour.
This is an extremely common cause of bad smells in:
- Guest bathroom sinks that are rarely used
- Holiday homes or properties left vacant
- Utility room sinks that are only used occasionally
- Floor drains in garages or laundry rooms
How to fix it:
The fix could not be simpler — run the tap for 20–30 seconds to refill the P-trap. The water seal will be restored immediately and the smell should disappear within minutes.
To prevent it from happening again in a sink that is rarely used, pour a small amount of water down the drain every two to three weeks. Adding a tablespoon of cooking oil to the water helps the seal last longer by reducing evaporation.
Cause 3: A Blocked or Slow-Draining Sink
What causes it:
A partial blockage in the drain — caused by accumulated hair (in bathroom sinks), soap scum, toothpaste residue, food particles, or a combination of all of these — creates a breeding ground for odour-producing bacteria. Slow-draining water sits in the pipe longer than it should, giving organic matter more time to decompose and produce bad smells.
The smell associated with a blocked or slow drain is typically musty and stagnant — a "blocked drain smell" that is distinctly different from the sharp rotten-egg sulphur of sewer gas.
How to fix it:
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Remove and clean the drain cover or plug: Hair and soap scum often accumulate just inside the drain opening, around and underneath the plug mechanism. Use a pair of needle-nose pliers, a drain hook, or even a bent wire coat hanger to remove accumulated debris.
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Use a drain plunger: A standard cup plunger can dislodge soft blockages. Cover the overflow outlet (the small hole near the top of the basin) with a wet cloth before plunging — this ensures the suction acts on the blockage rather than escaping.
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Drain cleaning solution: Commercial enzyme-based drain cleaners break down organic matter without the harsh chemical effects of caustic drain cleaners. Enzyme cleaners are safer for pipes and are a better long-term solution than chemical alternatives.
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A drain snake or hair catcher tool: A flexible drain snake or purpose-made hair catcher tool (widely available at hardware stores) can physically remove the blockage.
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Fit a drain hair catcher: A simple, inexpensive hair catcher fitted over the drain opening prevents hair and debris from entering the pipe in the first place. This is especially valuable in bathroom and shower sinks.
Cause 4: Mould and Mildew in the Drain or Around the Sink
What causes it:
Bathrooms and kitchens are high-humidity environments — exactly the conditions that mould and mildew thrive in. Mould can grow inside the drain pipe, on the underside of drain covers, inside overflow channels, and in the silicone sealant around the base of the sink. As mould grows and is disturbed by water flow, it releases spores and volatile organic compounds that produce a characteristic musty, earthy, or "damp" smell.
Mould inside the drain is particularly common in bathrooms where the drain receives warm water and organic matter (soap, hair, skin cells) regularly.
How to fix it:
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Clean the overflow channel: Your basin almost certainly has an overflow hole — a small opening near the top of the basin that prevents water spilling onto the floor. The channel behind this hole is one of the most neglected spots in bathroom cleaning, and it is a prime location for mould growth. Pour a diluted bleach solution (one part bleach to ten parts water) or white vinegar into the overflow hole and leave it for 30 minutes. Flush with clean water.
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Clean or replace drain covers: Remove the drain cover and scrub it with an old toothbrush and a diluted bleach or baking soda solution.
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Replace mouldy silicone sealant: If the silicone bead around the base of your sink or where it meets the wall has gone black with mould, scrape it out completely and re-apply fresh silicone sealant. This is a simple DIY task that makes a significant difference.
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Improve ventilation: Fit an extractor fan if the bathroom doesn't have one, or ensure existing fans are clean and operational. Good airflow dramatically reduces the humidity that mould needs to grow.
Cause 5: A Faulty, Cracked, or Missing P-Trap
What causes it:
Even when the P-trap contains water, it can allow sewer gas to pass through if:
- The P-trap has a crack or fracture in the pipe material — particularly in older plastic traps that become brittle over time
- The slip-joint connections at either end of the P-trap are loose and allowing gas to escape around the fittings
- The P-trap has been incorrectly installed (e.g. incorrectly angled, too long, or connected with the wrong fittings) and is unable to maintain a proper water seal
- The P-trap is absent entirely — in very old properties or after amateur plumbing work, the P-trap may have been removed and not replaced
How to identify it:
Inspect the P-trap visually. Look for:
- Visible cracks, discolouration, or mineral deposits on the trap body
- Moisture or staining around the slip-joint connections (indicating a slow leak)
- An incorrectly angled or poorly fitted trap
If the P-trap appears intact but the sewer smell persists despite the trap being full of water, the trap itself may be cracked and require replacement.
