Three seals, three different single quarter turns — and one of them must not be touched
Water on the pressure side behaves differently from water on the drain side, and that difference is the first free diagnostic. A drain leak appears when you use the fixture and stops when you do not. A supply leak is there all the time, tap running or not, because the pipe behind it is permanently at mains pressure. So “it is only damp after a shower” and “it is damp at eight in the morning with nothing running” are two different pages.
The second free diagnostic is a strip of dry tissue, and it is better than your eyes for a reason worth understanding. A drip lands, then runs along the pipe to the lowest point and hangs there. So the wet patch you can see is the destination, not the source, and tightening whatever is under that patch is how a small leak becomes an afternoon. Dry everything, then wrap a strip around the stem where it enters the body, a strip between the body and the wall, and a strip around the hose end. Wait ten minutes with nothing running. The one that wets first is the leak, and the others have just told you what to leave alone.
The third fact is what makes this a job you can do at all, and also what makes it easy to ruin. The valve stem is sealed by packing, not by torque. A ring of graphite or PTFE rope sits in a recess and the packing nut compresses it. It wants a quarter turn, and it wants it once. Past that, the packing is crushed, extruded or cut, and a weep becomes a leak that runs whenever the valve is open. The wall joint is sealed by a copper ring that deforms plastically around the pipe. It also wants a quarter turn, and it only deforms once — a second tightening does not make it tighter, it splits the ring. Both of these are the same mistake wearing different clothes: if a quarter turn did not fix it, more turns will not fix it, they will replace the fault with a worse one.
Three faults, three answers
The tissue around the stem wets first is the packing. The leak is a ring directly under the handle, and it is usually worse while you are turning the tap. Give the packing nut — the hex nut the stem passes through, not the handle — one quarter turn and retest. If it still weeps, or if the handle has become stiff and gritty, the packing has hardened and no amount of tightening will help: that valve gets replaced, which means shutting off the supply first.
The tissue against the wall wets first is the compression joint. One quarter turn on the nut, and if that does not do it, the joint has to come apart: new copper ring, or cut back and remake. What you must not do is keep tightening, because that ring has already been deformed once and a second attempt splits it rather than sealing harder. And if the pipe coming out of the wall is crusted and feels soft when you press it, stop — a pipe that has thinned to that point will snap inside the wall, and that is no longer a quarter-turn job.
The tissue at the hose end wets first is the cheapest of the three and usually not worth repairing. Inside the nut there is a rubber washer; when it hardens, flattens or goes missing, water comes out past the nut. A new washer costs pennies. But look at the hose itself while you are there, because the stainless braid is armour, not a seal — the water is held by an EPDM tube inside it, and that tube fails invisibly. Run your hand along the whole length: a soft spot, a bulge, or a section that squeezes flat is a burst waiting for a quiet afternoon. Braided hoses have a service life of about five years, and they are among the most common causes of sudden flooding in a house.