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.