Why do my drains stink?
26 June 2026
Why Do My Drains Stink?
That foul smell rising from your kitchen sink, bathroom basin, shower drain, or outdoor drain is one of the most unpleasant — and most common — plumbing complaints in South African homes. The odour can range from a musty staleness to a full sewage stench or sharp rotten-egg sulphur that permeates the entire room. And unlike a blocked drain, a smelly drain can be easy to ignore — until it gets much worse.
The good news: in most cases, a smelly drain has a specific, identifiable cause — and a straightforward fix. This guide covers every common source of drain odours in kitchen drains, bathroom drains, and outdoor drains, what each smell means, and exactly what to do about it.
What Type of Smell Is It? Start Here
The character of the odour tells you a great deal about its source before you investigate anything physically. Use this quick reference to narrow down the cause:
| Smell Description | Most Likely Source |
|---|---|
| Rotten eggs / sulphur | Dry P-trap or sewer gas rising from the drain line |
| Musty / stale / earthy | Mould, biofilm, or organic build-up in the drain |
| General "drain smell" | Grease, food, hair, and soap scum decomposing in pipe |
| Sewage / faecal odour | Blocked vent pipe, sewer line issue, or failed trap |
| Sour / chemical | Washing machine drain backflow or detergent residue |
| Intermittent sulphur — worse in wind | Blocked or partially blocked vent pipe on the roof |
| Outdoor drain smell strongest after rain | Blocked outdoor drain or sewer overflow |
Kitchen Drain Smells — Causes and Fixes
Cause 1: Grease, Food, and Organic Build-Up in the Drain
This is the most common cause of kitchen drain smells by a considerable margin — and it is entirely preventable.
Every time you wash dishes, rinse pots, or run water over food-covered surfaces, small amounts of cooking grease, food particles, oils, soap, and organic residue enter the drain. In the P-trap and along the pipe walls, this material accumulates over weeks and months into a thick, odorous biofilm. At ambient South African temperatures — particularly in Gauteng summers — this biofilm decomposes rapidly, producing sulphur compounds, ammonia, and volatile organic compounds that rise back up through the drain opening.
The smell is typically described as musty, rotting, or generally "drain-like." It worsens in warm weather and after the kitchen has been inactive for a period.
What to do:
- Boiling water flush (metal pipes only): Pour a full kettle of boiling water slowly down the drain. This melts grease deposits and flushes them through the pipe. For PVC plastic pipes, use very hot (not boiling) water to avoid softening the pipe material.
- Baking soda and white vinegar: Pour half a cup of bicarbonate of soda directly into the drain, immediately followed by half a cup of white vinegar. The fizzing reaction lifts and breaks down organic deposits on the pipe walls. Leave for 20–30 minutes, then flush with hot water.
- Dish soap and hot water flush: Squirt a generous amount of dish soap into the drain and follow with a full kettle of very hot water. The degreasing agents in dish soap cut through accumulated grease effectively.
- Clean the P-trap: Place a bucket under the curved pipe section below the kitchen sink, unscrew the slip-joint connections, and clean the P-trap manually. This is where the majority of grease and food residue accumulates.
Prevention: Never pour cooking oil, fat, or grease down the kitchen drain. Collect it in a container and dispose of it in the bin. Fit a kitchen sink strainer to catch food particles before they enter the drain. Run hot water for 30 seconds after every use of the sink.
Cause 2: The Kitchen Sink Overflow Channel
Almost every kitchen sink has an overflow opening — a small slot or series of holes near the top of the sink basin that prevents water spilling onto the counter if the sink fills too high. The internal channel behind this overflow opening is almost never cleaned, even by meticulous homeowners — and it is an extremely productive environment for mould, bacteria, and biofilm.
Water enters the overflow channel every time the sink is used and sits in the channel between uses. This creates a constantly damp, food-particle-enriched space where odour-producing bacteria and mould thrive unchallenged.
What to do: Pour a diluted white vinegar solution (undiluted if possible) or a diluted bleach solution (1 part bleach to 10 parts water) into the overflow opening. Leave for 30 minutes, then flush with clean water. Use a small bottle brush or a pipe cleaner inserted into the overflow opening to physically scrub the channel every few months.
Cause 3: Washing Machine or Dishwasher Drain Connection
In many South African homes, the washing machine and dishwasher drain hoses connect into the kitchen sink drain. If this connection is incorrectly installed — specifically, if the drain hose connects below the P-trap water level rather than above it, or if the hose lacks a proper high loop — the P-trap seal can be broken, allowing sewer gas to travel back through the appliance drain hose into the kitchen.
The washing machine drain hose itself can also accumulate residue, lint, mould, and detergent build-up — particularly if the hose has a low point or kink that traps standing water between wash cycles.
Signs this is the cause: The smell in the kitchen is notably worse directly after the washing machine or dishwasher completes a cycle. The odour seems to come from under the counter rather than from the sink basin itself.
