A stopcock leak deserves more attention than an ordinary drip
The internal stopcock controls the incoming mains water to the property, so a leak here sits on a pipe that is normally under pressure all the time. Even a slow seep can damage the cupboard, floor or wall around it, and the valve is the thing you may need in an emergency. First dry the valve and identify where the water starts. Common points are the spindle beneath the handle, the compression joints on either side or the valve body itself. Do not keep opening and closing an old leaking stopcock simply to test it; movement can make a marginal seal worse.
Water around the spindle can come from the gland
Many traditional stopcocks use packing around the spindle to prevent water escaping where the handle enters the body. That packing can dry, loosen or wear. A plumber may be able to service the gland if the rest of the valve is sound. The repair needs a light touch because overtightening can make the valve difficult to operate. If the spindle is badly corroded or the handle is already stiff, the condition of the whole stopcock becomes more important than the small amount of water currently visible.
Joint leaks may point to movement or corrosion
Compression fittings can seep if the pipe has been disturbed, if the olive no longer seals properly or if the pipework is under strain. In older properties, the incoming pipe may also be lead, galvanised steel or an early plastic material rather than modern copper. That changes how a replacement valve should be connected. A leak at the joint should not simply be covered with sealant. The fitting needs to be made sound while protecting the incoming service pipe from unnecessary twisting or bending.
Can you isolate the water upstream?
Repairing the main internal stopcock often requires the supply to be shut off before the valve. That may mean using an external stop tap at the boundary, meter box or pavement chamber. Access and responsibility vary between properties and water companies. Do not force a seized street valve with makeshift tools. A plumber will usually establish a safe way to isolate the service before dismantling the internal stopcock. In some homes a secondary full-bore valve can be added in a more convenient position once the main control is repaired.
Why replacing an old stopcock can be worthwhile
A valve that leaks, barely turns and sits in an inaccessible cupboard is not much use when a pipe bursts. If the stopcock is very old, replacement can improve reliability even if the immediate seep could technically be packed or tightened. The plumber should consider the condition and material of the incoming pipe before cutting anything. Where access is poor, moving the new isolation point slightly may make future emergencies much easier to manage. The aim is to leave you with a control that both seals and operates freely.
What to say when asking for help
Explain whether the leak is constant and exactly where it appears. Mention the pipe material if you know it, the age of the property and whether you can find an outside stop tap or water meter. A clear photograph showing several inches of pipe either side of the valve is better than an extreme close-up. Tell the plumber if the stopcock is boxed in, behind appliances or close to electrical equipment. Those access details can make a large difference to the time needed for what otherwise looks like a simple valve repair.
Make sure the main control still does its main job
After a stopcock repair, the valve should not merely be dry. It should also be capable of shutting the property supply off without excessive force. Ask the plumber to demonstrate that it closes and reopens cleanly, then check that normal flow returns at the taps. If the old valve was in a very awkward position, it may be worth discussing a second accessible isolation valve downstream, provided the incoming service is suitable. Label or remember the location so other people in the property can find it quickly. A stopcock is rarely thought about during normal life, but during a burst pipe it becomes one of the most important fittings in the house. A successful job therefore leaves you with both a sound joint and a dependable emergency control.
This page is general guidance about why is my stopcock leaking; the cause in an individual property depends on the actual plumbing arrangement.