On grade or at grade refers to a subfloor's elevation in comparison to the ground level (or, grade) outside the building. An on-grade subfloor is on par with or less than 18 inches above the ground. The average single-level home in Florida is built on grade with a concrete slab. On-grade subfloors are less likely to have moisture issues than below-grade subfloors. Certain flooring products can only be installed on grade or above grade.
See grade level for all three terms designating elevation relative to grade.