Question 25
Domain 1: Footings, Foundations, and CrawlspacesWhat is the minimum depth required for a residential footing in areas subject to frost?
Correct answer: C
Explanation
Residential footings in frost-prone areas must extend to "at or below the frost line" so freezing soil cannot lift or damage the foundation. This depth requirement prevents frost heave by placing the footing below the zone where the ground freezes.
Why each option is right or wrong
A. 6 inches below the frost line
B. 12 inches below the frost line
C. At or below the frost line
IRC Section R403.1.4 requires exterior footings to be placed at a depth of not less than 12 inches below undisturbed ground surface, and in frost-susceptible locations they must also extend below the local frost depth established by the authority having jurisdiction. The controlling condition here is the frost depth, so the footing must be set at or below that line to avoid uplift from freezing soil; the exact minimum is therefore the local frost line, not a fixed universal measurement.
D. 24 inches below grade