Rather than having one solid piece of hardwood, the engineered hardwood method uses layers of hardwood veneer to create a product that can range in thickness from 3/8" up to 9/16" thick. The wood veneer can range in thickness depending on the manufacturer.
In order to create an engineered hardwood, these veneer layers are stacked on top of each other with the grain of the wood facing perpendicular to each other. By doing this, the engineered hardwood becomes less susceptible to the effects of moisture and temperature change. Once the desired thickness is achieved, the boards are then cut into the correct board width. From there, the boards are then manufactured to have a tongue or groove on the edges. The final step is to add stain if necessary, and add a finish. Because of this, engineered hardwood is denoted as being dimensionally stable, as opposed to solid hardwood flooring, which is not.
Solid hardwoods are typically 3/4" thick, although some do come in 3/8" or 5/16" thicknesses. This type of hardwood flooring can be installed with a nail-down installation method over wood sub flooring. This type of hardwood is also very susceptible to the effects of moisture and temperature, because hardwoods expand and contract with moisture and temperature changes in the atmosphere.
Since hardwoods expand and contract in the width of the grain, this type of hardwood flooring is not recommended to be installed over a concrete slab, unless otherwise stated by the manufacturer.
There are some instances where 3/8" thick solid hardwood can be installed on a concrete slab.