The toilet cistern is the tank on the top of the toilet that holds the water for flushing and the flushing mechanisms. If it is leaking, it can cause issues with the functioning of your toilet and can damage the walls and floors around it. That's why it's critical to fix it as soon as you notice the problem.
Diagnose the Leak
You can't fix a leaky toilet tank if you don't know the problem, so first and foremost, inspect your toilet cistern to see where the water is leaking. If the water seems to be originating near the bowl or the center of the tank, it's possibly a leaking bolt gasket. If the water seems to be originating around the supply hose connection, then it's probably a stripped coupling or another type of bad gasket.
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Either way, once you have a good idea of what might be causing the leak, you need to turn off the water before making any repairs. This prevents the broken components from leaking any further. More importantly, it ensures that you won't deal with accidental floods or running water while you work.
When you're ready to begin, turn off the water to your toilet by locating the water supply valve on the wall behind or the floor beneath the toilet. Turn the knob on the end of the valve clockwise until it stops. If you have difficulty, use a wrench.
Repair the Coupling
Before proceeding with repairs, remove the top of the tank and flush the toilet to empty it. Next, find the coupling connecting the tank to the supply line or pipe and loosen it slightly. If the coupling gasket is worn down, which can happen over time, you will need to replace it. You can find coupling gaskets at any hardware store but make sure you get an exact match.
Next, inspect the threads of the supply inlet. If they are dirty, take a piece of steel wool and gently scrub them to remove any built-up grime or dirt. If the threads are damaged, wrap them in plumber's tape and then reconnect the coupling. Periodically, check to see if the tape is holding or if the threads need to be rewrapped. If the leak isn't coming from the coupling gasket or other connected parts or if these steps didn't solve the issue, move on to the next steps.
Fix the Tank
There may also be an issue with the cistern itself that is causing the toilet to leak. Before proceeding, disconnect the supply line and loosen the coupling connecting the tank and supply pipe. After these lines are sufficiently loose, use a wrench to loosen the nuts located on the underside of the tank. Inspect the bolts and gaskets to look for damage and replace them if you see any, as this could be the source of the leak. Once you have made repairs, replace the top of the tank, tighten the coupling and turn the water supply valve back on.
If these steps do not work to fix your leaking toilet cistern, you may need to call in a plumber or contractor. The issue could be more serious, involving the supply line or pipe or the toilet itself. If this sort of severe issue seems to be the case, it is best to let professionals handle it.