Sink Overflow Leaking

How your sink overflow is plumbed

Every sink with an overflow slot has a second route for water to reach the waste, and that route has its own seals. How it is built depends on the sink.

• Kitchen sinks, stainless or ceramic: the slot or round grille near the rim screws onto an overflow fitting behind the bowl, with a rubber washer squeezed between them. A flexible plastic overflow pipe runs down from it to the waste outlet, joining either at a banjo fitting (a ring clamped round the waste body) or at a side spigot on the waste, with its own seal.
• Bathroom basins: the overflow is a channel moulded inside the ceramic. It empties into the plughole through the slots in a slotted basin waste, so the only seals are the waste flange and the backnut washer under the basin.
• Double kitchen bowls: usually two overflows feeding a combined waste kit, which means more joints and more places for a slow leak.

A basin with an overflow must have a slotted waste. If someone has fitted an unslotted waste, the overflow water has nowhere to go and will leak out round the waste flange or sit and smell in the channel. It is a common result of fitting a new tap with the unslotted pop-up waste supplied in its box.

The two-level fill test

Put a dry tea towel or kitchen roll on the cupboard floor and clear the space so you can see every joint with a torch. Then fill the bowl in two stages.

• Fill to a few centimetres below the overflow slot and leave it five minutes. Water on the towel now means the waste flange, the waste body or something lower. The overflow is not involved.
• Pull the plug. A leak that appears as it drains is the trap, the waste connector or the pipe beyond it, not the overflow.
• Fill again, this time until water runs into the overflow slot, and hold it there briefly. A leak that starts only now is on the overflow route: behind the grille, along the overflow pipe or at the banjo joint.
• Dry everything and run a finger along the underside of each joint. The highest wet point is the leak; water runs down pipes and drips from the lowest point, which misleads people into tightening the wrong nut.

Fixing a leaking kitchen sink overflow pipe

The overflow is the least-tested seal on a sink, because most people rarely fill the bowl that far. It fails quietly for months and shows up the day someone leaves the tap running. The common faults, from most to least likely, are these.

• A loose overflow grille. The central screw works loose and the washer behind it stops sealing. Tighten it by hand with a screwdriver; overtightening can crack the plastic fitting behind.
• A perished overflow washer. Rubber behind the grille hardens and splits with age and hot water. It is replaced by unscrewing the grille from inside the bowl, which frees the fitting behind.
• A cracked overflow pipe. Items stored in the cupboard knock it, and old plastic goes brittle. The pipe is usually part of the waste kit and is replaced as a set.
• A loose or split banjo joint. The banjo ring is sandwiched between washers on the waste body; if the backnut has loosened or the washers have hardened, water escapes there when the overflow runs.

As a guide to market prices, an overflow washer costs a few pounds, and a replacement kitchen sink waste and overflow kit runs from about £5 to £30 at a merchant. Matching the kit to the sink matters more than the price: measure the waste outlet and check whether the sink has a round or slotted overflow.

When a bathroom basin overflow leaks

Because a basin overflow is part of the ceramic, a leak from it is almost always at the waste rather than the basin. Water that has come down the overflow channel reaches the plughole through the slots and can escape past a waste flange that is not bedded properly, or past the backnut washer underneath. Remove the waste, clean off old sealant, bed the flange on fresh silicone or the supplied washer, and line the slots up with the overflow opening before tightening. The guide to a leaking basin waste goes through that job step by step.

A crack in the ceramic channel itself is rare but does happen, usually after something heavy has been dropped in the basin or the basin has been overtightened to the wall. You will see a hairline crack running from the overflow hole, and water weeping from the back of the basin rather than from a joint. Sealants will not hold on that; the basin needs replacing.

If the basin overflow smells or grows black slime, that is not a leak but trapped soap and hair. A bottle brush and hot water through the overflow hole clear it. Persistent smells from the plughole are covered in sink smells.

When a leaking overflow needs someone tonight

A slow overflow leak you only trigger by filling the sink can wait: stop filling it past the slot until it is fixed. It needs attention sooner if the cupboard base has already swollen, if water has reached the floor void, or if you live above someone and the leak has gone unnoticed for a while. Chipboard kitchen plinths and cabinet bases swell quickly and do not recover.

It is also worth ruling out the obvious alternatives. Water under a sink that appears whether or not the sink is used is a supply pipe, tap connector or isolation valve, not the overflow, and it will not stop until the water is turned off. See a pipe leaking under the sink for that. Leaks while draining are the trap or a leaking waste pipe.

If you cannot find it or cannot fix it, our leak repairs service covers sink wastes, overflows and the pipework under them. The £60 call-out applies at any hour, and the repair price is agreed with you on site before any work starts. The engineer runs the same fill test, replaces the failed seal or the complete waste and overflow set, and checks the trap and waste run while the cupboard is empty.