Water under a water heater ranges from harmless (drain valve drip) to urgent (tank failure). The first step is always to identify the exact source before assuming the worst.
Likelihood: Most common, benign
The plastic or brass drain valve at the bottom of the tank can leak at its threads or packing — especially if it was recently used.
Fix: Try tightening the valve 1/4 turn clockwise. If it still drips, replace it ($5–10 brass valve).
Likelihood: Serious — don't ignore
The T&P (temperature & pressure) relief valve has a discharge pipe that should point down. If the valve is opening occasionally (from high pressure or temperature), water drips from this pipe.
Fix: If water is coming from the T&P discharge: check thermostat setting (reduce to 120°F), check water pressure (should be under 80 psi), then replace the T&P valve ($15–25).
Likelihood: Less common
The inlet and outlet connections at the top of the tank can develop slow leaks at the fittings.
Fix: Tighten fittings with a pipe wrench or apply plumber's tape and thread sealant.
Likelihood: Less common — older tanks
When a steel tank corrodes through, it leaks from the bottom seam. This is irreparable.
Fix: Replace the water heater. A leaking tank cannot be patched.
Dry the entire area with towels. Look at the drain valve (bottom), both pipe connections at the top, the seam around the tank base, and the T&P discharge pipe (side/bottom). Use paper towels to pinpoint the drip location.
Try turning the drain valve cap clockwise 1/4 turn. If the drip stops: done. If it continues, drain the tank (connect garden hose, open a hot tap, open drain valve), then remove and replace the brass drain valve.
If water is dripping from the T&P discharge pipe: do not cap this pipe. The valve is releasing excess pressure as designed. Check your thermostat (lower to 120°F if above that). Have a plumber check water pressure. Replace the T&P valve if it's more than 6 years old or continues dripping after pressure correction.
Feel along the base seam and look for rust streaks. If the tank itself is leaking: shut off the gas/power and cold water inlet. Call a plumber — the tank must be replaced.
If the leak is coming from the tank body (not a fitting or valve): yes, immediately turn off the gas or breaker and close the cold water shutoff. A leaking tank can fail suddenly. If the leak is from the drain valve or a fitting, you have time to diagnose calmly but fix it today.
Most tank water heaters last 8–12 years. Check the serial number — the first four digits often encode the manufacture year and week (varies by brand). If your tank is 10+ years old and leaking from the body, replacement is nearly always more cost-effective than repair.
Call a plumber immediately if: the tank body is leaking (irreparable, urgent), the T&P valve keeps releasing after correcting thermostat and pressure, or you need a full replacement installed.
ChatDIY can adapt this diagnosis to your exact fixture, tools, photos, and skill level.