Why this overlooked part of the house is now being blamed for persistent bad smells and bacteria

The hidden sink overflow: why this overlooked channel is being blamed for stubborn smells

You scrub the basin, blitz the taps and pour something fragrant down the plughole, yet a sour, slightly metallic smell creeps back the next day. It isn’t the bin. It isn’t the towels. It’s maddeningly hard to pin down.

Plumbing engineers and professional cleaners have started pointing to a culprit most of us never clean at all: the sink overflow. That slim, silent slit near the rim of your bathroom or utility sink is designed to prevent spills. It is also a perfect, protected incubator for biofilm and odours — and it is connected directly to your drain.

What the overflow does — and why it turns smelly so quickly

The overflow is a bypass channel that lets rising water return to the drain before the basin tips over. Because it sits above normal water levels, it stays damp rather than flushed, collecting toothpaste foam, soap residue, hair fragments and minerals. With little oxygen and almost no turbulence, a gelatinous biofilm establishes itself on the walls.

Unlike the main drain, which sees a decent flow during handwashing or cleaning, the overflow hardly ever gets rinsed. The result is a narrow, stagnant tube lined with organic matter — precisely the kind of environment where bacteria and yeasts thrive. When you run hot water, that warm, damp air burps out of the overflow aperture, and the sour note you can’t place fills the room.

Why the smell lingers even after a deep clean

If you’ve poured vinegar, soda or a branded cleaner down the plughole and still caught a whiff later, you’ve met biofilm resilience. Biofilm is a protective slime that bacteria produce to anchor themselves to surfaces. It resists many quick-cleaning routines by shielding microbes from chemicals and letting them regrow once the danger passes.

In bathrooms, the situation is aggravated by frequent humidity spikes and by the types of residues we generate. Toothpaste and mouthwash residues contain flavours and sweeteners that feed bacteria; hard water leaves scale that traps odour molecules; and exfoliating products add fine grit that sticks inside the overflow. Even if the basin looks pristine, the hidden channel remains a smelly time capsule unless you physically disrupt the film.

How to clean the overflow and drain safely

Start with access. Remove the pop-up plug if you have one, because hair and paste often catch on the pivot rod under the drain cap. Lift it out, rinse it in hot water with washing-up liquid, and brush away any dark slime on the underside. Set it aside while you tackle the overflow.

The gentlest, most reliable way to reach the overflow is with a flexible bottle brush or a thin, purpose-made drain brush. Dip the brush in hot, soapy water, insert it into the overflow opening and work it slowly in and out. Don’t force it; instead, twist to loosen the film and wipe the inner walls. You will likely pull out grey-brown residue you’ve never seen before.

To dissolve what the brush can’t dislodge, use a foaming cleaner that can travel both ways. A solution made with oxygen bleach (sodium percarbonate) is ideal for ceramic and enamel basins because it releases oxygen, which helps lift organic matter, and breaks down to oxygen and soda. Mix it per the label, pour a little into the overflow opening with a small funnel or a turkey baster, then follow with a splash of hot water to encourage foam.

Let the foam sit for at least ten minutes so it can work along the channel. Then brush again and flush the overflow by letting the tap run hot while you block the main drain with a plug and raise the water level near the overflow slot. When you release the plug, the rush of water will backflush the overflow and carry loosened debris away.

If you prefer a disinfectant finish, a mild hydrogen peroxide solution works well on ceramic and plastic without the harshness of chlorine. Apply, leave for a few minutes and rinse thoroughly. If you use bleach, keep it very dilute, avoid contact with natural stone or coloured silicone, and never mix bleach with acids such as vinegar. Gloves and good ventilation are sensible, especially in small bathrooms.

For stubborn cases, an enzyme-based drain treatment used overnight can help degrade the remaining film. Enzymes are slower than oxidisers but excellent at digesting the starches and proteins that form the biofilm matrix. Apply after your regular brushing, and let the product sit undisturbed until morning before running water.

