There are three basic types of concrete floor systems and, with their many modifications, they satisfy a wide range of requirements for floors in modern construction. In all of these systems, one slab itself forms the majority of the weight and includes a considerable amount of concrete which contributes little to the stiffness or strength of the system. Because of this, many ways have been devised to reduce the weight of the slab. One way is to use lightweight concrete. Another is to form voids of one type or another in that part of the slab where concrete is not really required. These voids can be formed in a number of ways, but this article will be concerned only with pan-type floor systems. Today there are several different types of forming systems for floor pan construction. These are the metal pans with straight and tapered sides; the metal domes for waffle slabs; molded fiber pans; fiberboard pans; molded fiberglass domes; and filler blocks. Metal pan forms come in several types according to the use required. The most widely used is the nail down flange type because of its flexibility and the simplicity of erecting the shoring and the centering to support it. Nail down flange type pans are especially recommended for ceilings which are to be later plastered or acoustically treated, since they cannot provide a quality architectural concrete surface for an exposed ceiling. A second type of metal pan is the adjustable type with no flanges. This type is usually chosen for exposed ceiling work because the soffit of the joints is formed smoothly without flange marks by setting it between the sides of the adjustable type form. The depth of the joist is controlled by nailing the metal form through its side into the soffit form of the joist. A third type of metal pan is the slip-in flange type which uses the same metal form as the nail down flange type. All the joints soffits in this system are formed by cutting to the exact width similar to use with the adjustable type no-flange pan. This system is also chosen for exposed ceilings because no-flanges mar the bottom of the concrete joints. The fourth type of steel form is the metal dome and this is used in two-way slabs, flat slabs and flat plates. These domes are usually formed with 3 inch wide flanges which when placed alongside each other form 6 inch wide joist bottoms. This eliminates defects in the finished concrete caused by lapping or a space between the pans which is typical with metal pan joints forms.