Wood flooring is a type of flooring made from the timber of hardwoods, or of spruce or hard pine. There are two basic manufactured types of hardwood. Wood flooring comes unfinished, and once installed is sanded, then finished on site. More modernly, the product is pre-finished in a factory. The products that are pre-finished are often a polyurethane finish that has added aluminium oxide, however some companies use titanium dioxide or other oxides instead. These metal oxide finishes are used in various types of floor coverings and increase the wear a hardwood floor can handle.
Solid Wood Flooring
Solid hardwoods are typically 3/4" or 19mm thick, although some do come in 3/8" (10mm) or 5/16" (8mm) thicknesses. Typically the wearing thickness, i.e., the thickness that can be sanded over the lifetime of the floor, above the tongue-and-groove portion, is approximately 7/32" approaching 1/4". This type of hardwood flooring can be installed with a nail-down installation method over wood subfloors. 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.
Engineered Wood Flooring
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" or 8mm up to 9/16" or 14mm thick. The wood veneer can range in thickness depending on the manufacturer, as will the top wearing thickness. In order to create an engineered hardwood, these veneer layers are stacked one on top of the other with the grain of adjacent layers oriented perpendicular to one other. 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. By doing this, the engineered hardwood becomes less susceptible to the effects of moisture and temperature change, because wood expands and contracts in the width of the grain direction. Therefore engineered hardwood is referred to as being dimensionally stable. Solid hardwood does not have dimensional stability because all of the grain runs in the same direction. Because of its dimensional stability, engineered hardwood can be glued directly to concrete above or below grade, as opposed to solid hardwood which cannot.