The frost line is 24 inches below the surface. Which describes the required depth of a footing?

The frost line is 24 inches below the surface. Which describes the required depth of a footing?

• A. above the frostline
• B. 12" below ground surface
• C. 18" below ground surface
• D. below the frost line

Answer: (D)

Footings must be placed below the depth that freezing reaches. This prevents frost heave, where freezing soil lifts foundations if the footing sits in the frozen zone. If the frost line is 24 inches, the footing depth needs to be deeper than that—to be below the frost line. Placing the footing within or above the freezing zone would leave it vulnerable to movement. In practice, builders often go slightly deeper than the frost line to add a safety margin, per local codes.

Footings must be placed below the depth that freezing reaches. This prevents frost heave, where freezing soil lifts foundations if the footing sits in the frozen zone. If the frost line is 24 inches, the footing depth needs to be deeper than that—to be below the frost line. Placing the footing within or above the freezing zone would leave it vulnerable to movement. In practice, builders often go slightly deeper than the frost line to add a safety margin, per local codes.