The hose bib at the bottom of the water heater seals by a rubber gasket compressing against a brass opening. When the rubber gasket is submerged in hot water for a period of time, the oil is leeched out of the rubber. When this happens, the gasket becomes hard instead of soft and pliable. The bib will not leak unless someone bumps or turns the handle. Then no matter how much you tighten it, it will not seal.
There is no need to call one of your local plumbing companies for this plumbing repair. These bibs are easy to replace and very inexpensive. The valve can be removed from the water heater and replaced in a matter of minutes.
You can always call Roto-Rooter should you have any questions.
