How to Get Rid of Smelly Drains in the Bathroom

How to Get Rid of Smelly Drains in the Bathroom

A smelly bathroom drain has one of three causes: biofilm buildup inside the pipe, a dry p-trap that is no longer blocking sewer gas, or a partial blockage accumulating organic matter. The smell from each is similar but the fix is completely different, and treating the wrong cause achieves nothing.

Biofilm smell is a general sulphur or rotting odour present most of the time. A dry trap produces a sharp sewage smell that appears suddenly, typically after a guest bathroom or downstairs cloakroom has not been used for several weeks. A partial blockage produces a smell that worsens after the bath or sink has been used, as water disturbs the decomposing organic matter caught in the pipe.

Identifying which one you have before reaching for any product takes two minutes and directs the solution accurately.

Understanding the P-Trap

Every bathroom sink, bath, and shower in the UK has a p-trap fitted directly below the waste outlet. This is the curved section of pipe, shaped like the letter P, that retains a small amount of water after each use. That water forms a physical barrier preventing sewer gas from rising back up through the drain into the room.

When a drain goes unused for several weeks, the water in the p-trap evaporates. The barrier disappears and sewer gas rises freely through the open pipe into the bathroom. This is a very common cause of sudden sewage smell in a bathroom that was previously fine, and the fix is simply running the taps for 30 seconds to refill the trap.

If a bathroom or cloakroom is used infrequently enough that the trap dries repeatedly, running the taps weekly prevents it from happening again. An infrequently used shower that consistently smells can also have a trap that dries between uses the same solution applies.

Cause 1: Dry Trap

Signs: Sewage smell appears suddenly. The bathroom has not been used in several weeks or the drain in question (shower, bath, or secondary sink) is used infrequently. The smell may be present constantly when in the room but absent in adjacent rooms.

Fix: Run all taps and the shower for 30 seconds each to refill the traps. Check the bath waste and shower waste specifically if those are the infrequently used drains. The smell should disappear within a few minutes as the trap refills.

For a shower tray that dries between uses, squirting a tablespoon of cooking oil into the drain occasionally helps the water in the trap evaporate more slowly, extending the time before it needs topping up again.

Cause 2: Biofilm Buildup

Signs: Persistent smell present most of the time, often described as sulphur, rotting, or musty. Not necessarily worse after the drain is used. Most common in the shower drain and the basin waste, where soap scum, hair, skin cells, and toothpaste residue accumulate over time.

Biofilm is a bacterial community that colonises the inner walls of waste pipes. It metabolises the organic matter that flows through the drain and produces hydrogen sulphide gas as a byproduct. This is the rotten egg smell associated with drains. Standard drain cleaners and bleach kill surface biofilm temporarily but do not prevent regrowth, which is why the smell returns within weeks of treatment.

Fix: Remove the drain cover and any hair or debris caught immediately below the outlet. This is the highest concentration of organic matter and the main feeding ground for the biofilm. A pair of rubber gloves, a cloth, and a bin bag are the tools for this stage.

Apply a purpose-made enzyme drain cleaner or biological drain cleaner not a caustic or bleach-based drain unblocker to the drain and leave overnight. Enzyme cleaners digest the biofilm layer itself rather than just killing the surface bacteria, which significantly slows regrowth. These are available in UK supermarkets and hardware shops for £4 to £12.

Pour a kettle of hot water down the drain the following morning to flush the digested material. Do not use boiling water if the drain pipework is plastic push-fit, which is the most common fitting in UK bathrooms a kettle that has stood for two minutes rather than boiling produces water hot enough to clean effectively without risk of distorting push-fit joints.

For persistent or severe biofilm, a mixture of bicarbonate of soda (two tablespoons) followed by white vinegar (half a cup) left for 20 minutes and then flushed with hot water shifts accumulated material effectively. This is a mechanical cleaning action from the fizzing reaction rather than a chemical breakdown, and it helps lift loose biofilm from the pipe walls before the enzyme treatment.

Monthly drain maintenance with a biological enzyme cleaner, particularly on shower and bath wastes, prevents biofilm from re-establishing to smell-generating levels.

Cause 3: Partial Blockage

Signs: Smell appears or worsens after the bath, sink, or shower is used. Water may drain slowly. Gurgling sounds as the water drains indicate air being displaced around a partial obstruction. The smell is often worse in the hour after use.

A partial blockage is typically accumulated hair, soap scum, and debris catching on the pipe wall, drain cover, or waste outlet of the bath. This material decomposes as it sits in a warm, damp pipe and produces the characteristic drain smell. Because it is not a complete blockage, water still drains but slowly, and each use adds more material to the accumulation.

Fix: Remove the bath waste or shower trap cover and extract whatever has accumulated. On most UK bath wastes this requires unscrewing the chrome waste cover with a flat-head screwdriver. A dedicated drain hair catcher tool or a straightened wire coat hanger with a small hook formed at the end pulls hair and debris from the pipe beyond the visible cover.

For a partial blockage further into the pipe, a drain rod set or a flexible drain cleaning brush reaches further. These are available from DIY shops for £10 to £30 and are reusable. Push-fit together and rotate as you feed the rod into the drain do not force through a curved section of the trap, which can dislodge push-fit joints.

After physically removing the blockage material, flush with hot water and apply an enzyme drain cleaner to address any biofilm that has established around the obstruction area.

When the Smell Comes From All Drains Simultaneously

If every drain in the bathroom, or multiple drains across the house, smells at the same time, the problem is not the individual drains. The source is likely the main soil stack vent or a damaged section of underground drainage.

UK Building Regulations require all soil stacks and drainage systems to be vented to atmosphere at roof level, which allows sewer gas to escape safely rather than building pressure inside the drainage system. A blocked or damaged soil stack vent prevents this and forces gas back through the water traps of multiple fixtures simultaneously.

