A common cause of a leaking water heater is a loose drain valve. Other common causes may be too much pressure inside the tank and an old or worn-out tank.
A loose drain valve is one of the most common reasons for a water heater to leak. These leaks are easy to fix by simply tightening the drain valve snugly with a wrench, being careful not to over tighten it. Water heaters can also leak when the tank builds up too much pressure inside. Excess pressure forces the water to leak out of the tank to reduce the pressure, usually because the temperature is set too high. Old tanks tend to rust and corrode as they near the end of their useful life. Eventually, the tank can fail, resulting in flooding in the house.