If a stopcock will not fully shut off the water, some water may keep flowing into the property even when the valve is closed. This can make it harder to control a leak or burst, and harder to carry out plumbing work safely, so it is usually worth having the valve repaired or replaced.
This kind of fault is often only noticed when the stopcock is needed most. Checking it now and then can help you find out before an emergency.
Why It Happens
Inside many stopcocks, a washer or seal presses onto a seat to stop the water. If the washer is worn, the seat is damaged, or limescale or debris is in the way, the valve may not seal completely.
Stopcocks that are rarely used can be more likely to have this problem, as build-up can form around the parts that seal when the valve is closed.
A stopcock that has been forced in the past may also have a damaged spindle or internal parts, which can stop it closing fully. These are possible causes rather than a diagnosis, as the valve needs checking to know for sure.
How to Tell
Close the stopcock gently by hand, without forcing it, then open the cold kitchen tap. A short burst of water is normal as the pipes empty. If water keeps running steadily, the valve may not be shutting off fully.
If the stopcock is stiff or will not turn far, do not use tools to force it. Some homes have more than one valve on the incoming pipe, so make sure you are testing the one that controls the whole supply.
What It Means in Practice
In an emergency, a stopcock that does not shut off fully may only slow a leak rather than stop it. If you need to stop the water completely, the outside stop tap may be needed. Your water supplier can advise on this.
If you rent your home, let your landlord or letting agent know, as they may need to arrange the repair.
It can also affect planned work. A plumber may need to isolate the supply elsewhere before repairing pipes, fitting new taps or making other changes.
Repair or Replacement?
Some stopcocks can be repaired by replacing a worn washer or other internal parts. Our stopcock repair page explains when a repair may be possible. If the seat or body is damaged, the valve is badly corroded, or parts are not available, replacement may be more practical.
Our page on replacing a faulty stopcock explains how stopcocks that will not shut off properly are assessed and when replacement may be suggested.
If your stopcock will not fully shut off the water, a plumber can check the valve and explain the options before it is needed in an emergency.