Abstract: A box spring assembly having a rectangular frame mounting a plurality of lateral support slats, formed wire torsion springs affixed to said slats and a metal grid interconnecting said springs. To minimize sag resulting from body weight, the support slats adjacent the lateral center line of the frame are first reinforced with U-shaped metallic bars which rest the wooden slats. In addition, the number or density of lateral slats adjacent this center line are increased leaving a lower density of slats adjacent the head and foot ends of the assembly. This arrangement of lateral slats permits reorientation of the formed wire spring units which further results in greater lateral stability of the box spring unit. The assembly is then completed by adding one or more insullating pads for cushioning and a fabric cover.