# Why does a sink overflow hole smell? Source: https://cantleyvidanjoraltyapi.com/en/blog/sink-overflow-hole-smell/ Published: 2026-10-17 ## In short - The overflow hole connects to the strainer below the plug through a channel inside the body of the sink. - The channel is rinsed with water very rarely, so the soap and toothpaste that splash into it stay damp and start to smell. - Warm water, washing-up liquid and a thin bottle brush clean the channel safely. - Do not pour chemical drain opener into the overflow hole: the fumes come out at face height and the product sits in the channel. - If the side slots of the strainer clog, the sink both smells and drains more slowly. A sink's overflow hole smells because the soap, toothpaste and dirt that splash into the channel behind it stay damp and are almost never rinsed away. Over time a slippery, dark film forms on the inner surface of this channel, and that film is what gives off the smell. People often blame the sink's drain, so the trap is taken off and washed, but the smell comes back. Unless the hole and the channel behind it are dealt with separately, the smell does not go away completely. ## How does the overflow hole work? Most porcelain sinks have one or two small holes on the front or back inner surface, just below the rim. This hole opens into a narrow channel inside the cast body of the sink. The channel runs downwards and reaches the body of the strainer, right below the plug. At this part of the strainer there are small holes or slots on the side. Water coming from the overflow channel enters the strainer through these slots and goes down to the trap. This way, if the tap is left running, the sink empties through this route instead of spilling over the edge. With the plug closed, air also moves in and out through this channel as the sink fills. ## What builds up inside the channel? While you use the sink, water and foam constantly splash into the hole, but the channel is only rinsed from top to bottom when the water rises all the way to the rim. That rarely happens. The moisture, soap and organic residue left in between create a suitable environment for bacteria. The part of the channel near the entrance is warm and dark, with no light. The film that collects here thickens over time, darkens in colour and gives off a damp, mouldy smell, which you notice especially when you lean over the sink in the morning. In the morning and evening hours, when the sink is used most, the channel hardly ever dries out, and this speeds up the growth of the film. - Soap and shampoo foam - Toothpaste and shaving foam residue - Short hairs, skin flakes and make-up residue - Limescale clinging to the channel wall ## How can you tell the smell comes from the overflow hole? A simple sniff test is enough. First lean in close over the plug, then over the overflow hole. If the smell is clearly stronger at the hole, the source is the channel itself. For a second test, pour a glass of warm water into the hole and watch it flow out through the strainer. If the water goes down easily, the channel is open and the problem is only the film. If the water waits in the hole or comes back up into the sink, the channel or the side slots of the strainer have narrowed. ## Cleaning the overflow channel safely Because the channel is narrow and winding, poking it with a stiff wire is useless and also scratches the glaze of the porcelain. Work with liquid and a soft brush instead. Some sinks have a removable plastic or chrome cover in front of the hole; if yours does, take it off first. Use warm water rather than boiling water. The plastic strainer part at the end of the channel and its seal do not like high heat. - Put warm water and a few drops of washing-up liquid in a plastic bottle and squeeze it slowly into the hole. - Push a thin, flexible bottle brush or pipe cleaner into the hole and move it back and forth several times. - Pour a teaspoon of baking soda into the hole through a funnel, add a little vinegar on top and wait for it to foam. - Finally, rinse with plenty of warm water and smell the hole again. - To stop the smell coming back, repeat this once a month, on the day you clean the sink. ## How do you clear the side slots of the strainer? The slots where the overflow channel joins the strainer sit right below the plug and cannot be seen from outside. With push-to-open plugs, the plug comes out when you turn it or pull it up; with chain plugs, the mouth of the strainer is already open. Once the plug is out, you will see small openings on the side of the strainer's inner wall. The area around these openings is usually covered with a grey, slippery film of soap. Wiping this film away with an old toothbrush and warm soapy water reopens the path between the channel and the strainer. When you have finished, pour a little water into the hole: if it flows freely through the slots, the cleaning has worked. - Before taking out the plug, put a cloth over the sink opening so small parts do not fall down the drain. - Wash the seal under the plug too; the smell often collects there as well. - Do not force the plug when you put it back, the mechanism should settle into its own seat. ## What should you not pour into the overflow hole? Pouring chemical drain opener into the overflow hole is a common mistake. Because the product sits in the channel, its fumes come out right at face height, and splashing drops can reach your eyes. Even if the channel does not clear, the chemical stays inside. Bleach suppresses the smell for a short time but does not strip off the film. If it is used on the same day as another cleaning product, it can also produce a harmful gas. Pushing hard tools such as a wire hanger, a toothpick or the tip of a knife into the hole is not a good idea either. A toothpick that snaps inside the channel or a brush bristle that comes off gives the soap film a new place to cling to and brings the smell back quickly. ## Why does a clogged overflow channel slow down the sink? When the side slots of the strainer fill with soap and hair, the overflow channel stops doing its job. As the sink fills and empties, the air inlet is closed, so the water runs more heavily and the drain gurgles. If the tap is left running, the water no longer goes into the channel but spills onto the counter. The reverse also applies. When the trap clogs, the water waiting in the strainer rises towards the channel, stands there and makes the smell stronger. If the smell and the slow drainage continue together even though you have cleaned the channel, the real narrowing may be in the trap or in the pipe inside the wall. In that case the strainer, the trap and the wall pipe should be dealt with together. ## Frequently asked questions ### How do you clean a sink overflow hole? Squeeze warm water and washing-up liquid into the hole and scrub the channel with a thin bottle brush. Then add a little baking soda and vinegar and rinse with plenty of warm water, which delays the return of the film. ### What does it mean if water comes back up through the overflow hole? It means the side slots of the strainer or the trap have narrowed. Because the water cannot move on through the strainer, it rises into the nearest empty space, which is the channel. ### Can a sink without an overflow hole smell? A sink without an overflow hole does not smell because of this channel, but it can still smell when its trap and strainer get dirty. In such sinks, looking after the plug and the trap is even more important. ### Do you need to block the overflow hole when using a plunger on a sink? Yes. If the overflow hole stays open, the pressure from the plunger escapes through it. Pressing a wet cloth over the hole closes it and lets the pressure reach the blockage. ## If the smell and the slow drainage continue together If the sink still smells or drains slowly even after you have cleaned the overflow hole, contact us on 0212 923 66 85 or on WhatsApp. We clear the strainer, the trap and the wall line without breaking tiles or walls. Phone: 0212 923 66 85 (tel:+902129236685) WhatsApp: https://wa.me/905315687185