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What is a Stopcock?

By Larry Ray Palmer
Updated May 17, 2024
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In plumbing terminology, a stopcock is a type of valve used to completely stop the flow of gas or water to a system. Sometimes called main plumbing valves, stopcocks control the flow of water or gas into a building's plumbing system from the main supply line. In areas served by a local utility provider, this valve is located outside of a building, often in the same general location as the meter. Depending upon the service provider, this valve may be locked to prevent unauthorized use and tampering. In the majority of modern systems, an additional stopcock may be installed inside the building to allow rapid disconnection of water or gas supply in the event of an emergency.

Stopcock valves are a form of inline plumbing valve with a rotating stop or plug. The plug is connected to a small handle that can turn to an open or closed position. In the majority of cases, this handle is a small, smooth, rectangular projection that requires a special tool, called a stopcock key, to create the leverage needed to turn the valve on or off. When turned clockwise, the line formed by the handle runs parallel with the main line and the service is turned on. When turned counter-clockwise, the line of the handle is perpendicular to the main line and the service is disconnected.

Due to the nature of plumbing systems, the local building codes in many areas require the installation of a secondary stopcock inside the building. This secondary stopcock is a protection device for the building owner in the event of damage to the plumbing system. These indoor stopcocks often have a different handle design than the outdoor stopcocks used by the utility service provider that makes them easier to use without the use of a special key or other tools.

While stopcocks do not require a great deal of maintenance, long periods of use in either the on or off position can result in the plug seizing and being unusable when the need for the valve arises. To avoid this issue, building owners are encouraged to test the function of secondary stopcocks on a regular basis. To perform this test, simply turn the valve handle to temporarily connect or disconnect service. If the valve handle does not rotate easily, maintenance or replacement of the valve may be required.

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