What to do: Ensure the washing machine and dishwasher drain hoses are installed with a proper high loop — the hose should rise as high as possible under the counter before connecting to the drain, preventing sewer gas from travelling backwards. An air gap device at the connection point provides additional protection. Clean the drain hose periodically with a hot water and white vinegar flush.
Bathroom Drain Smells — Causes and Fixes
Cause 4: A Dry or Empty P-Trap
This is the single most common cause of sewer smell — specifically the rotten egg or sewage odour — in a bathroom, and the simplest to fix.
The P-trap is the U-shaped or S-shaped curved pipe directly below every sink, basin, bath, and shower drain. It is specifically designed to hold a small amount of water at all times, creating a physical water seal that blocks sewer gases from rising up through the drain pipe and into the living space.
When a fixture is not used for an extended period — common in guest bathrooms, holiday homes, and utility sinks — this water seal evaporates. Without it, there is a direct open pathway from the sewer system into your bathroom.
The P-trap seal evaporates faster in hot, dry conditions — which makes this problem particularly common in Johannesburg during the dry, warm winter months.
What to do: Run the tap for 20–30 seconds to refill the P-trap. The smell should disappear within minutes. For fixtures that are rarely used, run water through them every 2–3 weeks to keep the seal intact. Adding a tablespoon of cooking oil to the water helps slow evaporation and extends the seal's longevity.
Cause 5: Hair, Soap Scum, and Biofilm in Bathroom Drains
Bathroom drains — particularly shower and bath drains — accumulate hair, soap scum, skin cells, shampoo residue, and conditioner over time. This organic material decomposes in the warm, damp drain environment, producing the musty, stagnant smell characteristic of a bathroom drain that hasn't been cleaned recently.
The drain cover itself, the underside of the drain plug, and the first section of drain pipe immediately below the opening are often coated with a thick black or grey biofilm that is the primary odour source.
What to do:
- Remove the drain cover and clean it thoroughly — both surfaces — with an old toothbrush and diluted bleach or white vinegar solution.
- Use needle-nose pliers or a drain hook to remove accumulated hair from just inside the drain opening.
- Pour a baking soda and white vinegar treatment down the drain (as described above) to address the biofilm on pipe walls.
- Use a drain snake to remove deeper hair and soap scum blockages.
- Fit a drain hair catcher over the shower and bath drain. This single, inexpensive step prevents the majority of bathroom drain build-up before it starts.
Cause 6: The Bathroom Basin Overflow Channel
Identical in mechanism to the kitchen sink overflow, but often even more neglected. The basin overflow channel accumulates toothpaste residue, soap, skin cells, and moisture — creating a mouldy, bacteria-rich environment that produces a characteristic musty or sour smell that seems to come from the basin itself rather than specifically the drain.
What to do: Use a small brush or pipe cleaner to physically scrub the overflow channel. Flush with diluted white vinegar or diluted bleach. This is a task that should be done monthly as part of bathroom cleaning — but is almost universally overlooked.
Cause 7: Mouldy or Cracked Silicone Sealant Around the Bath or Shower
Mouldy black silicone sealant around the base of the bath, at the bath-wall junction, or around the shower tray contributes significantly to bathroom odours. As mould grows in the silicone and is disturbed by water and air movement, it releases musty volatile compounds that are often mistaken for drain odours.
What to do: Scrape out all black or deteriorating silicone sealant completely using a silicone removal tool or a craft knife. Clean the exposed surface with diluted bleach to kill any residual mould. Apply fresh sanitary-grade silicone sealant. This is a straightforward DIY task that makes a significant difference to bathroom air quality.
Outdoor Drain Smells — Causes and Fixes
Cause 8: Blocked or Partially Blocked Outdoor Drain
Outdoor drains — including yard drains, patio drains, driveway drains, and garden stormwater drains — collect leaves, soil, organic debris, algae, and in many Johannesburg homes, car wash residue and garden product run-off. This organic material compacts in the drain chamber and trap, decomposes in the warm environment, and produces a range of unpleasant odours including musty, sulphurous, and sewage-adjacent smells.
The smell from a blocked outdoor drain is often most noticeable after rain — when water flushes into the drain and disturbs the decomposing material at the bottom, releasing trapped gases.
What to do:
- Lift the drain cover and inspect the chamber. Remove visible debris, leaves, and compacted material manually using gloves.
- Flush the drain with a strong jet of water from a garden hose to clear the drain channel.
- If the drain is connected to the stormwater system and is persistently slow or blocked, call a licensed plumber for professional high-pressure drain jetting.
- Clean outdoor drain covers monthly in Johannesburg's summer (October–April) when leaf fall and garden activity generate more debris.
Cause 9: A Dry Outdoor Floor Trap or Yard Trap
Many South African homes have floor traps or yard traps — small drain grates in outdoor areas, garages, laundry areas, and utility spaces. Like indoor P-traps, these contain a water seal that prevents sewer gases from rising. In an outdoor area that is rarely washed down — particularly during Johannesburg's dry winter season — the water seal in a yard trap evaporates, creating a direct pathway for sewer odours to rise.
