Slab-on-grade and frost-protected shallow footings

One-line orientation

Slab-on-grade is a thin ground-bearing slab built up from subgrade, capillary break, vapor barrier, and reinforcing; a frost-protected shallow footing lets you stay shallow in cold climates by insulating the perimeter instead of digging below the frost line.

Key points

• Slab-on-grade (SOG) — build-up, bottom to top:
  • Remove topsoil to reach stable subsoil/subgrade.
  • Crushed stone layer — the capillary break (stops liquid water wicking up).
  • Plastic sheet — the vapor / moisture barrier.
  • Welded wire reinforcing (WWF) or rebar grid on chairs for crack control.
  • Pour, screed, float; then cure (keep damp ~a week, cover with plastic).
  • Control joints pre-locate shrinkage cracks.
• Effective where frost depth is minimal — gives a solid base without deep excavation.
• Frost-protected shallow footing (FPSF):
  • Prevents frost heave by insulating the soil around the footing rather than digging deep.
  • Rigid foam insulation placed horizontally and vertically around the footing keeps ground temperature stable and prevents freezing.
  • Used in cold climates where the frost line is deep; reduces excavation depth, material, and cost while maintaining structural integrity.
• Frost line or frost depth: the depth to which soil freezes. Conventional footings generally extend below it because water can form expanding ice lenses in freezing, frost-susceptible soil and lift the foundation.

Confusions / comparison

	Slab-on-grade	Structural slab
Supported by	The ground, continuously	Footings / piers / beams (spans)
Carries load via	Bearing on soil	Bending between supports
Soil role	Essential — it holds the slab up	Not relied on beneath

	FPSF	Footing below frost line
Strategy vs frost	Insulate perimeter to keep soil unfrozen	Place footing below the freezing depth
Excavation depth	Shallow	Deep (below frost line)
Climate fit	Cold climates, cost/material savings	Cold climates, conventional approach

Related

→ Shallow foundation types (this topic): SOG within the shallow family · Shallow vs deep foundations (this topic): why shallow works here · Grade beam vs tie beam (this topic): beams that may thicken a slab edge · Concrete joints (Construction): control / expansion / construction / isolation joints in detail.