Block stacking usually consists of unit loads, stacked on top of one another. The height of the stack is typically limited to 4 m and is governed either by the crushing resistance of the load or by the stability of the forklift truck or stack.
Typically, unit loads are stored on timber pallets, in metal stillages or post pallets. It is usual, for ease of racking installation and block stack stability, to keep unit load dimensions and weight within close tolerances. Rolls of newsprint, bales and packaged goods handled by hydraulic clamps rather than forks are also considered as unit loads.