Two seals, two different jobs, and only one of them holds sewage
There is a wax ring squashed between the horn at the bottom of the bowl and the flange on top of the drain. Every flush pushes water past it, and it is the only thing keeping that water — and the sewer gas behind it — inside the pipe. Then there is a bead of caulk around the outside of the base where the porcelain meets the floor. That one keeps mop water and splashes from running underneath, and it stops the toilet rocking. It does not seal the drain, and it was never meant to.
Which is why the instinct to run a fresh bead around a leaking base is the one repair on this page that reliably makes things worse. If the wax ring has failed, water is already escaping under the bowl on every flush. Caulk it and you have not stopped the water — you have closed the only place it could be seen. It carries on into the subfloor, and the next evidence is a soft patch of floor, a black edge on the vinyl, or a stain on the ceiling of the room below. Clean water from a supply line is a nuisance. Sewage working into a floor structure is a different order of problem.
So the first question is not “what do I put on it” but “when is it wet”. Wet only during and just after a flush means pressurised flush water is escaping past the wax ring — that is the seal, and the toilet has to come up. Wet all the time, whether or not anyone flushes, and the water is clean: then something above is dripping down and pooling at the base, which fools almost everyone, because water always collects at the lowest point. And if it only appears in humid weather with condensation on the tank, nothing is leaking at all.
The second question is whether it rocks. Lean on the bowl and watch the joint. More than about a quarter of an inch — 6 mm — of movement means the closet bolts are loose, or the flange underneath is broken. A rocking toilet breaks and re-forms the wax seal every time somebody sits down, which is why a new ring fitted onto a rocking base fails again within weeks. Bolts first, then the seal.
Three faults, and the timing that separates them
- The wax ring. Water at the base only when flushing, usually discoloured, usually with a drain smell, and often a toilet that has been rocking for a while. The ring costs a few pounds and the toilet has to be lifted — there is no way around the lift, and no product that seals it from outside. While it is up, look at the flange: if it is cracked, or sits below the finished floor, a standard ring cannot bridge the gap and you need a thicker ring, a spacer, or a flange repair — which is the point at which the job changes shape.
- A leak from above. Clean water, present all the time, and the base is simply where it collects. Trace upward with a dry paper towel: the supply connection at the wall, the shut-off, the fill valve nut under the tank, the two bolts joining tank to bowl. Nothing here needs the toilet lifted, which is why misdiagnosing this as a wax ring is expensive in effort as well as money.
- A rocking toilet. The bolts are loose, or the floor is not flat, or the flange has cracked. Snug the bolts evenly — and gently, because porcelain cracks and a cracked base is a new toilet. If it still rocks with the bolts snug, the floor needs shimming, and the caulk line is what holds it steady afterwards. If it keeps loosening, the flange is the problem, not the bolts.
Two details that decide whether the repair lasts
Leave a gap in the caulk at the back. An inch of unsealed joint at the rear looks like a mistake and is the opposite: it is a weep hole. If the wax ring ever fails again, water escapes there and appears on the floor where you can see it, instead of being trapped under the base. Plenty of codes require it. And do not test before the caulk has cured — but do test before you caulk. Flush it several times with the toilet sitting on the new ring and the caulk still open, watch every joint, and only then run the bead. Caulking first and testing afterwards is how a mis-seated ring gets hidden behind a seal that looks finished.