The minimum coverage for concrete permanently placed against dirt or the earth is:

The minimum coverage for concrete permanently placed against dirt or the earth being three inches is based on structural and durability considerations. This coverage helps protect the concrete from moisture and soil-related issues, including erosion and freeze-thaw cycles. A greater thickness acts as a barrier to prevent soil moisture from penetrating into the concrete, which can lead to deterioration over time. Moreover, the three-inch coverage is often specified in building codes and industry standards to ensure proper support for the concrete, aiding its structural integrity and longevity.

In contrast, lesser thicknesses, such as two inches, one and a half inches, or six inches, may not provide adequate protection against soil moisture or structural support, and while six inches might offer a larger margin of safety, it is not the minimum called for in standard requirements. Hence, three inches is recognized as a practical minimum for ensuring durability when concrete is in direct contact with earth.