The average frost line depth in southern Pennsylvania is 36 inches. The frost line (also referred to as frost depth or freezing depth) is the average depth in which the ground water in soil usually freezes.
Building codes in some states take into account the frost line depth because, during freezing conditions, the frost in the ground can swell upwards, causing frost heaving. Frost heaving can move the foundation of a building, which in turn damages a building's structure. It is also important to know frost line depth if digging water lines. If a water line is buried above the frost line, it could freeze.