Engineered wood flooring is constructed to provide increased stability and moisture. The top layer is a thin sheet of natural wood and the core is constructed of layered plywood. Engineered Wood floors were developed for use over concrete slabs, but can be nailed down over a wood subfloor. The newest, thinnest engineered wood flooring use an advanced tongue-and-groove click system that locks them in place. These floors can be placed over a underlayment or directly over an older floor. This type of flooring can be used almost anywhere, on, above or below grade.