Abstract: In the method of the invention starting sheets are stiffened and straightened by pressing or rolling upon the sheet various stiffening patterns projecting from the plane of the sheet, the starting sheet being rolled or pressed from both sides by means of a roller or a press the opposite surfaces of which have the patterns, while the starting sheet is suspended by its lugs.An apparatus is disclosed having conveyors for conveying the starting sheets and a roller or press for embossing stiffening patterns upon the starting sheets, the roller or press having two parallel rollers or press plates, provided with regularly alternating depressions and corresponding protrusions, at least one of the press plates being movable in the lateral direction, the roller or press being mounted below the suspension conveyor for the starting plates in such a manner that a starting sheet suspended by its lugs from the suspension conveyor passes through the roller or press.