How do I build a retaining wall that handles New Brunswick's frost depth?

A retaining wall in New Brunswick must have its footing below the frost line — 1.2 to 1.5 metres deep — to prevent the entire structure from heaving and failing. This is the single most critical requirement for NB retaining walls, and it's where many DIY projects and even some contractor installations fail. A retaining wall that doesn't account for NB's frost depth will crack, lean, or collapse within a few winters.

For walls under 2 feet tall, a gravity wall system using interlocking concrete blocks (like Allan Block or Redi-Rock) can be built on a compacted gravel base without a poured concrete footing. Excavate a trench 6 inches below grade, lay 6 inches of compacted granular base, and bury the first course of blocks completely below grade level. The buried course provides stability, and the gravel base allows water drainage that minimizes frost heave. This approach works well for small garden terracing in NB.

For walls 2 to 4 feet tall, you need a more engineered approach. The base trench should extend 12–18 inches below grade with 10–12 inches of compacted gravel. Use a geogrid reinforcement system — layers of grid fabric that extend back into the soil behind the wall, anchoring it against the lateral pressure of frozen, expanding soil. In NB's climate, geogrid is essential at every 2 courses of block rather than the standard every 3 courses recommended in milder climates. The backfill behind the wall must be free-draining gravel, not native soil — NB's clay-heavy soils trap water that freezes, expands, and pushes the wall forward.

For walls over 4 feet tall, New Brunswick building codes typically require engineered design. This means a licensed engineer designs the wall with a poured concrete footing that extends below the frost line (1.2–1.5m), reinforced with rebar, and sized according to the specific soil conditions and wall height. The engineering cost ($500–$2,000) is a small fraction of the total project and protects an investment of $35 to $65 per square foot of wall face.

Drainage behind the wall is non-negotiable in NB. Install a 4-inch perforated drainage pipe at the base of the wall wrapped in filter fabric, surrounded by clear gravel. This pipe must daylight to a suitable outlet — not just dead-end behind the wall. The backfill zone (12–24 inches behind the wall face) should be clear 3/4-inch gravel, not soil, to prevent hydrostatic pressure buildup during spring thaw when snowmelt and rainfall saturate the ground simultaneously.

Build retaining walls in New Brunswick during late spring through early fall when the ground is workable. Never pour concrete footings when temperatures are below 5C or when frost is expected within 48 hours.

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