Solution:
P-trap replacement is a straightforward plumbing repair. For plastic traps (the most common type in South African homes), the components are widely available at hardware stores and the replacement can be done without specialist tools. Turn off the water supply, place a bucket under the trap, and replace like-for-like. If you are unsure, a licensed plumber can complete this repair quickly and inexpensively.
Cause 6: Blocked or Poorly Vented Drain Pipes
What causes it:
Every drain in your home is connected to a vent pipe system — a network of pipes that runs from the drain lines up through the roof of the building, open to the atmosphere. These vent pipes serve two critical functions:
- They allow air into the drain system, which enables water to flow freely without creating a vacuum (the "glug-glug" effect in a slow drain is a sign of inadequate venting)
- They allow sewer gases to escape harmlessly to the atmosphere through the roof, rather than building up pressure in the pipes and being forced back into the living space through drains, toilets, and traps
When vent pipes become blocked — by bird nests, leaves, debris, or even a dead animal — sewer gas can no longer escape upward and instead finds its way into the home through the path of least resistance: your sink drains.
Signs of a venting problem:
- A gurgling sound from the sink, bath, or toilet when other drains are used
- Multiple sinks or drains smelling simultaneously
- The toilet "gurgles" when you run the sink
- Slow drainage from multiple fixtures at once
- The sewer smell gets worse when it is windy (wind affects the pressure balance in the vent pipe system)
Solution:
Vent pipe blockages require roof access and are not a DIY job. A licensed plumber can inspect and clear blocked vent pipes. In some cases, additional venting may need to be installed if the existing system is undersized or incorrectly configured. A PIRB-registered plumber can assess and resolve venting issues and issue the required Certificate of Compliance.
Cause 7: Sewer Gas from the Municipal Sewer System
What causes it:
In some cases, particularly in older neighbourhoods with ageing sewer infrastructure, sewer gas can enter the home's drain system from the municipal sewer line. This is more common when:
- The municipal sewer main is blocked or under excess pressure, causing gas to back up into property drain connections
- The sewer line on the property has a crack, fracture, or root intrusion allowing gas to escape and migrate through the soil to the property's drain pipes
- The inspection chamber (manhole or rodding eye) on the property has a cracked or missing cover seal
Signs this is the cause:
- The smell is strongest near the floor drain, toilet, or lowest drain point in the house rather than specifically at the sink
- The smell is intermittent and seems to correlate with municipal sewer activity (e.g. worse after heavy rain when sewer systems are under pressure)
- Multiple properties in the street report similar smells simultaneously
Solution:
Report the issue to Johannesburg Water (0860 562 874) if you suspect a municipal sewer problem. For private sewer line issues on your property, a licensed plumber with a CCTV drain inspection camera can identify cracks, root intrusion, or joint failures and recommend the appropriate repair.
Cause 8: Washing Machine or Dishwasher Drain Connection Issues
What causes it:
In many South African kitchens, the washing machine and dishwasher drain hoses connect directly into the kitchen sink drain. If these connections are incorrectly plumbed — specifically, if the drain hose connects below the water level of the P-trap rather than above it — the P-trap cannot maintain a proper water seal, allowing sewer gas to travel back through the appliance drain hose and into the kitchen.
Additionally, the washing machine or dishwasher drain hose itself can accumulate residue, lint, detergent build-up, and mould — particularly if the hose is kinked or has a low point that traps standing water.
Signs this is the cause:
- The smell in the kitchen is worse after the washing machine or dishwasher has completed a cycle
- The drain hose runs along the floor with a dip or kink rather than looping up high before connecting to the drain
- There is a musty or sour smell that seems to come from under the kitchen counter rather than the sink basin
Solution:
Ensure the washing machine and dishwasher drain hoses are connected with a proper high loop — the hose should loop up as high as possible under the counter before connecting to the drain, creating a break that prevents sewer gas from travelling back through the hose. Alternatively, fit an air gap device at the connection point. Clean the drain hose periodically with a hot water and vinegar flush.
A Quick-Reference Diagnosis Guide
| Smell Type | Most Likely Cause |
|---|---|
| Rotten eggs / sulphur | Dry P-trap or sewer gas |
| General "drain smell" / musty | Organic build-up, grease, or food residue |
| Sewage / faecal odour | Blocked vent pipe or sewer line issue |
| Damp / earthy / mouldy | Mould in drain, overflow channel, or silicone |
| Sour / chemical | Washing machine drain hose or detergent residue |
| Intermittent sulphur — worse in wind | Vent pipe blockage |
Step-by-Step: How to Fix a Smelly Sink Yourself
Work through these steps in order before calling a plumber:
Step 1 — Run the tap for 30 seconds. If the P-trap has dried out, this alone will fix it.
