frost line(redirected from frost lines)
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The depth of frost penetration into the soil.It varies from area to area,depending on the soil quality in a particular location, but can average 3 to 4 feet below the surface. Building foundations should penetrate below the frost line in order to avoid the shifting caused by frost heaving. Many local building codes allow an exception with the use of a frost-protected shallow foundation, which uses insulation and drainage techniques to raise the frost line to just below the surface.