A stopcock (also called a stop valve or main stop valve) is a valve fitted in a water supply pipe that can be turned off to shut off the flow of water to all or part of a building. The main internal stopcock is typically located under the kitchen sink or in a cupboard near where the water supply enters the building. Turning it fully clockwise closes it; anticlockwise opens it. All occupants of a building should know where their stopcock is located - it is the first thing to turn off if a pipe bursts.

Where is the stopcock in a house?

Most UK homes have two stopcocks. The internal stopcock (the one you control) is usually under the kitchen sink, in a ground-floor bathroom, in a utility room, or under the stairs - wherever the mains supply enters the building. The external stopcock (boundary stopcock or external stop valve) is owned by the water company and is located at the property boundary, usually under a small metal cover in the pavement or driveway. The external stopcock requires a special square key to operate.

My stopcock is stiff - can I force it?

Do not force a stiff stopcock - the valve stem can shear, leaving the valve permanently stuck in one position and requiring replacement. Old brass stopcocks often seize up if left open for years. Apply penetrating oil and wait a few minutes, then try gentle pressure. If the valve is very old or visually corroded, it is worth replacing it proactively before it fails in an emergency. Replacement of the internal stopcock is a straightforward job for a plumber and may require a brief water shutoff at the external stopcock.

What is the difference between a stopcock and an isolating valve?

A stopcock is a full-flow valve in the main supply pipe, designed to isolate the whole building or a large section of it - typically operated with a hand wheel or large handle. An isolating valve is a smaller quarter-turn valve (usually requiring a flat-blade screwdriver to operate, with a slot on the head) fitted directly to individual appliances - under the toilet cistern, under the basin hot and cold feeds, at the washing machine connection, and at the boiler. Isolating valves allow individual appliances to be serviced without turning off the whole building supply.

Plumbing & Heating noun

Stopcock

/ ˈstɒpkɒk /

Also known as: stop valve, main stop valve, water isolation valve

Definition

A stopcock (stop valve) is a valve in a water supply pipe that can be closed to shut off the water supply to all or part of a building. Every building must have an accessible internal stopcock to allow the water supply to be isolated in an emergency or for maintenance. It is typically located where the mains supply pipe enters the building - under the kitchen sink, in a utility room, or in a ground-floor cupboard.

In practice

A traditional stopcock has a brass body with a hand wheel or lever. Turning it fully clockwise closes it; anticlockwise opens it. Modern equivalents often use a quarter-turn ball valve design that is more reliable and less prone to seizing. The internal stopcock controls the entire building supply. Individual appliances have their own smaller isolating valves - typically quarter-turn slot-head valves under each appliance - that allow one fixture to be isolated without cutting the supply to the whole building.

Stopcocks that are never exercised often seize in the open position. Testing yours occasionally - a slow turn to nearly closed and back - prevents this. The external stopcock (boundary valve) is owned by the water company and is in a metal box in the pavement or driveway, operable with a square-section stopcock key available from plumbers' merchants.

Building Regulations

Relevant PartPart G - Sanitation, hot water safety, and water efficiency

RequirementAccessible stopcock must be provided at the building entry point

Water RegulationsWater Supply (Water Fittings) Regulations 1999

Approved Document G and the Water Supply (Water Fittings) Regulations 1999 require that every water supply to a building includes an accessible stopcock where the supply enters. The stopcock must be operable without tools, in an accessible position, and protected from frost. Water Regulations (not Part G Building Regulations) also require individual isolating valves on WC cisterns, appliances, and hot water equipment to allow maintenance without cutting the whole supply.

Full Building Regulations guidance

Stopcock

Just so you know...