Composite slabs consisting of steel decking combined with concrete have been used in building applications for several years. Some advantages of such a system are fast construction, elimination of formwork and ease of providing services. A new type of steel decking is described which has additional features. These include the ability of the slab to attain the full theoretical load capacity with full composite action, the provision of a simple system by which ceilings and service ducts can be suspended from the underside of the deck and a smooth soffit surface suitable for the application of fireproofing finishes.