A stopcock that will not turn the water off completely usually has a worn or damaged washer inside, or debris and scale stopping the washer from sealing against its seat. WaterSafe notes that when a stop tap will not shut off fully, the internal washer may need replacing, which involves removing the headgear and is a job more suited to a plumber or experienced DIYer.
Sometimes the stopcock is fine and the water is coming from another source. Checking this first can save time.
Signs the Stopcock Is Not Sealing
After turning the stopcock off, open the cold kitchen tap. A small trickle that slows and stops is normal, as the water left in the pipes drains out. A steady flow that does not stop suggests water is getting past the valve.
If you have a water meter, watching whether it still moves with the stopcock closed and all taps off can also help.
Note how long you waited and how strong the flow was. These details help a plumber judge whether the valve is barely sealing or letting through a lot of water.
Common Reasons It Lets Water Through
The washer inside a traditional stopcock presses down onto a seat to stop the flow. If the washer is hardened, split or flattened, it cannot seal properly. Grit or limescale on the seat can stop even a good washer from closing fully.
The stopcock may also not be turning far enough. A stiff valve can feel as though it has reached the end of its turn when it has not. Never force it further, as this can damage the spindle.
Some newer homes have a different type of valve, such as a quarter-turn lever valve. These can also fail to seal if the internal ball or seals are worn or scaled, though the parts involved differ from a traditional stopcock.
When the Water Comes From Elsewhere
Some homes have more than one supply pipe, or a shared supply that feeds several properties. If the stopcock serves only part of the home, other taps may still run when it is closed.
Stored water can also keep flowing. If you have a cold water tank in the loft, some taps and toilets may be fed from that tank rather than directly from the mains, so they will keep running for a while after the stopcock is shut.
If you are not sure how your home is supplied, a plumber can trace the pipes and show you which valve controls what.
Repair or Something More
A worn washer or damaged seat can often be dealt with as a repair, renewing the parts inside while the outside stop tap is closed. If the valve body is corroded or cracked, or the seat is badly worn, stopcock replacement may be the more practical option.
A stopcock that cannot shut off fully leaves you exposed if a pipe bursts. If yours keeps letting water through, our team can carry out repairs on a faulty stopcock in Oxford and check that it seals properly once the work is done.