What to do: Pour water into the outdoor floor trap to restore the water seal. In dry periods, do this weekly for outdoor traps in areas that do not receive regular water from rain or hosing. A small amount of cooking oil added to the water helps slow evaporation in exposed outdoor locations.
Cause 10: Sewer Line Problems — The Serious Cause
If the drain smell is strongest at low points — outdoor floor drains, ground-level bathroom drains, or the lowest drain in the home — and is persistent despite cleaning and trap refilling, a more serious underlying sewer line issue may be present.
Possible sewer line causes include:
- A cracked or broken sewer pipe on the property, allowing sewer gas to escape through the soil and enter the home or outdoor area
- Tree root intrusion into the sewer line — roots crack pipe joints and allow gas to escape. Common in Johannesburg's older suburbs where large established trees are prevalent
- A collapsed pipe section causing sewage to pool in the soil, producing persistent ground-level odours
- A blocked or faulty inspection chamber (manhole) on the property with a damaged or missing seal on the cover
Signs of a sewer line problem:
- Smell is persistent despite all cleaning and maintenance
- Smell is strongest outdoors, at ground level, or over a specific area of the garden
- Multiple drains in the home smell simultaneously
- Wet, boggy, or unusually lush patches in the garden
- Gurgling sounds from multiple drains when water is used
What to do: Call a PIRB-registered plumber for a professional drain inspection. A licensed plumber with a CCTV drain inspection camera can identify cracks, root intrusion, joint failures, and collapsed sections within the sewer line. A high-pressure water jetter can clear root intrusion and blockages. Any crack or structural failure requires professional repair, and the plumber must issue a Certificate of Compliance (CoC).
The One Cause Affecting All Drains: A Blocked Vent Pipe
Every drain system in a South African home is connected to a vent pipe — a pipe that runs from the drain lines through the roof of the building and opens to the atmosphere. This vent pipe serves two critical functions:
- It allows air into the drain system so that water can flow freely without creating a vacuum
- It allows sewer gases to escape harmlessly to the atmosphere above the roof level, rather than building up in the pipe system and being forced back into the home through drain openings
When a vent pipe becomes blocked — by a bird's nest, accumulated leaves and debris, a dead animal, or incorrect installation — sewer gases can no longer escape upward. They instead take the path of least resistance: back down into the pipe system and up through your drain openings.
Signs of a blocked vent pipe:
- Gurgling or bubbling sound from the toilet, basin, or other drains when water is used
- Multiple drains in the house smell simultaneously
- The sewer smell is intermittent and seems to worsen when it is windy outside
- Slow drainage from multiple fixtures at the same time
What to do: A blocked vent pipe requires roof access and professional equipment. This is not a DIY task. Call a PIRB-registered plumber to inspect and clear the vent pipe. In some cases, the existing vent pipe may be undersized or incorrectly configured and may need to be extended or augmented.
When to Call a Plumber for a Smelly Drain
Most smelly drain problems respond to the DIY treatments above. Call a licensed, PIRB-registered plumber immediately if:
- The smell is a strong, persistent rotten egg or sewage odour that does not improve after cleaning and refilling the P-trap
- Multiple drains in the home are smelling simultaneously
- You hear gurgling sounds from one drain when another is used
- The smell is strongest outdoors or at ground level — indicating a possible sewer line issue
- Anyone in the household experiences headaches, nausea, or dizziness near the drain — these are symptoms of hydrogen sulphide or methane exposure; ventilate immediately and call a plumber
A safety note: Sewer gas is not merely unpleasant. It contains hydrogen sulphide (H₂S) — the rotten egg gas — which at low concentrations is only irritating, but at elevated concentrations in enclosed spaces is toxic and potentially fatal. It also contains methane, which is odourless, colourless, and explosive. A persistent, strong sewer gas smell warrants urgent professional investigation.
Your Drain Smell Prevention Routine
Weekly:
- Run water through all drains and traps in the home — including guest bathrooms, utility sinks, and outdoor floor traps
- Clear outdoor drain covers of leaves and debris
Monthly:
- Baking soda and white vinegar treatment for kitchen and bathroom drains
- Clean basin and kitchen sink overflow channels with vinegar or diluted bleach
- Remove and clean drain covers in showers and baths
Every 3 months:
- Clean the P-trap under the kitchen sink manually
- Descale kitchen and bathroom tap aerators
- Inspect outdoor drain chambers for accumulated debris
Annually:
- Professional drain inspection and maintenance — a licensed plumber can jet drains clean and inspect the vent system
- Check all silicone sealant for mould — replace where necessary
Need a Plumber in Johannesburg for a Smelly Drain?
If your drain smells persist after following this guide — or if you suspect a blocked vent pipe, cracked sewer line, or tree root intrusion — Joburg Plumbers connects you with verified, PIRB-registered plumbers and drain specialists 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.
Most drain odour problems are fixable quickly, affordably, and permanently — once you know the cause.
This article is intended as general guidance only. Always use a PIRB-registered plumber for drain repairs, vent pipe work, sewer line inspections, and any plumbing installation in South Africa. If you suspect sewer gas exposure, ventilate the space immediately before taking any other action.