What is the required depth of a footing if the frost line is 24 inches below the surface?

The required depth of a footing in regions where frost is a concern is typically established to ensure that it extends below the frost line. The frost line, also known as the freeze line, is the depth at which the ground is expected to freeze during the coldest months. When footings are placed at or above this line, they risk experiencing soil movement due to the freeze and thaw cycles, which can lead to structural problems.

By placing the footing below the frost line, which in this case is 24 inches, it mitigates the risk of frost heaving and provides a stable foundation that is less susceptible to the movements caused by changing temperatures. Thus, ensuring that footings are constructed below the frost line is crucial for maintaining the integrity and durability of the structure over time.

This approach emphasizes the importance of adapting construction techniques to local climatic conditions in order to prevent future structural issues. The other options suggest alternatives that do not adequately address the risks associated with frost heaving, which is why they do not meet the requirements for proper footing depth in frost-prone areas.