Step 2 — Clean the drain opening. Remove the drain cover or stopper and clear out any visible hair, soap scum, or debris.
Step 3 — Baking soda and vinegar treatment. Half a cup of baking soda followed by half a cup of white vinegar. Leave 15–30 minutes, then flush with hot water.
Step 4 — Clean the overflow channel. Pour diluted white vinegar or diluted bleach into the overflow hole and leave for 30 minutes. Flush with clean water.
Step 5 — Flush with hot water and dish soap. Run the hottest water your tap produces for two minutes with a generous squirt of dish soap.
Step 6 — Inspect and clean the P-trap. Place a bucket underneath, remove the P-trap, and clean it manually. Reinstall and check connections are tight.
Step 7 — Check the condition of silicone sealant. Replace any black or mouldy sealant around the sink base and splashback.
Step 8 — If the smell persists, call a plumber. Persistent sewer smells — particularly rotten egg or sewage odours that don't respond to the above — indicate a venting problem, cracked pipe, or sewer line issue that requires professional diagnosis.
Smelly Sink Repair Costs in South Africa
| Repair / Service | Typical Cost Range |
|---|---|
| DIY baking soda & vinegar treatment | Free |
| Enzyme drain cleaner (product) | R80 – R250 |
| P-trap replacement (DIY) | R60 – R200 (parts only) |
| P-trap replacement (plumber) | R400 – R1,200 |
| Drain unblocking (plumber) | R500 – R1,500 |
| Overflow channel clean (plumber) | R300 – R800 |
| Vent pipe inspection & clearing | R1,500 – R4,000 |
| CCTV drain inspection | R2,000 – R5,000 |
| Sewer line repair | R3,500 – R15,000+ |
Prices are indicative and vary by area and plumber. Always request a written quote.
How to Prevent a Smelly Sink — Ongoing Maintenance Tips
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Never pour cooking oil or grease down the drain. Collect it in a container and dispose of it in the bin. Grease is the single biggest contributor to drain build-up and bad odours.
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Use a hair catcher in bathroom sinks and showers. Hair is the primary cause of bathroom drain blockages and the odours that follow.
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Run hot water for 30 seconds after every use. This flushes organic matter through the pipe before it can accumulate.
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Clean your drain monthly with a baking soda and white vinegar treatment — it takes less than five minutes and prevents odour-causing build-up.
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Clean the overflow hole monthly. Pour white vinegar into it and leave for 30 minutes before flushing. This is the most overlooked maintenance task in the bathroom.
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Run unused sinks regularly. Guest basins and utility sinks should be run for 30 seconds every two to three weeks to keep the P-trap water seal intact.
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Inspect silicone sealant annually. Replace any mouldy or cracked sealant to prevent mould growth from contributing to odours.
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Schedule a professional drain clean annually if you have persistent odour problems. A plumber can jet the drain clear and inspect the vent system, catching problems before they worsen.
When Should You Call a Plumber for a Smelly Sink?
Call a licensed, PIRB-registered plumber if:
- The smell is a strong, persistent rotten egg or sewage odour that does not improve after cleaning and refilling the P-trap
- Multiple sinks or drains in the home smell simultaneously
- You hear gurgling from the toilet or other drains when you use the sink
- The smell is accompanied by slow drainage that does not respond to DIY clearing
- You suspect a cracked pipe, missing P-trap, or venting problem
- Anyone in the household experiences headaches, nausea, or dizziness near the affected sink — evacuate and ventilate the space immediately
Need a Plumber in Johannesburg for a Smelly Drain?
If you've worked through this guide and the smell persists — or if you suspect a more serious plumbing problem — Joburg Plumbers connects you with verified, PIRB-registered plumbers across all areas of Johannesburg, including Sandton, Randburg, Roodepoort, Midrand, Fourways, Soweto, Boksburg, Germiston, Kempton Park, and across the East Rand, West Rand, Joburg North, and Joburg South regions.
Don't live with a smelly sink — most causes are fixable quickly, affordably, and permanently.
This article is intended as general guidance only. Always use a PIRB-registered plumber for drain repairs, P-trap replacement, and venting work in South Africa.