Abstract: A plurality of modular units formed of ducts and horizontal load-supporting webs each form part of a ventilating floor structure with the ducts and webs being interconnected to adjacent modular units for forming a total floor. The ducts have upwardly diverging sidewalls for strength and a duct of one modular unit is provided with a bead that forms half of a joint to be slid within a downwardly opening recess forming the other half of the joint on an adjacent web. The recess is provided with a downwardly and laterally curving guide flange so that the bead can be rolled into the recess as the modular unit having the bead is rotated into position. The interconnected ducts and webs are underlayed with foam insulation which is carried up into each web for additional insulating thickness the joint is essentially waterproof from water passing into or through the ducts.