If the frost line is 1.0 m, what is the minimum footing depth?

• A. 1.2 m
• B. 1.0 m
• C. 0.8 m
• D. 1.5 m

Answer: (A)

Foundations must be placed below the frost line to prevent frost heave, with a safety margin to account for variations in frost depth and soil conditions. The frost line is the depth to which ground freezes in winter, and putting footings below that zone helps keep the structure stable during freeze-thaw cycles. When the frost line is 1.0 m, a common practice is to add a margin—often about 0.2 m—to ensure the footing stays below the deepest freezing that could occur. That yields a minimum footing depth of 1.2 m. A depth exactly at the frost line would be risky because freezing conditions can extend deeper in some years, while shallower depths would be above the frozen zone and prone to movement. Going deeper than 1.2 m is not necessary for the minimum requirement. So, the minimum footing depth is 1.2 m.