Abstract: For fulling, the mineral fiber sheets (11) are moved in the direction of the arrow by means of two endless studded belts, only the lower one (15) of which is visible in the drawing. By means of the first fulling tool (29), pre-fulling to a depth of about 8 cm is effected. With the second fulling tool (30), fulling to a depth of about 11 cm is effected. During the fulling process, the region of the mineral fiber sheet (11) not belonging to the fulling zone is supported on the bottom and top, so that a change in the fibrous structure of the material takes place only in the peripheral region that subsequently forms a compressible zone (23). The fulling is done from the side, or in other words in the same direction as the later compression upon installation of the mineral fiber sheet between two rafters.