Frost line & frost-protected shallow foundations
“Frost line” is defined as the lowest level below the ground surface to which a temperature of 32 F extends. The factors determining the depth of the frost line are air temperature and the length of time the temperature is below freezing, as well as the ability of the soil to conduct heat and its level of thermal conductivity. Frost lines vary significantly throughout the country, ranging from five inches in the deep south to 100 inches in the uppermost northern regions. The frost-free depth for shallow foundations depends on the frost line set for the particular locality of construction.
IBC Section 1809.4 requires a minimum depth of 12 inches for footings below the undisturbed ground surface. This is a very general statement; however, 1809.4 does invoke Section 1809.5 which is more specific and is detailed in the narrative box at the bottom of the page. ASCE 32-01, “Design and Construction of Frost-Protected Shallow Foundations,” can be downloaded for free simply by Googling the name of the document. In addition to code compliance criteria for frost-protection, each jurisdiction across the country tends to establish their own minimum requirements related to the depths of frost lines. There is a table in the International Residential Code (IRC) that most jurisdictions use (Commentary figure R301.2-9) to help determine depths. The IBC code and the IRC leave it to individual jurisdictions to determine frost line depths. The IRC table can be used, but it is not mandatory.
The narrative box at the bottom of the page does not list the exceptions to Section 1809.5, which follow.
Exceptions: Free-standing buildings meeting all of the following conditions shall not be required to be protected.
- Assigned to Risk Category I
- Area of 600 square feet or less for light frame construction or 400 square feet or less for other than light frame construction
- Eave height of 10 feet or less
IBC Chapter 18, Section 1809.5 Frost Protection. Except where otherwise protected from frost, foundations and other permanent supports buildings and structures shall be protected from frost by one or more of the following methods:
- Extending below the frost line of the locality
- Constructing in accordance with ASCE 32-01
- Erect on solid rock
Shallow foundations shall not bear on frozen soil unless such frozen condition is of a permanent character.
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