In Quebec, winter doesn't just chill the air: it makes the ground work. Frost heave is one of the most common causes of movement in slabs and shallow foundations. Understanding how it works explains why our buildings are designed the way they are.

How frost lifts the ground

Counterintuitively, frost heave isn't only caused by water freezing in the soil. The most powerful effect comes from the formation of ice lenses: in a frost-susceptible soil, water migrates toward the freezing front and forms growing layers of ice that push up everything above them — a slab, a footing, a porch. On thaw, these lenses melt and the soil drops back down, often unevenly.

This cycle is why a garage slab lifts in winter and drops in spring, or why a porch shifts from year to year.

At-risk soils

Not all soils react the same way:

  • silts and some clays are frost-susceptible: they hold water and feed the ice lenses;
  • clean stone and gravel drain well and are not very frost-sensitive;
  • water and poor drainage always make the phenomenon worse.

Three ingredients are needed: a frost-susceptible soil, water and cold. Reducing one of these factors can significantly reduce the risk.

How we protect against it

Design in Quebec aims precisely at neutralizing frost. The main strategies:

  • Place footings below the frost line. Footings must bear below the depth of frost penetration — often on the order of 1.2 to 1.5 m across much of Quebec, depending on the region, municipal requirements and project conditions.
  • Insulate. For slabs-on-grade and some garages, an insulation skirt (horizontal perimeter insulation) limits frost penetration under the slab.
  • Drain and backfill properly. Good drainage and a non-frost-susceptible backfill (clean stone) deprive frost of the water it needs.

These choices aren't improvised: they depend on the soil, the configuration and the building's use.

When to worry

A slab that moves seasonally, cracks that open in winter and close in summer, doors that stick in the cold: all signs of frost-related movement. To be distinguished from an evolving foundation crack, which is a different diagnosis. For new construction, frost protection is an integral part of sound foundation and slab design.

Noticing suspicious movement at your property? Contact our team in Quebec City and Lévis.

This article is provided for general information. Frost-protection depth and applicable measures must be confirmed against the Quebec Construction Code and municipal requirements.