All timber used for grain fittings shall be of good sound
                           quality and of a type and grade which has been proved to be satisfactory
                           for this purpose. The actual finished dimensions of the timber shall
                           be in accordance with the dimensions specified below. Plywood of an
                           exterior type bonded with waterproof glue and fitted so that the direction
                           of the grain in the face plies is perpendicular to the supporting
                           uprights or binder may be used provided that its strength is equivalent
                           to that of solid timber of the appropriate scantlings.