Abstract: A floor-ceiling system for multi-story buildings which utilizes a bottom membrane having significant tensile strength supported on the building frame, such membrane being either cast in place and supported by shoring or prefabricated and self-supported, with void-producing duct forms regularly arranged and supported on the membrane, and a poured-in-place plastic material covering the membrane and duct forms to create structural elements alternating with the duct forms to form a substantially finished floor-ceiling system when the plastic material cures. The duct forms may be provided with access openings to the floor or ceiling or with lateral access openings interconnecting each other. The duct forms may be left-in-place forms or inflatable tubes removed when deflated after the material cures. Finishing materials for the floor or ceiling such as carpeting or acoustical tile may be secured directly to the essentially planar bottom or top surface of the system.