Q. I’m building a detached deck in my backyard and want to pour concrete footings for the posts to stand on. How deep do I need to dig before I can pour? –Kevin H., Greenville, SC
A. Hey Kevin, that has a lot to do with where you live, because the most important factor involved is the “frost line”. That’s the depth at which frost no longer forms.
In moderate to cold climates, frost forms in the ground when air temperatures drop below freezing. Where winters are very cold (and long), frost reaches even farther below the ground surface.
How does that affect my footings?
This is a problem for builders, because frost can “heave” the structures that we build. When moisture in the ground freezes, that freezing creates lens-shaped swells in the soil. Of course, these swells get even bigger as unfrozen moisture below them is drawn up to the cooler pockets.
Virtually all major structures, like houses, decks or highways, are built on poured “footings. These footings are essentially concrete pads. They give foundations and posts something solid and stable to sit on.
When frost heaves these footings, whole portions of the structure above will be thrown out of whack. (A good article here on both frost heave and subsidence.)
Frost line and geography
Obviously, it’s necessary to set your footings below the frost line of your particular region. Also, this means the bottom of your footings.
Here in balmy North Carolina, our winters are fairly brief, and not severe. Our frost line is 12” down. In chilly places like Boston, or Milwaukee, the frost line extends as far down as 48”!
So, in NC, 12″ is technically the safe depth for the bottom of a footing. But you really can’t go wrong digging a few inches deeper. And, there are other factors involved, like soil density. It’s best to look at your local code language.
Here’s a link to the International Code Council’s requirements.
And a link to ConcreteNetwork that puts it all in slightly more human terms.
What should my footings look like?
There are important considerations regarding the shape of your footing holes.
Inspectors insist on “square walls”. That is, footing walls that are truly vertical, not tapered at the bottom. This is because frost can grab the angled edge of your footing and heave it. For a good illustration of exactly how this happens, check out the video below.
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