The idea of prefabrication as a theory of construction (at least as applied to lighter materials such as wood) did not appear until panel construction itself was suggested by the availability of new materials like wallboard and plywood. In the 1930's, improvements in plywood manufacturing (mainly the first waterproof exterior glue) and the development of plywood construction systems had a large influence on the growth of prefabrication as a whole. Since manufacturers could mass-produce it in large sheets, plywood became the preferred material for "stressed skin" construction. This method uses specialized gluing and wood framing to create large panels with both an interior and exterior surface. Such a panel acts as a box girder and is generally stronger and stiffer than conventional wood framing.