On a roof tank you should find the tap on the top pipe.
Roof tank ballcock.
They work by rising with the water level in the tank which in turn activates a lever which then shuts the valve.
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Before you go any further check that there is no water in the ballcock float.
This part is often known by the generic term ballcock although strictly speaking a ballcock is an older form of this valve that is rarely installed anymore.
There should be no water gushing out.
Make sure the water is turned off by taking off the lid of the cistern or tank and pushing down on the ballcock arm.
How to change the ball valve ballcock in your cold water storage tank in the loft.