Stopcock
Plain English. Practical detail. UK context.
A stopcock is a valve used to shut off a water supply, commonly the incoming mains supply to a home. People also say stop tap. Its location and the area it isolates matter more in an emergency than its appearance. A small service valve beneath a basin may isolate only one fitting, not the entire property.
Last reviewed: (editorial review against the public sources listed below).
Know which supply it controls
An internal stopcock is often near where the water service enters the building, but there is no universal location. Converted buildings and flats can have unusual or shared arrangements. An external stop valve may be the water company’s responsibility or serve several properties. Ask the water supplier or a competent plumber if ownership or operation is unclear. Do not turn an unidentified street valve merely because it is the nearest cover to the front door.
Prepare before a leak happens
Find the accessible internal shutoff, label it clearly and make sure household members know where it is. Keep access clear when fitting cabinets, boxing in services or storing belongings. If operation is stiff, corroded or uncertain, arrange a plumber’s assessment rather than forcing the handle. A valve that has not been used for years can leak or fail when disturbed. A suitable maintenance check is easier to plan than an emergency repair during a flooding incident.
Common mistakes
Closing the incoming stopcock does not necessarily drain every pipe or isolate stored water already in a cistern or cylinder. The arrangement depends on the system. Turning a valve without understanding it may leave hot-water or heating equipment needing further action. Do not assume an outside tap isolates the whole house, and do not confuse a gas emergency control with a water stopcock. If water reaches electrical equipment, avoid contact and obtain appropriate emergency help rather than reaching through water to operate switches.
Replacement, alternatives and requirements
A plumber may propose a replacement stop valve or another suitable isolation arrangement, with accessible positioning and compliance with relevant water fittings rules. Water Supply (Water Fittings) Regulations apply in England and Wales; Scotland has Water Byelaws and Northern Ireland has its own regulations. Approved products and correct installation both matter. A quarter-turn valve can be convenient in the right application, but the fitting’s suitability and full system arrangement should be checked. At handover, confirm what the valve isolates and whether any other valves are needed for stored supplies.
Frequently asked questions
Is stop tap another word for stopcock?
Yes, in everyday UK usage. Confirm which valve and supply someone means.
Does it turn off hot water too?
Not necessarily immediately. Stored supplies and the plumbing layout affect what continues to flow.
Should I force a seized stopcock?
No. Forcing it can cause damage or a leak; arrange competent assessment and replacement where needed.
Sources and further reading
Public sources and further reading. Sources checked: 8 November 2026. Consult the original for details and later changes.