This is a plumbing investigation job. A drainage engineer can inspect the soil stack and underground drainage with a CCTV camera for £100 to £300, which identifies damage, root intrusion, or vent blockages that cannot be diagnosed from the drains themselves.

For any persistent drain smell that does not respond to the fixes above, or that returns rapidly after treatment, the InstantCost plumbing cost guide covers 2026 UK plumber rates including drain unblocking at £100 to £250, CCTV drainage surveys, and when a drain problem is large enough to warrant an emergency call-out versus scheduling a daytime visit at the standard £40 to £80 per hour rate.

Bathroom drain smell is often accompanied by slow drainage, and the two problems share a common cause in most cases. Our related guide on how to remove bathroom sealant covers the associated bathroom maintenance task that often comes up when drain access requires lifting the bath panel or inspecting around the bath waste connection.

Common Mistakes and What to Do Instead

Mistake: using bleach as a long-term drain smell solution. Bleach kills surface biofilm immediately but does not prevent regrowth, which re-establishes within two to three weeks. Enzyme cleaners address the biofilm at a deeper level and slow regrowth significantly more effectively.

Mistake: pouring boiling water into plastic push-fit drainage. Boiling water can soften and distort push-fit pipe connectors, the most common type in UK bathroom drainage. Hot-but-not-boiling water from a kettle left to cool for two minutes achieves the cleaning benefit without the distortion risk.

Mistake: treating a dry trap with drain cleaner. A dry trap does not need cleaning it needs refilling. Running the tap for 30 seconds is the entire fix. Drain cleaner into an empty trap is wasted product that sits in the pipe and may damage the trap material.

Mistake: ignoring slow drainage alongside the smell. Slow drainage and drain smell together mean a partial blockage, not biofilm. The blockage must be physically removed before any cleaning product is effective.

Mistake: using caustic drain unblocking chemicals on a partial hair clog. Caustic chemicals dissolve organic matter but are not safe for all pipe materials and can damage rubber seals in older UK plumbing. A drain tool that physically removes the hair is faster, safer, and more effective.

Mistake: assuming drain smell is always a drain problem. Simultaneous smell from multiple drains points to the soil stack vent or underground drainage rather than individual pipe biofilm. Individual drain treatment will not fix a venting problem.

What It Costs in the UK

Figures based on InstantCost.co.uk's 2026 UK plumbing pricing data and vary by region, drain condition, and provider.

Staying Safe

Biological enzyme drain cleaners are low-hazard products safe for household use. Caustic drain unblockers those labelled as heavy-duty or professional strength are a different category entirely. They are alkaline chemicals that generate heat on contact with water and cause severe burns to skin and eyes. If you choose to use a caustic drain product, wear rubber gloves and eye protection, do not lean over the drain opening, and follow the label instructions exactly.

Never mix drain cleaning products. Bleach combined with an acidic drain cleaner produces chlorine gas. Enzyme cleaners combined with bleach are inactivated the bleach kills the bacteria in the enzyme product and renders it useless.

Keep children and pets away from any drain treatment area until the product has been flushed and the drain is clear. Store all drain cleaning products in their original labelled containers, high or locked, away from children.

If drain work requires lifting or moving the bath, removing the basin pedestal, or accessing under-floor pipework, confirm the structural consequences before proceeding. A bath that is moved without isolating the water supply first causes flooding; a basin pedestal that is removed without isolating the supply first does the same.

UK drainage and plumbing is subject to the Building Regulations Approved Document H for drainage and waste disposal. Any alteration to waste pipe routing, trap configuration, or soil stack connections requires compliance with these regulations. Straightforward drain cleaning and unblocking does not constitute notifiable building work. A persistent drainage fault that requires structural investigation should involve a qualified drainage engineer.

FAQ

Why does my bathroom drain smell like sewage? The three most common causes are a dry p-trap no longer blocking sewer gas (fix: run the tap for 30 seconds), biofilm buildup producing hydrogen sulphide inside the pipe (fix: enzyme drain cleaner), or a partial blockage of decomposing organic matter (fix: physical removal and enzyme cleaner). If all drains smell simultaneously, the cause is likely a blocked soil stack vent.

How do I stop my bathroom drain smelling? Identify the cause first. If the drain is in an infrequently used bathroom, refill the trap by running the taps. If the smell is general and persistent, use a biological enzyme drain cleaner monthly. If it worsens after use and drainage is slow, remove the physical blockage with a drain tool.

Does bleach get rid of drain smell? Temporarily. Bleach kills surface biofilm bacteria and provides immediate odour improvement, but does not address the biofilm layer in the pipe wall. Regrowth occurs within two to four weeks. Biological enzyme cleaners provide longer-lasting improvement by digesting the biofilm itself.

What is the p-trap and why does it matter for drain smell? The p-trap is the curved pipe section below every bathroom drain that retains a small amount of water after each use. This water physically blocks sewer gas from rising through the pipe into the room. If the trap dries out through non-use, the gas rises freely and produces a sharp sewage smell. Refilling it by running the tap resolves this immediately.

How much does a plumber charge to unblock a drain? UK plumbers charge £100 to £250 to unblock a drain in 2026, depending on the severity, the method required (rodding, jetting, or CCTV), and the region. Emergency out-of-hours drain clearance attracts a £100 to £200 call-out fee on top of the hourly rate of £80 to £150.

When should I call a plumber for a smelly drain? Call a plumber if the smell persists after trying the appropriate fix for the identified cause, if multiple drains smell simultaneously suggesting a soil stack or underground drainage problem, if there is visible damage to drain components, or if drain rods or tools have failed to clear a partial blockage.