The different shut-off valves in a cold water system, and where the water company's pipework ends and yours begins.
Stop valve, gate valve and service valve
A stop valve (stopcock) shuts off the whole incoming supply. It seals against the flow, so fit it with its arrow pointing the way the water flows. It is the valve used where the main enters and at the property boundary. In an emergency, such as a burst or split pipe on the mains-fed side, close the internal stop valve first: it should be easy to reach and free to turn, and it shuts off everything after it. The boundary stop tap belongs to the water company, usually sits under a small cover outside and needs a special key, so it is the fallback if the inside valve will not close. Pipework fed from a storage cistern keeps running after the stop valve is shut, until you close the cistern's own outlet valve or the cistern empties.
A gate valve opens to a full bore with no restriction, so it suits low-pressure pipework such as a gravity feed from a storage cistern. It has no set flow direction and is turned with a wheel; it is not meant for fine flow control.
A service (servicing) valve is a small in-line valve, often turned with a screwdriver, fitted close to one appliance or fitting. It isolates that single item for repair or replacement without draining the rest of the system.
Pillar and mixer taps
A pillar tap is a single tap body that delivers one supply, hot or cold, to its own outlet on a basin or bath. A mixer tap blends the hot and cold supplies together so they leave through one shared spout.
Communication pipe and supply pipe
The underground pipe from the water main to the boundary of the property is the communication pipe. It belongs to the water company, which is responsible for it.
The supply pipe normally continues from the property boundary towards the building and is the owner’s responsibility. A water company stop tap at or near the boundary usually marks the dividing line, but where it sits does not decide who owns what. Check shared supplies, and pipes that cross other land, with the water company and the property records. The internal stop valve (main stopcock) is normally already on the customer’s supply pipe.
Draining the system
A drain-off cock (draining tap) is fitted at the lowest point of the pipework. Water collects there under gravity, so opening it empties that part of the system for a repair or before a hard frost.
Where the stopvalve goes, and keeping valves usable
The guidance to the Water Fittings Regulations asks for a building's stopvalve to be inside the premises, above floor level, near where the supply pipe enters, fitted so that closing it stops the supply to every point of use. Where one pipe supplies two or more premises, it needs a stopvalve that each occupier has access to. Valves may sit in a duct or cupboard behind a hinged door or removable cover, but that cover must stay visible; it must not be covered by carpet, tiles or wallpaper. Stop valves, servicing valves and drain taps should be labelled to show which part of the system each one controls, so the right one can be found for maintenance or to drain down before cold weather.