The four common faults
Runs continuously. Flapper, usually. If a new flapper does not fix it, check the fill valve and the float height - an overflowing tank drains down the overflow tube and looks identical.
Weak or incomplete flush. Clogged rim holes, a flapper closing too early, or a partial blockage in the trapway.
Water on the floor. Where it comes from decides the severity. Condensation on the tank in a humid bathroom is cosmetic. Water at the base is a failed wax seal and needs pulling and resetting. Water from the supply connection is a fitting.
Rocking. Loose closet bolts at best; a compromised flange or rotten subfloor at worst.
Do not flush
“Flushable” wipes, kitchen roll, cotton buds, dental floss, sanitary products. Wipes in particular do not break down and are the most common cause of main-line blockages.
When to call
Call when the toilet rocks, when the floor around it is soft, when the same blockage returns, or when more than one fixture is backing up - that last one is a main-line problem the toilet is only reporting.
Common questions
Why does my toilet run constantly?
Nearly always the flapper. It hardens with age and stops sealing, so water trickles from the tank into the bowl and the fill valve keeps topping it up. A flapper is a cheap part and a ten-minute job.
How much water does a running toilet waste?
Enough to show up clearly on a bill. A silent trickle you cannot hear can still run continuously - put a few drops of food colouring in the tank, wait ten minutes without flushing, and look in the bowl. Colour in the bowl means it is leaking.
When is a rocking toilet serious?
Immediately. Movement breaks the wax seal below, and what leaks out goes into the subfloor rather than across it, so you find out when the floor is already damaged.