Bad Smells From Drains: Causes and How to Fix Them
A bad smell rising from a sink, bath or toilet is one of the most disliked household problems, partly because it travels far beyond the room where the drain sits. Drain smells are never normal, but they are also rarely a sign of disaster. Most causes are mechanical (a dry trap, a broken vent, a layer of biofilm) and have straightforward fixes. This guide walks through the four common smell types we hear about across north London, what is causing each one, and what works to clear them. It also covers why pouring chemical drain cleaner on a smell often makes the situation worse.
Common smell types and what they mean
The first step is identifying the smell. Different chemistry produces different odours, and the source narrows down quickly once you put a name to what you are smelling.
Sewage smell
A clear sewage smell, sometimes described as a damp, faecal, mildly farmyard scent, means sewer gas is escaping into the room. The most likely cause is a dry trap or a broken vent system. Sewer gas is mostly methane, carbon dioxide and trace hydrogen sulphide; it is unpleasant but not generally dangerous at the concentrations found in households.
Rotten egg smell
The sharp, eye-watering sulphurous smell of rotten eggs is hydrogen sulphide, a by-product of bacteria breaking down organic matter in low-oxygen conditions. Hydrogen sulphide usually means there is a build-up of biofilm or sediment somewhere in the drainage system, often in a section that drains infrequently.
Musty or earthy smell
A musty smell suggests moisture in walls, under flooring or behind boxing-in, rather than the drain itself. It can be triggered by a slow leak on a waste pipe, a defective seal at the back of the toilet, or condensation in a poorly ventilated bathroom. The drain may be the symptom or the source, depending on context.
Ammonia smell
An ammonia smell, particularly around toilets, usually points to old urine residue under the toilet rim, in tile grouting nearby, or in a poorly cleaned drain. It can also appear when cat or pet urine has reached an unsealed flooring section. This one is rarely the drain itself.
Cause 1: Dry P-trap or U-bend
Every drain in the property has a water-filled trap that seals against sewer gas. When a sink, bath, shower or floor gully sits unused for several weeks, the water in the U-bend evaporates and the seal disappears. Sewer gas rises freely into the room. This is the single most common cause of sudden sewage smells in second bathrooms, utility rooms, basements and holiday properties.
The fix is simple: run water in every fixture for thirty seconds, including floor drains, washing machine standpipes and rarely-used basins. Add a teaspoon of cooking oil to floor gullies that evaporate quickly; the oil floats and slows evaporation. The smell usually clears within minutes of refilling the trap.
Cause 2: Broken or blocked vent stack
Vent pipes equalise air pressure during drainage. When the vent is blocked at the top, or cracked somewhere in its run through the property, draining one fixture sucks water out of nearby traps (siphonage) and lets sewer gas through. Symptoms include gurgling fixtures, drainage that briefly slows then resumes, and intermittent sewage smells in rooms that previously had none. Vent stack problems usually need a drainage engineer to identify and address, particularly if access is poor.
Cause 3: Biofilm build-up
Inside every waste pipe, bacteria gradually colonise the walls as a thin film. Over time the film thickens, traps organic matter and starts to produce hydrogen sulphide. The smell tends to be worst in showers, washing machine standpipes and kitchen sinks (anywhere warm water and organic residue meet regularly). It usually intensifies as the water flows because the bacterial layer gets disturbed.
The fix is mechanical, not chemical. A weekly flush with very hot water and bicarbonate of soda slows the growth. A more thorough clean involves removing the strainer, scrubbing the visible pipe with a bottle brush, and pouring a kettle of near-boiling water down the drain. Persistent biofilm in drains beyond the trap usually responds well to professional high-pressure water jetting, which scours the pipe wall in a way no household method can replicate. This is often the first step we recommend during drain cleaning in N16 for properties dealing with recurring smell problems.
Cause 4: Blocked main drain
A partial blockage in the main household drain can let sewer gas escape upstream of the restriction. If the smell is accompanied by slow drainage across multiple fixtures, or by water visible in the boundary inspection chamber, the main run is the source. Clearing the blockage usually resolves the smell within hours, although a stubborn deposit may need camera investigation to confirm full clearance.
Why DIY chemical cleaners make smells worse
Caustic soda-based drain cleaners are a popular first response to bad smells, but they often make the underlying situation worse.
- They kill the bacteria temporarily, but the dead biomass remains as a food source for the next colonisation, often making the smell return stronger.
- They damage older lead, copper and cast-iron pipework common in central and north London Victorian housing.
- They do not address mechanical issues like dry traps or broken vents.
- They can react with sulphurous compounds in the pipe to produce more hydrogen sulphide, not less.
The safe sequence is mechanical first (refill traps, clean strainers, flush with hot water), then biological (bicarbonate of soda, enzyme-based maintenance cleaners), then professional jetting if the smell persists. Caustic cleaners belong well down the priority list, and many drainage engineers will not work on pipework recently flooded with concentrated caustic without protective gear.
When the smell needs a professional
Persistent drain smells, particularly those accompanied by gurgling, slow drainage, or visible damp patches on walls or ceilings, are worth investigating properly. A CCTV survey identifies cracks, broken vents, biofilm hotspots and partial blockages in a single visit, allowing targeted intervention rather than guesswork. For older properties with original lead or cast-iron internal pipework, professional inspection is usually safer than trial-and-error DIY. Greater London Drainage covers north London for diagnosis and treatment, including wider plumbing service work where the smell turns out to be a plumbing rather than a drainage fault.
Are drain smells a health risk?
Sewer gas is unpleasant, and at high concentrations hydrogen sulphide can cause headaches, nausea or eye irritation. The concentrations found inside a normal household, even with a dry trap, are well below the levels typically associated with measurable harm, but small children, older adults and people with respiratory conditions may be more sensitive to the irritation. Persistent smells should not be ignored, particularly in basement flats and ground-floor properties where ventilation is poorer. If a household member experiences worsening headaches or breathing symptoms that coincide with the smell, ventilate the property thoroughly, identify the source promptly, and avoid sleeping in affected rooms until it is addressed. This is rarely an emergency, but it is not something to tolerate week after week either.
Smells from external gullies and inspection chambers
Not every drainage smell originates indoors. Open yard gullies, ungrouted inspection chamber covers and cracked downpipes can all release sewer gas around the outside of the property. Walk the perimeter on a still, mild day; smells are easier to locate when there is no wind. Common external sources include silted-up gullies that no longer hold a water seal, missing or perished gully gratings, and cracks in the bedding around the chamber lid. Re-bedding a chamber cover with proper sealant, replacing a damaged gully grating, or clearing a silted gully usually resolves the smell within minutes. For terraced properties where the smell travels between gardens, neighbour cooperation often speeds the diagnosis.
Final thoughts
Drain smells fall into a small number of categories with well-understood causes. Start with the simplest fix (running water through every drain to refill traps) before assuming the worst. If the smell returns within a week of mechanical cleaning, the cause is downstream of the trap and the right step is professional inspection rather than another bottle of caustic cleaner. Most smell complaints we attend across north London are resolved on the first visit once the underlying mechanism is identified, often without disruption to the household.
