Plumbing Basics · 6 min read · Beginner

How to Isolate the Water Supply Before a Plumbing Job

Find and use local isolation valves or the internal stop tap safely before touching taps, traps or other plumbing fittings.

Key points & steps

Identify whether the job can be isolated locally using service valves under a basin, sink or appliance. If local isolation is unavailable or unreliable, locate the internal stop tap.
The internal stop tap is often under the kitchen sink, but it can also be in a utility room, airing cupboard, downstairs WC, garage or near where the mains supply enters the property.
Turn the relevant valve slowly to the closed position and do not force a seized or damaged valve.
Open the affected cold tap to confirm that flow has stopped before disconnecting anything. Allow residual pressure and trapped water to drain.
Protect the work area with a towel and bucket because pipework and fittings can still contain water after isolation.
When the work is finished, reopen the supply slowly and inspect every disturbed joint for leaks immediately and again after the area has been used.
If the internal stop tap will not close properly, is leaking, is badly corroded or appears fragile, stop and use a competent plumber rather than forcing it.

Know where your internal stop tap is before an emergency, not only when a leak occurs.
External stop taps may belong to the water supplier and should not be treated as a routine DIY isolation point.
A valve handle position alone is not proof of isolation—confirm by opening the relevant tap and checking that the flow stops.