Frost can easily push the soil up 6, or more, inches. As each ice lens forms, it pushes the lenses above it higher up into the air. These different areas of water transform into ice lenses within the soil. The liquid water in the soil often is in layers much like a sandwich has layers of meat, cheese, lettuce, and tomato. This extra volume is pushed up into the air, not down into the compacted soil. Liquid water expands by nine percent when it freezes solid. This causes the soil just below the frozen layer at the surface to also freeze. If the air temperature is well below 32 F, then that soil temperature can easily start to mimic the air temperature. When below-freezing weather begins, the soil that’s in contact with the air starts to freeze. The uplift force is tremendous and can do significant structural damage to your deck. Should the soil beneath the footing freeze, the uplift force created by the expanding freezing water can lift the footing and anything on it up into the air. Frost heaving can and does lift things up, things that weigh thousands of pounds. Just as you don’t want to build on poor soil where the weight of the building will cause unequal downward settling which can destroy the structure, you don’t want the structure or your deck to lift up into the air unequally. When you build any structure, your goal is to ensure the foundation of the structure doesn’t move over time. Why is the Frost Line Important in Construction? In order to achieve a suitable lateral barrier, you may need to dig your foundation much deeper than on a flat surface. If you are building into the side of a steep hill or beside a retaining wall you will have to maintain the minimum distance to grade not only vertically but also laterally.
Furthermore, you must install footings at least 12” below unsettled soil. Always check with your local building inspector to see what is required in your area.
If you live in a warm climate, like South Florida, where frost rarely ever occurs, you may be allowed to place your footings on grade, but in very cold regions, like Minnesota and Canada, you will need to dig down 96 inches or more. The depth you are required to set your frost footings will vary from region to region across the country. Once the bottom of your footings is buried below the frost line, the residual heat in the ground below the footing will ensure the soil below the footing will not freeze in the winter. The frost line is the deepest depth at which the moisture present in the soil is expected to freeze. In warmer parts of the country, the frost depth may be less than one foot. In the coldest climates in the USA, the frost depth can be deeper than eight feet. You may wonder how deep the frost line is. You then must consult a frost depth map or reach out to your local building inspector to find out how far you need to dig down to get below the frost line depth in your town or city. First and foremost, you need to have good soil conditions that can support the concentrated load placed on each deck pier. Your deck footing depth is controlled by two things.