I am often asked while on the job site why our fence post holes are so deep and wide, and why we use so much concrete. The answer is simple, we don’t want our posts to move, ever.
Fence post Frost Heave
If you drive around your neighborhood keeping an eye on fences you will almost certainly see examples of frost heave and you might not even know it. When you see one or two post in a line of fence it can some times be installer error, they just set the post to high, but most likely its frost heave. Frost heave occurs when moisture in the ground reaches temperature below thirty-two degrees fahrenheit. As we know from middle school science class this is the temperature that moisture freezes and turns to ice.
When moisture turns to ice it expands. As it expands it pushes the ground, or any thing else above it up. You will often see this in concrete next to your house the ground underneath the concrete freezes and expands and pushes the concrete up. Where your house with a foundation under it will stay at the same elevation.
In relation to fence posts the way to prevent frost heave is simply to go below the frost line, the maximum depth to which the ground freezes, which in our area of Ohio is 36″ below grade. If you fail to get your fence post foundations down to 36″ the repeated freeze thaw cycle may push your posts up and possibly allow dirt to work its way underneath the concrete resulting in frost heave.