A number of bladders are stacked one upon another and in fluid flow communication with one another. A fluid introduced into one of the bladders causes that bladder and the remaining bladders to inflate with resulting increase in the height of the stack. An inflexible support panel is seated on top of the uppermost bladder. The panel may be attached to the uppermost bladder or may be detached from it. The panel has a flat upper surface which remains generally horizontal as the bladders inflate while the stack rests upon a flat horizontal surface.
