How to Fix a Toilet Cistern That Is Leaking Underneath

1. Turn off the water supply to the toilet. There is usually a valve behind or underneath the toilet that you can turn to do this.

2. Flush the toilet to empty the cistern.

3. Disconnect the water supply line from the cistern.

4. Remove the cistern lid. There are usually two or three bolts at the back of the cistern that you can remove to do this.

5. Check the cistern fill valve for leaks. The fill valve is the part that controls the flow of water into the cistern. If it is leaking, you may need to replace it.

6. Check the cistern overflow tube for leaks. The overflow tube is the tube that allows excess water to escape from the cistern. If it is leaking, you may need to replace it.

7. Check the cistern wax seal for leaks. The wax seal is the seal between the toilet bowl and the cistern. If it is leaking, you may need to replace it.

8. Reinstall the cistern lid and reconnect the water supply line.

9. Turn on the water supply to the toilet and check for leaks. If there are still leaks, you may need to call a plumber for help.

Here are some additional tips for fixing toilet cistern leaks:

- When replacing the cistern fill valve or overflow tube, use plumber's putty to create a watertight seal.

- If you are replacing the toilet bowl wax seal, use a wax ring that is the correct size for your toilet.

- Tighten all bolts and nuts securely.