A simple routine that actually keeps smells away

Once the overflow is clear, it doesn’t take much to keep it that way. Make a habit of a weekly hot, soapy flush: fill the basin with the hottest water your tap provides, add a squirt of washing-up liquid, and let it drain once the water level kisses the overflow opening. The brief moment of contact helps rinse the channel.

Every fortnight, give the overflow a quick brush during your usual bathroom clean. It takes less than a minute when it’s not heavily fouled. Rinse the plug mechanism as you go; that undercap gunk is another stealthy smell source, and it also slows drainage.

If you have hard water, consider a descaling run monthly. A non-acidic descaler compatible with your sink material will reduce mineral build-up that traps odours and supports film regrowth. Keep a small turkey baster or squeeze bottle under the sink so you can direct cleaners precisely into the overflow without splashing benchtops or mirrors.

Why keeping the trap wet matters

Another reason smells seem to “come from nowhere” is a dry trap. The U-shaped section of pipe under your sink is designed to hold water, sealing your room from sewer gases. If a guest bathroom or utility sink goes unused, that water evaporates, and the characteristic eggy smell drifts up. A weekly run of the tap for 20–30 seconds keeps the trap charged and odours at bay.

Strong cooking aromas or seasonal humidity don’t help, but they rarely produce that metallic-foul note on their own. When people say the smell is worse after they clean, it’s often because they stirred the biofilm without flushing it or because the trap water was disturbed. A thorough rinse after any cleaning treatment resets the system.

When the odour isn’t the overflow

There are other hidden crevices worth checking if the smell persists. The rubber splash guard on a kitchen waste disposal, if you have one, is notorious for trapping grease and food slurry. Lifting it to clean both sides, or running a dedicated brush across the flaps with hot, soapy water, eliminates a surprising amount of odour.

Dishwasher filters collect a film that smells like wet dog when neglected. Pull out the filter, wash it with hot water and detergent, and wipe the sump area. While you’re there, run a maintenance cycle with a dishwasher cleaner to purge hidden channels. Around the door, the soft gasket can harbour milky residue; a toothbrush and a little peroxide clears it.

Shower drains and bath overflows mimic the sink problem on a larger scale. Hair and conditioner residue create thick mats that hold bacteria. Removing the cover, pulling the hair trap and brushing the inner walls change the odour profile dramatically. If the bathroom has a floor drain you never use, pour a cup of water into it monthly so its trap doesn’t dry out.

Simple precautions for different materials and systems

Most ceramic and enamel basins tolerate oxygen-based cleaners, mild detergents and peroxide without issue. Natural stone, sealed concrete or bespoke finishes can etch or discolour if exposed to strong chemicals. If your sink is stone, stick to neutral pH cleaners and mechanical brushing, and test any new product on an inconspicuous spot.

Homes on septic systems should avoid heavy, frequent use of chlorine bleach in drains. Occasional small amounts used carefully are typically fine, but enzymes and oxygen-based cleaners are kinder to the tank’s microbial balance. Whatever your system, never mix cleaning agents, and rinse between products to prevent unwanted reactions.

The pay-off

Clearing the overflow and its neighbours removes more than a smell. It reduces the moisture-loving film that can harbour microbes, improves drainage speed and cuts down on the tiny flies that breed in gunk. The job costs pennies, requires no special kit beyond a slim brush and a baster, and returns a bathroom that actually smells clean after you clean it.

Many people don’t realise that a few minutes spent on the hidden parts of the sink make the rest of the bathroom easier to keep fresh. If you’ve been battling a persistent odour that seems to hide from your efforts, the overlooked channel by the rim is the place to start. Once you’ve seen what comes out of it the first time, you won’t forget to add it to your routine.

• About
• Latest Posts

Winfrith

Latest posts by Winfrith (see all)

• Why professional cleaners say most people in the UK are cleaning their homes completely the wrong way - January 20, 2026
• How one common cleaning habit is secretly making British homes smell worse instead of fresher - January 20, 2026
• Why thousands of UK households are only now realising their homes are never truly clean despite constant effort - January 20, 2026