What is the Frost Line?

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Image showing Frost line underground

The depth to which water in soil will freeze in the winter is known as the Frost Line. The depth of the frost line is essential because it influences the construction of buildings and roads; since water expands as it freezes, infrastructure above the frost line can be prone to burst pipes, cracked foundations and similar problems. Typically, the frost line is the depth to which ice or frost penetrates the ground. 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 (plant life) and land use could also play minor roles in determining the exact depth.


The depth of frost line ranges in the U.S from about 3 to 6 feet. Below that depth the temperature stays constant 50° F. Alternatively, in Arctic and Antarctic locations, the freezing depth is so deep that it becomes year round permafrost, and the term “thaw depth” (the level down to which the permafrost soil will normally thaw each summer in a given area) is used instead. Permafrost soil is the soil at or below freezing point of water for two or more years.


How to determine the Frost line depth?

The simplest 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 time of winters. In the majority of cases, local building inspector offices should also have information concerning the frost line. This local source of information is often the most precise because it is fine tuned to a specific area in a way no national map could be.


Will the frost line vary place to place?

The frost line can vary greatly from one location to another. For instance,

  • 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 in the northern counties, but in southern counties; the depth of the frost only reaches 40 inches (101.6 cm).

Some frost maps will show an average frost depth for the state, but this is not greatly reliable information and should not be the basis of any decision because of the variance described above.


What is the significance of Frost line?

Al though the extent of frost penetration is not relevant to many people, it is significant 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 are above the frost line, the pressure applied on the post or footing will be upward. This could push the post or footing out of the ground or even responsible for major structural damage in buildings. So, always confirming that these structures are placed at least several inches below the frost line lessens this unease.


What is the role of frost line in the Building codes?

The placement of footings and posts is of such prime concern that is often written in the local building codes, a set of rules that must be followed to satisfy the minimum acceptable levels of safety and non-building structures. Those who do not follow the building codes regarding the frost line in an area may have to 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.

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