Knowing how to shut off a toilet valve is something everyone should know. When something goes wrong and the toilet is overflowing it is not the time to learn how or, find out it doesn’t work.
Toilets normally work quite reliably and may not need to be shut off for many years. But, there are many people every year that have problems that can cause large amounts of water damage to occur if water cannot be shut off quickly. Situations like the fill valve failing and not shutting off where the tank overflows, the tank breaking when someone leans back on it or, a supply line breaks, you need to be able to shut off the water in a hurry and not wait for a plumber to respond to your call and shut off the water.
Go to your toilet, close the seat lid, and kneel in front of the toilet facing it. Under the tank on the left side the shank of the fill valve comes out of the bottom of the tank. Connected to this tank is a supply line that either goes down to an angle stop valve coming out of the wall or, to a straight stop valve coming out of the floor. This stop valves should turn off the water to the toilet.
You should try turning off the valve several times a year and this will ensure that it works for a long time. You should also try it just to make sure that it works and does shut off the water. When these valves sit without being used for a long time they often get very hard to turn. The rubber packing bonding to the metal on the stem causes this. Often the rubber will tear the first time it is turned and the valve will start to drip around the stem. If it starts to drip you may be able to tighten the packing nut using two wrenches, tighten just enough that the dripping stops and do not over-tighten it or it will get too hard to turn.
Some of the newer valves are ¼ turn valves and they do not have sticking problems. If your valve is bad and will not shut off replace it with a ¼ turn valve.