Where this starts
A tap has three separate sealing jobs and they fail independently. It has to stop the flow when closed, it has to stop water escaping around the part you turn, and it has to stay watertight where it joins the pipework. The symptom looks the same from across the room and the three have nothing in common.
SO LOOK, WITH THE TAP DRY. Wipe everything down, close the tap, and watch. Water forming at the spout is the shut-off. Water appearing around the handle or seeping from the base as the tap is used is the gland or the O-rings. Water under the basin with a dry tap above is a tail connection, a flexible hose, or the waste — none of which is fixed by anything done to the tap head.
WHICH TYPE OF TAP decides the part entirely. A traditional tap with a handle that turns several times closes onto a rubber WASHER against a brass seat, and it is repairable for the price of a washer. A quarter-turn lever tap closes with a CERAMIC DISC CARTRIDGE, which is a sealed component replaced whole and matched to the make. Telling somebody with a lever mixer to fit a new washer is the commonest wasted trip in domestic plumbing.
THE SEAT IS THE PART EVERYONE FORGETS. A washer seals against a machined brass seat, and that seat corrodes and scores over decades — so a tap that drips again within weeks of a new washer usually has a damaged seat rather than a bad washer. The seat can be re-cut with a tool, which is why a stubbornly dripping traditional tap is not automatically a new tap.
The reason to bother: a drip is a small number that compounds. A steady drip is measured in litres a day, and on a metered supply it is visible on a bill long before anyone finds it annoying — and if it is the hot tap, the water was paid for twice, once to buy and once to heat.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
A perished washer on a traditional tap
Most commonA drip from the SPOUT with the tap closed, on a tap whose handle turns through several revolutions, often needing more force to stop the drip than it used to.
How to check: How far the handle turns. Several turns means a washer; a quarter turn means a cartridge, and the parts are unrelated.
A worn ceramic cartridge on a lever tap
Most commonA drip from the spout on a quarter-turn lever tap, sometimes with the lever feeling gritty or loose, and no washer to be found when it is opened.
How to check: Whether the tap is quarter-turn. A ceramic cartridge is replaced whole and is specific to the make and model.
A damaged or scored valve seat
CommonA traditional tap that drips again within weeks of a new washer, sometimes with visible pitting on the brass seat inside when the head is off.
How to check: Whether a new washer lasted. A washer failing quickly is being cut by the surface it seals against.
A failed gland or O-ring around the spindle
CommonWater appearing around the HANDLE or at the base while the tap is running, rather than dripping from the spout when closed.
How to check: Whether it leaks when running or when closed. When running and around the handle is a moving-part seal.
A connection under the basin
CommonWater in the cupboard or on the floor with the tap and its base dry, often at a flexible hose, a compression nut or a shut-off valve.
How to check: Dry everything, put paper underneath, and see where the first mark appears. Under the basin is not the tap.
Supply pressure higher than the tap is rated for
Less commonSeveral taps dripping or wearing out quickly, often with noisy pipework and appliances filling fast, on a mains-fed system with no pressure reducing valve.
How to check: Whether more than one tap does it. One tap is that tap; several is the supply pressure eating the seals.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
Spout, handle, base or underneath are four different components, and the whole repair depends on which one it is.
- 2
Several turns is a washer tap, a quarter turn is a ceramic cartridge, and buying the wrong one is the commonest wasted trip in domestic plumbing.
- 3
Closed and dripping is the shut-off valve; running and seeping around the handle is a spindle seal, and neither fix helps the other symptom.
- 4
It finds a connection leak that is being blamed on the tap, and it tells you which joint rather than which cupboard.
- 5
One tap is a worn part; several at once points at supply pressure, which is one change rather than several repairs.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where the isolating valve will not close or there is no isolating valve on that tap, since the alternative is stopping the whole supply and working against time.
- Where several taps are failing together, which points at supply pressure and is a change to the incoming installation rather than a repair.
- Where the leak is under a basin or a bath and has been running long enough to wet the floor structure, because the damage is then the floor rather than the fitting.
- Where the tap is on an unvented hot water system or a combined mixer fed by two different pressures, both of which have specific requirements a general repair can breach.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.