The nut does not seal. It only clamps.
A slip joint is a very simple idea, and almost every mistake on this page comes from not knowing it. Two pipe ends meet face to face. A rubber or nylon washer sits between them. A nut screws on and squeezes that washer flat — and the water is sealed inside the washer, against the two faces. The thread does nothing except supply the clamping force. That is why a slip joint is designed to be tightened by hand: the washer needs to be squeezed, not crushed.
Which is why PTFE tape does nothing here, and can make it worse. Tape is for tapered threads — NPT and its relatives — where the thread itself is the seal and the tape fills the spiral gap. A slip joint is a straight thread over a washer; there is no spiral gap to fill. What the tape does do is make the nut slide more easily, so it is easier to turn it past the point where the washer is already fully compressed — and it leaves shredded tape in the thread, which stops the nut seating next time. Plumbers are unanimous on this and it is still the single most common thing people add to a joint that was already leaking.
The torque has a ceiling, and it is a quarter turn. Hand-tight, then a quarter turn with pliers. Past that, the plastic nut starts to deform and then to crack — and a cracked nut leaks no matter how tight it is, while still feeling tight. This is how a two-minute job becomes a new trap assembly: the nut cracks, the leak gets worse, and now you are buying the whole kit. If a joint is fully tight and still weeping, more force is not the answer. The washer is the answer.
And before any of that: find out which side you are working on. The drain side only has water in it while water is flowing, and then it drains away. The supply side is at 2 to 4 bar all day and all night. Those are different failures with different risks, and dismantling a sound trap because you assumed the wrong side is how people turn a drip into a puddle.
Three sources, in the order you should suspect them
- The slip nuts on the trap. The commonest by a wide margin, and the cheapest: about half of them stop leaking with nothing but a quarter turn. The washer inside has lost its compression — from vibration, thermal cycling, or someone knocking the pipe while reaching for the cleaning products. If tightening does not stop it, the washer has hardened or been installed with the taper facing the wrong way.
- The supply side: the angle stop and the braided hose. Rarer, and the one that does real damage, because it runs under pressure. Look for a dry paper towel wrapped round the valve handle coming back damp — that is a stem leak, and it means the packing has gone. Braided hoses have a service life of about five to seven years; when the inner tube perishes the braid holds the shape and hides the failure until it lets go all at once.
- The drain flange and the overflow. The one that gets misdiagnosed, because the water appears at the trap and the fault is 30 cm above it. If water runs down the outside of the tailpiece, the leak is at the sink opening, not at any nut. The old plumber’s putty or gasket under the flange has dried out and the water is going round the seal instead of through the hole.
The one test that replaces all the guessing
Dry every joint with a cloth. Wrap a dry paper towel around each one — the two slip nuts, the supply connections, the base of the strainer. Then use the sink normally once, and come back and look. The first towel that is wet is your source. It sounds too simple to be worth saying, and it is worth more than every other diagnostic on this page combined: it is the difference between tightening one nut and taking three apart to find out which one it was.