What is the minimum depth for footers below undisturbed soil?

The minimum depth for footers below undisturbed soil is 12 inches. This depth is established to ensure that the foundations are placed below the frost line and are not susceptible to movement caused by frost heave or soil consolidation. A 12-inch depth typically provides adequate support for most residential structures while also respecting local building codes and standards which aim to ensure longevity and stability in foundation construction.

In many jurisdictions, the 12-inch depth is the standard because it helps mitigate risks associated with the seasonal changes that can affect the soil's density and moisture content. Foundations placed too shallow may be subject to shifting during thaw cycles, which can lead to structural problems over time.

This standard may vary depending on specific local conditions, building codes, or other factors like soil type and climate, but in general building practice, 12 inches is widely accepted as the minimum necessary depth for footers in undisturbed soil to provide a sound foundation for a residential building.