Hardwood flooring is constructed in two ways, solid and engineered each having their own positives and negatives. Solid hardwood was originally designed for structural purposes, being installed perpendicular to the houses joists. As building code updated and technology increased, solid hardwood was refined into what it is today.
Although the flooring had improved since the first iterations, there was always the glaring issue of moisture. Depending on what environment the solid wood was being installed into problems would arise.