The present invention is a method of making a backerboard having a fiberglass mesh on one side, and an impervious reinforcement membrane on the other side. The backerboard incorporates a low density, high compressive strength concrete core having an upper principal surface and a lower principal surface. The upper principal surface of the core is covered by a fiberglass mesh reinforcement layer, itself covered and bonded to the core by a thin layer of Portland cement. The lower principal surface of the backerboard is covered with a high tensile strength, impervious reinforcement membrane.