An all truss foundation unit, method of making the foundation unit, and a foundation unit kit. The foundation unit is formed of multiple cross trusses interconnected by bridging top and bottom side rails. This results in a flat surface on both the top and the bottom of the foundation unit. The kit is formed of a plurality of the trusses and four side rails, with the trusses and side rails being oriented together with their longitudinal dimensions aligned in a generally parallel orientation and with at least some of their flat sides in contact with one another. When the foundation unit is constructed, a truss locator guide is provided on at least two of the side rails, and the cross trusses are oriented in a parallel, spaced relationship in accordance with the truss locator guide.