A Footing Depth and Frost Line Question
I recently received this question from a reader:
Dear Mr. Hirsch,
My wife and I are planning on building a house with a partial basement in North Dakota. The frost line is 48 inches deep. The footing requirement is 16″ wide and 8″ high. Does that mean the top of the footing must be 4 feet below the frost line and does the concrete stoop require a footing below grade also?
Here is my answer:
Building Codes usually require that the bottom of the footing must be below the frost line, not the top. But check with your local Building Department to be sure of how they enforce it. As a matter of practicality, I would suggest you place the footing deeper than just the minimum depth required by the Building Code. An extra eight inches down will not cost much more and it will give you a bit of added protection against frost heave during an exceptionally cold winter.
Frost heave is a serious problem if it occurs. If the soil beneath a footing freezes, it will expand. And that expansion is so strong, it will lift that portion of the building. If that happens, you will get cracks in your foundation and possibly end up with jammed doors, cracked windows and walls, broken pipes, and other movement problems in your house.
I would suggest you also extend the footing beneath the stoop below the frost line to keep it from rising with a frost heave in winter. If the stoop is totally detached from the house, you could possibly use a shallow footing and allow it to heave in winter and settle back in the spring without any permanent damage. But I would not want that to happen in my own house. Also, check with your local building department. The Building Code may require a 48″ deep footing under stoops.
Hope that helps. Good luck with your new house.
I’m always happy to answer questions from readers. If you have one, please send it along. I answer as many as I can.