The purpose of a frost-depth chart or map is to show the depth at which water in the soil freezes or frost penetrates the ground, information that affects the cost and construction of buildings, roads and bridges. Some jurisdictions incorporate this information into building codes.
This information is important because water expands when it freezes and exerts pressure upward, and this expansion severely compromises the stability and safety of anything built at or above this level. Building codes in many areas reference these charts to prevent “frost heaving” or damage that occurs when freezing water in the ground causes a building’s foundation to move. Many code enforcement agencies require builders to build foundations below the frost-depth line for this reason.
Another reason for knowing the frost depth of an area is to determine how deep to lay underground water pipes. Installation of pipes above the frost-depth line may cause the pipes to freeze and burst. Pipes installed above the line may require insulation.
In addition, deck builders need to know the frost depth to properly excavate for footings. Since the footings support the weight of the deck, the structure can topple if the builder does not set the footings to a depth below this line.