Wherever the ground freezes in winter, special precautions must be taken in how to construct a fire pit. As with any kind of patio or stonework, frost heaves can cause damage. The way to prevent this is to provide adequate footing in the fire pit construction.
Frost disrupts the stability of soil. The force can be strong enough to crack stonework that has not been carefully designed. Over a few winters freezing weather will enlarge these cracks. Besides ruining the appearance of an important landscape feature, this reduces the safety of the pit. Use the same precautions as in constructing walls, patios or any kind of mortared stonework outdoors. The pit must withstand both extreme heat and simple cold weather.
The required depth of the foundation depends on geographical location. More specifically, it is a matter of how deep frost is expected to penetrate during the winter. This can be discovered on the internet from a map that shows winter temperatures and necessary footing depths. The value will be given in inches.
Unlike the concrete footings for a deck, the fire pit should be built on gravel. This allows rainwater to drain from the base. Although not as stable as concrete, gravel absorbs the disturbance caused by frost. That reduces stress on the stonework.
In preparing the site for building a fire pit, mark a circle on the ground to indicate the outside of the wall. Next excavate the entire pit within the circle down to a depth of six inches. The base of the stonework will eventually start at that six-inch depth, and the inside of the pit will be backfilled with gravel to ground level.
Now dig a trench around the inside edge of the circle. It should be as wide as the wall and deep enough to prevent frost heaving. In most cases the bottom of the trench should be 12 inches below ground level, but more where winter frost is deep.
Finally, fill the trench with fine gravel. Use a level to ensure the entire site is level on all sides. With this foundation laid, you are ready to proceed with building the wall. This will ensure a stable fire pit, attractive and functional for years to come.