Footing Depth Below Grade
Footing depth below grade | Look how high the ground is to the left of the footing form. The bottom of the footing will be 48 inches below grade. The footing forms are set and it's ready to pour the concrete. Note the continuous steel bars and how they are raised up above the soil. Copyright 2019 Tim Carter
"The footing depth below grade is determined by two things: when you encounter strong soil and making sure you're below the local frost depth in your area."
Tim Carter - Founder | AsktheBuilder.com
Footing Depth Below Grade - Two Factors Control the Depth
The footing depth of your garage, house, or building depends on where you're building and how strong the soil is on your lot.
Cold climates create frozen soil in the winter. Water expands in the soil creating ice lenses. These layers of frost will lift a building out of the ground. Structural failure is guaranteed if the lifting is not equal around all sides of the building. It rarely is.
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This footing is on solid ground and it's going to be below the local frost line. Check with your building inspector to see how far down you need to dig. Copyright 2019 Tim Carter
How Far Down Will Frost Penetrate?
Frost can penetrate as deep as 6 feet or deeper. It's hard to believe, but it's true.
The depth depends on how severe the winter cold is in the area you're building. Look at the following drawing to get an idea of different frost levels in different parts of the USA:
The frost depth can be as shallow as 6 or 12 inches if you live in the deep South in the USA. Extreme northern Minnesota and Michigan's upper peninsula can have frost depths approaching 8 feet. Copyright 2019 Tim Carter
What is Strong Soil?
Strong soil is that which will support the weight of the building being placed on it. Topsoil is not strong. Any soil with organic waste or debris in it is not strong.
Dense clay soils that are red, brown and light brown are usually very strong. Your local building inspector or a soil engineer will gladly inspect the soil to ensure it will not sink under the weight of the building.
Should the Footing Be Level?
All sections of the footing should be level. A building constructed on a sloped lot will have a stepped footing so all sections of the footing are below the frost line. Look at the following photo to see a stepped footing.
This is a great example of a stepped footing. Note how all horizontal sections are level and the vertical faces are plumb. This makes setting the foundation forms very easy. Copyright 2019 Tim Carter
