To solve a problem wherein the demand for plates such as sheathing roof boards used for roof backing and wall backing in wooden building is sharply reduced by the rise of various face materials represented by imported construction panels and domestic coniferous plywood in recent years, and a domestic material lumbering system is forced to be sluggish and requires a new utilization method of plates equal to product performance and workability of face materials such as South Sea material construction panels.

Plates are fully dried. Plates finished in predetermined dimensions such as 12 mm in thickness, 9 cm in width and 200 cm in length are dipped in a high temperature liquid phase in which natural fat-and-oil-based fatty acid or wax which is solid at ordinary temperature is melted by heating to carry out waterproofing/rot-proofing treatment. Only the single sides of two adjacent plates are then stuck with an adhesive sheet to join the width. A third plate is then attached, and an adhesive sheet is stuck to the back face. Plates are alternately and additionally stuck to the front and rear in sequence to integrate ten narrow plates up to a width of 90 cm, for instance, to form a face material. Dipping in the high temperature liquid phase is excludable.