The frost line is the depth to which ice, or frost, penetrates into the ground. Though the factor is often an afterthought in terms of general weather information, it is very important for building codes. The frost depth is determined by a variety of factors including the temperature, the length of time the air temperature remains below freezing, and the level of moisture in the ground. Other factors, such as exposure, the amount of vegetation and land use could also play minor roles in determining the exact depth.
The easiest way to determine the frost depth for an area is to consult a frost line map. The map serves as a general reference, and finding a specific zone should provide an accurate idea of what to expect even in the worst of winters. In most cases, local building inspector offices should also have information regarding the frost line. This local source of information is often the most accurate because it is fine tuned to a specific area in a way no national map can be.
The frost line can vary greatly from one location to another. For example, in the United States, there is no frost line set for Florida, but it can be as deep as 60 inches (152.4 cm) in some northern sections of the lower 48 states. In Minnesota, the maximum is 60 inches (152.4 cm) in the northern counties, but in the southern counties, the depth of frost only reaches 40 inches (101.6 cm). Some frost maps will show an average frost depth for the state, but this is very unreliable information and should not be the basis of any decision because of the variance just described.
Though the extent of frost penetration is not relevant to most people, it is important because the posts and footings of buildings must be installed in the ground below the frost line. If the underground portions of the supporting structure is above this line, the pressure exerted on the post or footing will be upward. This could push the post or footing out of the ground or even be responsible for major structural damage in buildings. Making sure these structures are placed at least several inches below the line alleviates this concern.
In fact, the placement of footings and posts is of such prime concern that it is often written into the local building code. Those who do not follow building codes regarding the frost line in an area may find they must make extensive adjustments to the project before it is approved for use. While this may be an inconvenience, it saves the consumer the trouble of having to make repairs on a job that was done incorrectly from the beginning.