A sheet stacker including a sheet conveyor arrangement and a stacking bay, wherein sheets delivered by the sheet conveyor arrangement are formed into stacks; the stacking bay including a stacker platform. The stacker platform is provided with a vertical lifting and lowering movement with respect to a stationary supporting structure and is controlled to move downwards while a stack of sheets is formed thereon. The stacker platform supports a stack conveyor configured and controlled to perform at least an evacuation motion, to remove a completed stack from the stacking bay. The evacuation motion of the stack conveyor is oriented such that a completed stack is moved from the stacking bay onto an evacuation conveyor arranged under the sheet conveyor arrangement.