Solid hardwood floors are made of pieces of solid wood, usually ¾” thick, and are available in many widths. Solid hardwood floors are less dimensionally stable and because of this, are more susceptible to humidity and temperature changes than engineered wood floors. Solid wood floors are generally only installed above grade over approved wooden subfloors and must be either nailed, stapled, or glued down. Solid wood floors can generally be sanded and refinished several times.