a machine for stacking rectangular sheets of relatively rigid material such as panels , counters , doors , plastic sheets , or the like which has an input conveyor for receiving the panels as they are placed horizontally thereon , an elevator conveyor pivotally connected to the discharge end of the input conveyor on a horizontal axis for raising and lowering movement , a discharge conveyor at the outer end of the elevator conveyor , a conveyor drive motor for driving the conveyors , elevator motor for moving the elevator conveyor about its pivot axis , and a switch on the discharge conveyor for activating the elevator motor as a panel is discharged therefrom to raise the elevator conveyor a selected distance prior to discharge of the next panel . the machine also includes a mechanism on the input conveyor for straightening any panel disposed thereon prior to its movement onto the elevator conveyor .