The average depth of the frost line in Iowa is 58 inches. In the northern part of the state, the frost line can be up to 70 inches deep, while in the southern part the line may be less than 40 inches deep.
The frost line, or frost depth, is the depth at which moisture in the ground does not freeze. Below that line, the ground temperature averages around 50 degrees and water stays liquid. The frost line varies greatly across the United States and within individual states, so an individual must consult a frost map and check with her local building inspectors to know the specific frost depth for her area.
Builders, plumbers, city planners and others in construction and related industries must know the depth of the frost line in their areas because buildings' footings and decks' posts must be sunk below it. As water freezes, it expands and puts pressure on whatever surrounds it. If a building's footing is above the frost line, the pressure from the ice pushes upward on it, eventually causing structural damage to the building. Construction projects must meet local building codes, including the ones for frost depth, before contractors may start building.