What is the difference between a stopcock, an isolation valve and a service valve?

They all shut off water, at different points and for different purposes — and knowing where each one is before you need it is one of the more valuable pieces of household knowledge.

The external stopcock (stop tap). Outside the property, usually under a small metal or plastic cover in the pavement or driveway, sometimes shared between houses. This is the water company's valve, controlling supply to the property. It often needs a long key to operate, and in an emergency it is the last resort.

The internal stopcock. The main shut-off inside the house, controlling all incoming mains water. Typically under the kitchen sink, but also found in downstairs cloakrooms, utility rooms, garages, under stairs or beneath floorboards near the front of the house.

This is the one to find now, before an emergency. Turn it clockwise to close.

A crucial maintenance point: stopcocks seize if never operated. A valve untouched for fifteen years frequently will not turn, or shears off when forced. Exercise it twice a year — turn it fully off and on again — which keeps it free and confirms it works.

Isolation valves (sometimes called service valves) are small in-line valves on individual pipe runs, letting you shut off one appliance or fitting without cutting water to the whole house.

They are typically a small brass fitting with a slotted spindle turned with a flat-blade screwdriver — the slot parallel with the pipe means open, across the pipe means closed.

Where they are found: under basins and sinks on the tap tails, at toilet cisterns, at washing machine and dishwasher connections, and on radiator feeds.

Why they matter: replacing a tap washer or a toilet fill valve becomes a five-minute job rather than draining the system.

Also worth locating: the drain-off valve at the lowest point, the gate valves on tank-fed systems in older houses, and the shut-off for the outside tap, which should be closed in winter.

Label them once found.