It's leaking. Where
from matters most

Do these two things first

Shut off the cold water supply. There is a valve on the pipe entering the top of the heater. Closing it stops the tank refilling as it empties, which is what turns a leak into a flood.

Turn the heater off. On gas, turn the control to Pilot or Off. On electric, switch off the breaker — an electric element that fires in a partly empty tank burns out in seconds.

That is enough to make it safe and stop the loss. Everything below is diagnosis, and you can do it with a torch and a dry paper towel.

Find the source with a paper towel

Dry everything thoroughly, then press a dry paper towel at each point in turn and check which one comes away wet. Water runs down and pools at the bottom regardless of where it started, so the puddle tells you almost nothing about the source.

Work top to bottom, because gravity means anything wet high up explains everything below it.

• The inlet and outlet fittings on top. Threaded connections and flex lines loosen and corrode. Usually a repair, sometimes just a tighten.
• The T&P relief valve on the side or top, and the pipe running down from it. Either the valve is failing or it is discharging because it has been asked to.
• The drain valve at the bottom. Plastic ones in particular weep after being opened for a flush. Cheap to replace.
• Around the base of an electric heater's element ports. Element gaskets fail; the element is replaceable.
• The tank itself — no wet source above, water simply appearing at the base. That is the lining.

The T&P valve, which is the one to take seriously

Temperature and Pressure relief is the safety device that stops a water heater becoming a pressure vessel. It opens if the water exceeds roughly 210°F or 150 psi, and it is the reason water heaters do not fail catastrophically.

So a T&P valve that is discharging is doing one of two things: failing, or telling you something true. The second possibility is the one that matters.

Thermal expansion is the usual cause here. Water expands as it heats, and in a closed system — one with a check valve or pressure regulator on the incoming main, which most homes have — that expansion has nowhere to go, so pressure climbs until the relief valve vents it. The fix is a properly sized expansion tank, not a new relief valve.

Never cap, plug or tighten down a discharging T&P valve. It is the one component on the heater whose defeat is genuinely dangerous.

When it is the tank

The steel is glass-lined, and the anode rod exists to corrode in the lining's place. Once the anode is spent, corrosion attacks the tank, and it works from the inside outward. By the time water appears you are seeing the end of a process that has been running for a while.

That is why a leaking tank is not repaired. There is nothing accessible to repair — the failure is inside a sealed vessel, and welding or patching a pressurised, glass-lined tank is not a thing anyone reputable does.

The warning that usually precedes it is rusty hot water while the cold runs clear. If that happened months ago and now there is water on the floor, those are the same story.

It is also why a heater in its second decade with a weeping tank is a replacement conversation rather than a repair quote, and we will say that plainly rather than fit parts to it.

Condensation, and the false alarm worth knowing

Not every wet water heater is leaking. A gas heater running with very cold incoming water — which is to say, in winter, or right after heavy use has emptied the tank — can condense moisture on the outside of the tank, which then runs down and puddles.

The tell is that it appears during or just after heavy hot water use and dries up when the tank has recovered. A genuine leak does not care what you are doing.

If a heater is new and doing this, it is often nothing. If a heater is ten years old and doing this, do not talk yourself into the comfortable explanation — dry it, watch it, and check again when nobody has run hot water for a few hours.