Find it before you fix it

What the job involves

Working out where it is actually coming from before anything gets cut open. That means isolating things one at a time and watching what stops: hot side or cold side, mains or heating, is it only when the shower runs, does the pressure gauge drop overnight with everything off. Ten minutes of that saves a great deal of somebody's floor.

Then the fix. A joint remade, a section of pipe cut out and replaced, a valve or a tap swapped, a rad valve repacked, or a run re-routed if what is there is in a daft place to begin with.

The ones that come up most

• Compression joints under a bath or a kitchen unit, weeping quietly for months
• Radiator valve glands, which show as a rust ring on the floor rather than a puddle
• Boiler pressure dropping with no visible water anywhere — usually a rad valve or the expansion vessel
• Mains under a drive or a lawn, which shows as a wet patch that never dries and a meter that will not stop
• Condensate pipes freezing and splitting in a cold snap
• Immersion or cylinder connections, which tend to go all at once

Before anybody arrives

Find the internal stop tap — usually under the kitchen sink or near where the main comes into the house — and turn it clockwise. If it is a heating leak rather than a mains one, turning the boiler off and closing the valves either side of the offending radiator will hold it. Catch what you can in a bowl and get the electrics away from it.

Why it matters

A slow leak does far more damage than a dramatic one, because nobody deals with it. Water tracking into a joist end or the back of a stud wall does not announce itself until something is rotten, and by then the plumbing is the cheap part of the repair.