How far below grade should footings ideally be placed?

Footings should ideally be placed at or below the frost line to ensure structural integrity and prevent issues related to frost heave. The frost line is the depth at which the ground is expected to freeze during the winter months. If footings are not placed below this line, they are at risk of being affected by the expansion and contraction of frozen soil, which can lead to significant damage to the foundation and the overall structure of the building.

Placing footings at or below the frost line provides stability by ensuring that the soil remains stable and undisturbed during freeze-thaw cycles. This practice also helps prevent water accumulation and reduces the risk of erosion around the foundation. It is essential for maintaining the longevity and durability of the structure, particularly in regions where freezing temperatures are common.

The other choices suggest specific depths that do not take into account the variability of frost lines between different geographical locations, as the frost line can vary significantly based on climate and soil conditions. Therefore, adhering to the guideline of placing footings at or below the frost line is the most reliable approach for construction safety and effectiveness.