Abstract: A walled structure forms an inner protected area and is made up of prefabricated roof, floor, side and end wall members. These members have jointed overlapping connection with adjacent members with peripheral edges thereof having offset surfaces from each other across the seam of the joint to provide non-continuous burglar-proof seams which lead from the exterior of said vault to the inner protected area. The joints are arranged to provide a minimum of exterior seams. The members of the vault are secured together by disconnectable weld plates for the purpose of disassembling the structure in temporary installations.