The three that are hiding damage
Putting these first because they are the ones people live with.
Water at the base of the toilet. Usually the wax seal between toilet and flange has failed, so every flush puts a small amount of waste water under the floor. It is not an emergency this hour; it is genuinely damaging over weeks. And if the toilet also rocks, do not simply tighten the bolts — tightening against a corroded flange cracks the porcelain, turning a seal repair into a new toilet.
Tile that sounds hollow when tapped. Tap around the shower enclosure; a hollow note means the tile has debonded from its substrate, and water is generally why. Re-grouting will not fix it, because grout is not what keeps water out — the membrane behind it is.
A soft patch, a stain, or a musty smell that returns after cleaning — on the floor, on the wall behind the shower, or on the ceiling of the room below. That is water that has already passed the waterproofing.
All three share a pattern: the visible symptom is minor and the thing causing it is not.
Worth knowing
Bathrooms are frequently above living space or on a slab, so a slow leak has somewhere expensive to go. That is why a small stain on a ceiling below a bathroom is worth investigating immediately rather than repainting.
Toilets
Running constantly is nearly always the flapper — hardened rubber, or its seat scaled up on our water. Food colouring in the tank, no flushing, colour in the bowl within fifteen minutes confirms it. It is also frequently the largest single item on a water bill, so it is worth fixing the week you notice it.
Weak flush. Check the tank water level against the marked fill line, then the slack in the flapper chain. If both are right, the rim jets under the bowl edge have probably scaled over.
Slow to drain. A flange plunger — the type with the extra sleeve — not the flat cup kind, which cannot seal the outlet.
Repeat clogs in one toilet point past the fixture, at the branch line or venting. A properly cleared toilet stays clear.
More detail in common toilet problems.
Showers and tubs
Weak or uneven spray is almost always scale in the showerhead nozzles, not supply pressure. Soak it in white vinegar for a few hours before concluding anything else.
Dripping when off, or a stiff handle — the valve cartridge. Stiffness is the early warning worth acting on, because stiff handles get forced and forcing damages the valve body, which means opening the wall.
Temperature wandering, or going hot when a toilet flushes — the pressure-balancing spool is not compensating. That is a scald-protection feature failing.
Slow draining — hair bound with soap scum, which is what soap becomes in hard water. A hair catcher prevents nearly all of it. Chemical cleaners will not clear a hair mat and damage older pipe.
Gurgling when a toilet flushes is not a shower problem at all — that is a main line symptom.
Sinks and faucets
Where it drips tells you what failed. From the spout is the valve mechanism. From the base of the spout is the O-rings. From the handle is the packing. From under the sink with the faucet dry is the supply line or shutoff — and that last one is the version that quietly ruins a cabinet base.
Weak flow at one tap — the aerator. Unscrew, soak in vinegar, refit. On our water this is the most common single-fixture complaint and it costs nothing to fix.
Slow drain — pull the trap and clean it. Bucket underneath, hand-tight on reassembly, and check for drips for a full minute before walking away.
Check under every sink monthly with a torch. You are looking for the slow drip that has been going for months and the white mineral crust left by one that evaporates between visits.
What hard water does to a bathroom specifically
Most of the recurring bathroom complaints here trace back to one cause.
Scale forms on valve sealing faces, so cartridges drip and handles stiffen sooner. It blocks showerhead nozzles and aerators, producing what feels like a pressure problem. It builds on toilet flapper seats and in rim jets, causing running and weak flushes.
And soap scum — the film on the glass and the mat in the drain — is a hard water product rather than a cleaning failure. It is what soap becomes when it meets calcium and magnesium, which is why it returns so quickly.
None of that makes a bathroom unusual; it just means components here reach the end of their life sooner than the manufacturer assumed. A softener extends every one of them, which is worth weighing against replacing the same parts repeatedly.
See what hard water costs for the numbers.