Your toilet tank is sealed from leaking by a round rubber gasket that is seated upon the back of the bowl. Because the tank is held in place by only two bolts, it is subject to movement by larger individuals leaning against the tank as they read or contemplate the meaning of life on the toilet, or children inexplicably shaking the tank back and forth
This leaning or rocking motion can eventually wear away the gasket, causing water to drip onto the floor behind the toilet. Luckily, it is rather simple to replace the gasket. However, you must not have an aversion to getting wet or handling a slimy, wet gasket. A little bit of contortionist skills may also be helpful, as you navigate under and behind the toilet to complete the task.
What you will need
A new gasket
Toilet tank gaskets vary in size, so it would be best to remove and clean the old gasket, then take it to a home improvement store to match it to the correct replacement gasket.
This type of pliers have jaws that are offset to one side instead of straight, and will lock in place when the handles are completely closed.
Large flat head screwdriver
Rags to absorb water from tank and to cover floor
Removing the old gasket
You will need to drain the tank of water, so first turn off the supply valve that controls the flow of water into the tank. Remove the tank lid and flush the toilet, holding the flush handle down until no more water will flush.
Absorb the rest of the water in the tank with a rag, wringing it into the bowl until the tank is dry.
You will see two flat bolt heads at the bottom of the tank. Beneath the toilet are two nuts and washers that hold the bolts in place to secure the tank to the toilet.
Lean behind the toilet (it's not always easy in a small bathroom with a corner toilet) and secure the locking pliers around one of the bottom nuts. You will need to adjust the width of the pliers' jaws by turning the round knob at the base of one of the handles.
When the pliers are secured, loosen the corresponding bolt from inside the tank with your flat head screwdriver. The locking pliers may turn, but will stop when hitting the wall or the side of the toilet.
When both bolts are removed, you can remove the tank. You will need to rock it back and forth as you lift it to separate it from the old gasket.and bowl.
When it is removed, place it on rags on the floor. Remove the old gasket (it may be slimy and disgusting) and clean it with a rag. Take it with you to buy the new gasket. Take the bottom nuts along also, and buy wing nuts of the same size.
Installing the new gasket
Place the gasket, larger side down, onto the bowl, pressing it down as much as possible. Next, place the tank onto the gasket. The tank will need to be fully seated on the gasket, so you may need to rock it slightly as you push the tank onto the gasket. Seat the tank as level as possible (a stick level or even a phone app level would be helpful in assuring the tank is level).
Place the bolts into the openings in the bottom of the tank. They will have black rubber washers attached, which will be pressed against the bottom of the tank as the nuts on the bottom are tightened.
Instead of using the old nuts, use the newly acquired wing nuts under the bowl to secure the bolts in place. Don't forget to include the plastic washers that were removed with the old nuts before adding the wing nuts.
Using the wing nuts will allow hand tightening, which may be difficult with the old square nuts because their location under the tank makes it hard to get a good grip. Hand tighten only until the tank is secured against the bowl. Do not over-tighten or you may crack the porcelain tank.
Put on the lid, turn on the water, clean up your mess, and you're finished. For more information or assistance with plumbing issues, contact a company like Knights Plumbing & Drain.