The modular lumber stacker utilizes a single fork and board width sensor to load layers of boards with varying widths onto a lift table. The single-fork loader allows a fork lift to readily access the opposing side of the lift table to remove a multi-layer stack of boards. An adjustable unloading stop may be used to justify the stack by distributing the dead space in each layer between the boards to justify the stack to avoid jagged stack edges and improve stacking stability.