Basin Waste Leaking

Slotted, unslotted, pop-up and click-clack

Every basin waste is a threaded body that drops through the plughole, with a flange on top and a backnut underneath clamping it to the ceramic. The difference that matters most is the slot. Basins with an overflow, the small hole near the top of the bowl, need a slotted waste: the body has a slot in its side that lines up with the channel inside the ceramic so overflow water can drain away. Basins without an overflow need an unslotted waste with a solid body.

Fit an unslotted waste to a basin with an overflow and the overflow has nowhere to go, so any water that reaches it sits in the channel and eventually finds its way out around the waste. Fit a slotted waste to a basin with no overflow and water pushes through the slot and out under the backnut every time the basin drains. Both are common after a DIY replacement.

On top of that choice, the waste can be a plug and chain, a click-clack that you press to open and close, a flip-top, or a pop-up operated by a rod behind the tap. A pop-up adds a pivot seal where the rod enters the waste body. That seal is a leak point of its own and drips only when the basin is draining.

Pin down the leak before you buy anything

Clear the cupboard, dry the waste, the trap and the tap hoses, and put a dry tissue against each in turn. Then work through this order.

• Leave the basin unused for 15 minutes. If something under the basin is wet with nothing running, it is not the waste at all. It is the tap connection or hoses, which is covered in a leak under the bathroom sink.
• Close the plug and fill the bowl half way. A wet tissue under the flange and backnut means the waste seal.
• Fill until water enters the overflow. A leak now, but not at half full, points to the overflow channel and the slot area.
• Pull the plug. A drip only while it drains is the pop-up pivot, the trap joints or the pipe beyond. If it is past the trap, see a leaking waste pipe.

Hold a small mirror and torch under the basin rim. Water can run along the underside of the ceramic and drip from a point well away from where it is escaping.

Refitting or replacing the waste

Turn the tap isolation valves off so nobody runs water while the waste is out. Undo the trap from the waste tail, then undo the backnut. On an old waste the backnut is often fused with scale, and a basin wrench or grips on the nut while someone holds the flange from above helps. Lift the waste out and clean every trace of old sealant and washer from the ceramic around the plughole, both on top and underneath.

A new basin waste typically costs £6 to £30 for a standard click-clack and more for a designer finish. Fit the washers in the order shown in the instructions: most kits have a thin washer under the flange and a thicker one under the basin. On a slotted waste, many fitters run a bead of sanitary silicone around the body at the slot, so water from the overflow channel is forced into the slot and cannot track down the outside of the waste. Line the slot up with the overflow channel before tightening.

Tighten the backnut firmly by hand, then a little more with a wrench while holding the flange straight. Ceramic cracks if you lean on it, and a crack around the plughole ends the basin’s life. Refit the trap, let any silicone cure for the time on the tube, and test with the same fill and drain sequence.

The trap and what else to check

Basin traps are 32mm. Building Regulations Approved Document H sets a minimum water seal of 75mm for a washbasin trap, and bottle traps are the usual choice because they are compact and easy to clean. The joints are plastic nuts with washers, tightened by hand. If a trap nut drips, undo it, clean the washer and its seat, and refit. A trap that has been overtightened in the past may have a hairline crack in the nut, and a new bottle trap typically costs £5 to £15.

While the waste is out, look at the overflow hole. Soap scum and hair build up inside the channel and smell, and if the overflow is partly blocked, water backs up and finds its way around the waste. Flush it through with hot water and a bottle brush before refitting.

If the basin is on a pedestal, check it is actually fixed to the wall. A basin that moves when you lean on it keeps working the waste and trap joints loose, so fixing the brackets can be the real repair.

When to hand it over

Most basin waste leaks are small, slow and only happen when the basin is used, so they can wait for a convenient time if you stop using the basin. Call someone sooner if a vanity unit is already swelling or the floor is soft, if water is coming through a ceiling below, if the backnut will not move without the basin shifting on its fixings, or if you have found a crack in the ceramic.

A plumber will confirm the leak point, fit the correct waste for the basin, reseal it and refit the trap in one visit. Our leak repairs service covers this. The £60 call-out applies at any hour and the repair price is agreed with you on site before any work starts; see pricing.