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Water heater leaking

Where the water is coming from tells you almost everything. A leak from the top is usually a repair. A leak from the tank itself is not.

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The short version

From the top: Usually a loose connection or a valve. Often repairable.
From the bottom: The drain valve, or the tank itself. A leaking tank means replacement.
Do now: Turn off the cold water supply to the heater and the power or gas.

Smell gas near the water heater? Leave the house first. Call your gas company or 911 from outside. Do not switch anything on or off on the way out.

Safe to do yourself

Turn the cold water valve above the heater off.
Switch off the breaker for an electric heater.
Set a gas heater's control to off or pilot.
Put a pan or towels under the leak and photograph it.

Stop and call

Water is coming from the tank body or seams.
The relief valve is releasing water.
The leak is near wiring or the gas control.
There is rust in the water or on the tank.

Where water heaters leak

Connections on topThe supply fittings can loosen or corrode. Water runs down the side and looks like a tank leak.
The relief valveThis valve opens when temperature or pressure is too high. Water from it is a warning, not a nuisance.
The drain valveThe valve at the bottom can drip. It can often be tightened or replaced.
The tankOnce the steel tank rusts through, no repair holds. The heater is replaced.

What a plumber will do

Find exactly where the water is coming from.
Test the relief valve and the incoming pressure.
Repair the fitting or valve if that is the cause.
If the tank has failed, quote a replacement and the permit that goes with it.

Never cap or plug a relief valve that is dripping. It is the safety device that keeps the tank from over-pressurizing.

Related guides

Sources

US Consumer Product Safety Commission: tap water scalds