Below grade refers to a subfloor's elevation in comparison to the ground level (or, grade) outside the building. A below-grade subfloor is lower than the ground. Basements and sunken rooms are below the ground or grade. Tampa, FL is about 30 feet above sea level. So, there are virtually no homes with basements in Tampa Bay. In contrast, some homes have a sunken room. Below-grade subfloors are more likely to have moisture issues but are less likely to have unwanted movement or flexibility. Certain flooring products are not recommended to be installed below grade. Ceramic tile and porcelain tile are normally suitable below grade.
See grade level for all three terms designating elevation relative to grade.