Wash basin waste leak

[John]

Underneath. Between the nut and the bottom of the basin

[F]

I've installed a new wash basin but have got a very slight leak from the waste after the plug has been pulled - not before.

I've fully tightened the large nut which clamps a large washer on the waste to the basin from underneath but there is still the odd drip from it.

Is it time for silicone? If so, where? Between the large washer and the sink?

[Alex]

remove waste and throw away the washers and replace with plumbers mate ,trim back excess after the backnut is tight

[F]

Don't have any Plumber's Mait in stock, but I do have silicone so I've used more this time in the manner suggested by John.

If I can't get a seal that way then it's off to the shop tomorrow for some Plumber's Mait.

[The Natural Philosopher]

Ah. Then it is NOT - despite what others have said - a problem with the waste to sink seal.

Its PROBABLY in the trap area or the trap to waste connection.

You need to ID where exactly it is coming from. In genenral for screw fittings and the like, the rubber seals should be adequate..a lot depends on what type of trap assy. you have. Some metal ones are pretty vile and need PTFE tape or plumbers mait and hemp, if you are like that) to seal properly.

From WHAT though.

If no drip with plug in place and a full sink, its NOT the waste itself.

If that was the problem, yes. I prefer to put the silicone on the topside of the sink though.

A lot depends on the overflow arrangements..some overflows are led in the china itself (for a Belfast sink) to the trap hole: You are expected to use a trap with a slot cut in it and the primary seal on these IS at the base of the trap assembly, not inside the sink itself.

Then it is a question of sealing both - the top so that the sink doesn't slowly empty, and the bottom so that as you emoty te sink the water doesn't drip.

If its any consolation I have installed a couple of sinks, and three basins in the last four years. Not one has been leak free on first assembly.

As I said, use PTFE tape on crappy screw areas, and silicone where it's a washer against a plate situation.

But the very first thing is to work out exactly which joint is leaking.

[F]

It seemed to be running down the outside of the waste screw thread from the area of the large nut under the basin. Possibly from between the large washer and the nut or basin. Certainly from above the trap assembly.

Copious amounts of silicone now applied and waiting to be tested once it's dried.