Sink Overflow Leaking or Smelly? Causes and Fixes

Updated July 2026 · Emergency Plumber Chelsea

It helps to know how your overflow is built, because basins and kitchen sinks do it differently. On a bathroom basin, the overflow is usually a slot or a ring of small holes near the top of the bowl, and the water runs down a channel moulded inside the ceramic to the plughole. The waste fitting in the plughole has slots in its side that line up with that channel. On a kitchen sink, the overflow is a separate outlet in the bowl with a flexible plastic hose behind it, running down to a ring-shaped connector, often called a banjo, clamped onto the waste under the plughole.

A smelly overflow is the most common complaint and the easiest to fix. The inside of the overflow channel never gets properly rinsed, so soap scum, toothpaste, hair, grease and food residue coat it and bacteria breed there. The smell is often worse on a basin that is rarely filled, such as in a guest bathroom, and it can attract small drain flies. Our guide to drain flies explains why they keep coming back.

To clean a basin overflow, push a thin bottle brush or overflow brush into the slot and work it up and down, then flush the channel with a jug of hot water and washing-up liquid poured slowly into the overflow. A few repeats usually shift most of it. A kitchen sink overflow is easier: undo the overflow hose from under the sink, rinse and brush it out, clean the outlet from inside the bowl, and refit it. Avoid mixing chemical cleaners, and never pour bleach and an acid-based cleaner into the same channel. If the smell comes from the plughole rather than the overflow, the trap under the sink or the drain beyond is the likelier source; our guide to why a drain smells covers those causes.

A basin that smells even after cleaning may have the wrong type of waste fitted. A basin with an overflow needs a slotted waste, so the water in the overflow channel can drain through the slots into the plughole. If an unslotted waste has been fitted, the channel has nowhere to drain, water sits in it and goes stale. The opposite mistake causes a different problem: a slotted waste in a basin without an overflow can let water escape from the slots and leak under the basin. This is surprisingly common after a basin or tap has been changed, and the cure is the correct waste.

A leak under a kitchen sink that only appears when the sink is full, or when you empty a full bowl, often comes from the overflow. The banjo connector relies on seals that harden and crack over time, the clamp nut can work loose, and the overflow hose itself can split, particularly behind a sink where it gets knocked by bottles and bins. To find it, dry everything under the sink with kitchen roll, fill the bowl to the overflow, and watch the hose, the banjo and the waste nut. Our guide to a leak under the kitchen sink helps you separate an overflow leak from a leaking trap, tap connection or dishwasher hose.

On a basin, a leak from the overflow usually appears as water on the floor below the basin or a damp patch on the wall behind, but only when the basin is filled high. The cause is typically the seal between the waste and the ceramic, or a crack in the ceramic channel itself. Reseating the waste with fresh sealant or a new washer often cures it. A cracked basin cannot be repaired properly and needs replacing, especially if the crack reaches the overflow channel. If the basin leaks even when it is not filled high, our guide to a leak under the bathroom sink goes through the tap connectors, waste and trap.

Water coming up out of the overflow, or the overflow gurgling when you empty the sink, is not really an overflow fault. It means the waste below the sink or the pipe beyond it is partly blocked, so water backs up and finds the easiest way out. In a kitchen, a dishwasher or washing machine sharing the same trap can push dirty water back up the sink and out of the overflow when it pumps out. Clear the blockage and the overflow behaves. Our guides on how to unblock a sink and a dishwasher not draining cover the usual causes and fixes.

If the overflow gurgles or smells while another appliance drains, or the sink gurgles when a toilet is flushed, the problem may be with venting or a shared drain rather than the sink itself. In Chelsea mansion blocks, several flats often share the same soil and waste stacks, and a blockage lower down in the building can show up as gurgling and slow draining in more than one flat. Our guide to why a toilet or drain gurgles explains how to tell a local blockage from a stack problem, and when to involve the managing agent.

There are a few things not to do. Do not seal up an overflow with silicone or tape to stop a smell: it removes the protection against a flooded bathroom when a tap is left running, and the smell will come back through the plughole anyway. Do not keep tightening a plastic banjo nut until it cracks. And do not ignore a slow overflow leak into a kitchen cupboard, because chipboard base units soak it up quietly and swell, and in a flat the water can find its way into the ceiling below.

When to call a plumber: if the overflow keeps leaking after you have tightened the connections, if you need a new waste or overflow kit, if the basin is cracked, if water keeps coming up out of the overflow after you have cleared the trap, or if the damage has spread into the unit or the floor. These are usually short jobs, but getting the right waste and seals first time saves repeat visits.

Emergency Plumber Chelsea fixes leaking and smelly sink and basin overflows, replaces wastes and overflow kits, and clears the blockages behind them across Chelsea, Knightsbridge, South Kensington, Sloane Square and Fulham. We explain what we have found and the options before any work starts, and there is no call-out fee on jobs over 3 hours. See our blocked sinks and kitchen repairs pages, or call 020 3002 6667.

Quick tips

• A smelly overflow is usually trapped soap and grease. Brush the channel and flush it with hot water and washing-up liquid.
• Find a kitchen overflow leak by drying under the sink and filling the bowl to the overflow while you watch the hose and banjo.
• Basins with an overflow need a slotted waste; basins without one need an unslotted waste.
• Water coming up out of the overflow means a blockage further down, not a faulty overflow.
• Never seal an overflow shut to stop a smell. It is there to stop a flood.

FAQs