People are most familiar with wood flooring as solid one-piece boards. Most solid-wood flooring is not recommended for below-grade installations because it can shrink and expand, resulting in gaps or warping. As an attractive alternative, consider engineered wood, which consists of two or more layers of wood laminated together—similar to plywood but not to be confused with laminate flooring. The top, or wear layer, is hardwood veneer, and the lower layers are usually softwood. The engineered flooring is usually suitable for below-grade installations because it shrinks and expands less than solid-wood flooring.
For comfort underfoot or to span an uneven concrete slab, install a wooden subfloor. The easiest way is to install a subfloor panel system, otherwise install 5/8- or 3/4-inch exterior-grade plywood sheets are nailed to a grid of "sleepers"—pressure-treated 2X4s laid flat to help keep the finished height within the 90 inches required by building codes. Correct any unevenness in the concrete by placing shims beneath the sleepers. Fill spaces between the sleepers with rigid foam insulation before nailing the plywood in place. The result is a smooth, even subfloor that will accept most types of flooring.