The frost line, also referred to as the frost depth, is the depth at which the groundwater in soil is expected to freeze. Frost lines depend on an area's climatic conditions, including the heat transfer properties of materials in the ground (such as soil) and nearby heat sources.
Frost lines vary by latitude and are deeper closer to the poles. According to the Federal Highway Administration, the maximum frost depth ranges between zero to eight feet in the contiguous United States.
Generally, the coldest U.S. states have the deepest frost lines. Alaska's is the deepest at 100 inches, followed by Minnesota with 80 and North Dakota with 75. On the other hand, the hottest U.S. states have more shallow frost lines. Florida and Hawaii are zero inches, Louisiana and South Carolina's are four inches, followed by California and Georgia with five.
Frost lines can be important for building construction and are taken into consideration when laying a foundation. When water transitions from liquid to solid, it expands 9% in volume. Frost heaving, which is the upward swelling of soil during freezing conditions, can move the foundations of buildings, severely damaging them. For this reason, building codes will consider frost lines and build the foundations below the line. Additionally, water and sewer pipes need to be placed below the frost lines as well.