In every attic there should be an isolation valve to the tank so there should be no reason to turn the mains water supply of until the emergency plumber arrives to fix the plumbing.
Overflow tank in attic leaking.
At the very least shingles.
Once done refill the tank with cold water.
Take the lid off each tank and check how high the water level is in each one.
These type of water leaks are easy to find but also easy to fix if your good at a bit of diy plumbing.
Flush the toilet to drain the water.
The water level in a tank is too high so it is running out the overflow.
A leaking overflow on the tank is a common cause of a faulty ball cock which is also known as a float operating valve part 1 or part 2.
This is a valve located on the water pipe that.
Water leaks occur through roof failure ice dams and water condensation or frost build up inside an attic.
The most common leaks coming through the ceiling from the attic would be the float operating valve followed by a burst pipe due to the cold or a joint on the pipe work that may of been knocked at some point.
If the leaking overflow pipe is at loft level or protrudes from the eaves of the roof then the source is either the cold water storage cistern or the smaller feed and expansion cistern that supplies the boiler if you have one.
Use a flat head screwdriver to turn the screw a quarter turn clockwise.
In all cases if you do not know which is the problem cistern then a visual inspection will tell you.
Rainy weather causes the first type of leak to be discovered.
Go into the loft and take the lid off each tank.
There are normally two but can be virtually any number depending on the size and type of system you have.
Your central heating system might also benefit from a powerflush to clear out any sludge debris or rust that s blocking the water from flowing properly.
When removing the sludge try not to disturb the water too much as it could end up making its way down the overflow pipe.