A method and apparatus for mounting a water heater on the roof of a building or at another location outside of the building. A roof curb surrounds a hole in the roof, and a base member is mounted to the roof curb. Alternatively, the base member is mounted on a concrete pad at ground level. A water tank is mounted to the base member with the longitudinal axis of the water tank extending horizontally. Support rails are mounted to the base member, and wedge-shaped supports are mounted to the support rails. The wedge-shaped supports are interconnected with tank mounted brackets that are mounted to the sides of the water tank. Thermally insulated spacers are interposed between the wedge-shaped supports and the tank mounting brackets.