What should the footer's minimum depth generally exceed?

The footer's minimum depth should generally exceed the frost line to ensure structural stability. The frost line, also known as the frost depth, is the depth below the ground where the soil remains at or below the freezing point during winter months. If footers are placed above this line, they risk heaving or shifting due to freezing temperatures, which can cause cracks and other structural issues in the foundation above. By ensuring that the footer's depth exceeds the frost line, the building is better protected against these potential freeze-thaw cycles, leading to a more stable and durable foundation.

In contrast, while considering other options, the surface layer of soil may change and is subject to disturbance and erosion, so relying on it for footer depth can be problematic. Similarly, the drainage layer varies depending on the design and purpose but does not necessarily provide a stable environment for footers. The foundation wall itself does not dictate the depth required for the footer; rather, it sits on the footer and depends on it for support. Therefore, the frost line is the most critical measurement for ensuring that footers are placed at an appropriate depth.