What is a frost wall?
A frost wall is a foundation wall that extends below the frost line to prevent ground heave and settling caused by freeze-thaw cycles in winter.
In Ontario, a frost wall forms the below-grade portion of a garage foundation, dug deep enough to sit below the seasonal frost line where soil freezes and thaws. When water in soil freezes, it expands, causing the ground to heave upward. Without a frost wall extending below this frost line, a garage foundation can shift unevenly, cracking concrete slabs, misaligning door frames, and creating structural problems over time.
The Ontario Building Code sets minimum frost depths based on geography. Most of southern Ontario requires foundations to extend 4 to 5 feet below grade, though northern regions and areas with poor drainage may need deeper footings. Builders must determine the correct frost depth for a specific property location before design and construction begin.
A frost wall typically consists of concrete or concrete block extending from the footing (which sits at or below frost depth) up to finished grade level. The wall carries the weight of the garage structure while the below-grade portion anchors it in stable, undisturbed soil below the frost line. This prevents seasonal ground movement from destabilizing the building. For garages built in Ontario, garage builders size and depth the frost wall according to local frost depth requirements and soil conditions to ensure a durable, level foundation.