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What is the Frost Line?

Ken Black
Ken Black

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.

Scientist with beakers
Scientist with beakers

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.

Discussion Comments


@ PelesTears- The state must spend a boatload of money on road repairs. Is that why there are so many dirt roads crisscrossing the state?


@ PelesTears- The effects of frost heaves must be why you can't find a house with straight walls or square corners over thirty years old in that state. I lived in Vermont for ten years, and I painted houses for a living. I would paint exteriors in the summer and interiors in the winter. I used to hate doing older farmhouses because there was always so much work. I often found myself scraping lead paint from the exterior and repairing rotten sills and eaves.

This was never as bad as the interiors though. The walls were so warped that you needed buckets of wall mud to smooth out damage. If the house had horsehair plaster, the work was even worse. Corners were often warped into funny angles, and brickwork often needed repair. It is so much easier to paint the flat, straight houses you find in the desert South West.


The frost line is an important factor in any type of construction in Vermont. frost heaves wreak havoc on concrete foundations, roads and foundation slabs. It seems like a road has to be torn up and paved every three to four years because the constant freezing and thawing of the ground causes expansion, cracks, dips and potholes. After a while foundations to homes begin to fail and slabs need to be re-poured. My parents spent around ten grand repairing the foundation on their barn because the shifting ground had caused the concrete to buckle.

It got to the point that weasels were building nests under the concrete, and hiding eggs that they had stolen from our chickens. The eggs would rot and the entire barn began to stink. We must have found a few hundred eggs when we finally ripped the concrete slap up.

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      Scientist with beakers