Jargon: Frost Line

Frost line and frost line (Wikipedia)

I was listening to the radio program Living on Earth and heard about The Home Ground Project which collects evocative definitions in their series The Language of Landscape. I heard Eva Saulitis of Homer, Alaska define frostline in a wonderful way, new to me. [Listen here.] It prompted me to give you my definition as well.

In architecture and construction foundations are critical to the success of any project. In this context the frost line is the depth below the surface that the ground freezes. I understand that the term originates from visual inspection of groundwater wells. You could actually see frost down the walls of the well.

Knowing the depth of the frost line is important in foundation design because soils increase in volume when they freeze (just like that soda that exploded in the freezer). If the primary foundation elements are above the frost line your whole house will move with the freeze and thaw cycle. If it is too expensive to dig below the frost line your structural engineer can look into shallow foundations intended to work with the freeze thaw cycle. For practical purposes the local building code may set an official frost line based on the worst freeze in the past 99 years.

If you are interested in more definitions from Home Ground: Language for an American Landscape check out their website The Home Ground Project or listen to previous entries from Living on Earth.