What Is the Frost Line Depth in Wisconsin?

Wisconsin's frost line depth — the maximum depth to which ground freezes in winter — is 48 inches in most of the state, as specified in the Uniform Dwelling Code (SPS 321). Some southern counties, particularly along the Illinois border, may accept a 42-inch frost depth. This measurement is critical because all structural footings (for homes, decks, garages, pole barns, and other permanent structures) must extend below the frost line to prevent frost heave, which can crack foundations and shift structures. The frost line requirement applies to all footing types: poured concrete footings, concrete block foundations, deck post footings, and pier foundations. When you apply for a building permit in Wisconsin, the building inspector will verify that your plans show footings at or below the required frost depth. The footing inspection — conducted before concrete is poured — is one of the most critical inspections in the building process. If your footings are too shallow, you will be required to dig deeper before proceeding. Wisconsin's 48-inch frost line is among the deepest in the contiguous United States, which directly affects construction costs. Deeper footings require more excavation, more concrete, and more labor. For deck construction, this means each post footing requires a hole at least 48 inches deep and typically 12 to 16 inches in diameter. For new home construction, the foundation walls must extend at least 48 inches below grade. Frost-protected shallow foundations (FPSF) are an alternative allowed under certain conditions in the UDC, using rigid foam insulation around a shallower footing to prevent frost penetration, but these require engineering analysis. Municipalities do not have authority to reduce the frost depth requirement below what the UDC specifies, though some may require deeper footings in areas with specific soil conditions. Always confirm the required frost depth with your local building department before designing your project.

Check Your Municipality

Permit requirements, fees, and exemptions vary by municipality. Verify the specifics for your location.