Abstract: Concrete slab foundation and method of construction in which ground soil is graded to form a building pad, a plurality of plastic domes having top and side walls are placed in a rectilinear array on the pad, and concrete is poured over the domes to form a monolithic structure consisting of a gridwork of criss-crossing ribs between the side walls and a horizontally extending deck above the top walls. The concrete is prestressed with longitudinally extending tendons in the ribs, and the plastic domes are left in place as a permanent part of the foundation to serve as a moisture barrier at the under side of the deck and the lateral faces of the ribs.