supplying an aqueous suspension of fibers to a forming zone of a headbox, the headbox being positioned over a forming fabric and comprising a first chamber, a second chamber, and an adjustable roof, the first chamber and the second chamber each comprising a plurality of nozzles adapted to supply the aqueous suspension of fibers to the forming zone, the adjustable roof configured to adjust over a range of movement such that in a first mode, the forming zone is defined by the adjustable roof, the forming fabric, and at least the first chamber, and in a second mode, the forming zone is defined by the adjustable roof, the forming fabric, and at least the second chamber;

adjusting the adjustable roof from the first mode to the second mode.