What gravel base is needed under a garage floor slab in New Brunswick?

A garage floor slab in New Brunswick requires a minimum 6-inch compacted granular B gravel base, and 8–12 inches is better in areas with poor drainage, high clay content soil, or elevated water tables. The gravel base is not just a construction step — it's the difference between a garage floor that stays flat for 30 years and one that cracks and heaves within a decade.

NB's frost depth of 4–5 feet means the soil beneath your garage floor freezes and thaws every year. When water-saturated soil freezes, it expands. That expansion — frost heave — pushes upward against anything above it, including your concrete floor slab. A properly installed granular base does two critical things: it drains water away from beneath the slab so there's less water to freeze, and it provides a stable, uniform bearing surface that reduces differential movement.

Granular B (also called crusher run, bank gravel, or processed gravel) is the standard material for slab sub-base in NB. It's a well-graded mix of crushed stone and fines that compacts tightly and drains well. Avoid using topsoil, native clay, or organic material beneath a slab — these compress under load and retain moisture.

What the Process Looks Like

For garages in areas with known drainage problems — low-lying properties in Riverview, Dieppe, or the Moncton lowlands, river valley locations in Fredericton or Miramichi — a perimeter French drain around the garage or a layer of 4-inch clear stone (drainage stone with no fines) as the bottom layer beneath the granular B significantly improves drainage performance.

The cost for gravel base preparation in NB runs $2–$4 per sq ft for material and compaction. For a 500 sq ft two-car garage, that's $1,000–$2,000 — a small investment relative to the total project cost that has an outsized effect on how long the floor lasts. New Brunswick Concrete can connect you with garage slab contractors who understand NB soil conditions and will prepare the base properly.