Foundation Below Frost Line

One type of building movement that we can prevent is frost heaving. Frost heaving is caused by water freezing in the soil beneath a building's foundations. Phase change expansion of the water can cause the soil to expand, lifting the building slightly. Larger amounts of lifting can occur due to the growth of long vertical crystals of ice under the foundation under certain conditions of temperature and moisture.

1. Building codes generally require that the bottom of a foundation be placed at a level below the deepest level to which the ground freezes during a severe winter; consult the applicable building code to find out how-deep this is.

2. Isolated pier foundations are economical and effective for decks, porches, and small wooden structures. A post hole digger is used to excavate for each pier. The concrete should not be cast directly against the rough sides of the hole, however, because frost can heave upward against the rough sides of the pier. A smooth fiber tube form should be used to cast piers whose sides are smooth above the frost line. Similarly, a foundation wall should be cast in smooth forms, not directly against the walls of the trench.

3. Though most building codes in North America do not allow it at this ume, many buildings in cold European climates arc built on foundations that are quite shallow. An exterior layer of plastic foam insulation allows interior heat that escapes from the building to warm the soil beneath the foundation and keep it from freezing. This detail should not be used for an unheated building, such as a barn or storage building.

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