How thick should a garage floor slab be in New Brunswick?

A residential garage floor slab in New Brunswick should be a minimum of 4 inches thick, and 5 inches is strongly recommended if you park heavy vehicles, use the space as a workshop, or plan to apply an epoxy or polyaspartic coating. Thicker is better — the cost difference between 4 and 5 inches is modest, but the improvement in durability and resistance to cracking under NB's frost-heave conditions is meaningful.

The NB Building Code doesn't mandate a specific thickness for garage floor slabs the way it does for structural foundations, but industry best practice and local contractor experience both point to 4 inches as the minimum and 5 inches as the practical standard for anything beyond a basic, light-use garage. If you're planning to store a motorhome, run a woodworking shop with heavy equipment, or add a vehicle lift, 6 inches with heavier rebar is worth the extra investment.

The gravel base matters as much as the slab thickness. In New Brunswick, where frost penetrates 4–5 feet into the ground, a well-compacted granular base is essential to prevent frost heave from cracking the slab. A minimum 6-inch compacted granular B gravel base beneath the slab is standard, with 8–12 inches in areas with poor drainage or high water tables (common near the Saint John River valley in Fredericton or low-lying areas in Riverview and Dieppe). The gravel allows water to drain away from beneath the slab rather than freezing and lifting it.

Reinforcement for NB Garage Floors

A 4–5 inch garage slab should include 6x6 welded wire mesh at minimum, supported on chairs at mid-slab height (not laid on the ground — mesh on the ground ends up at the bottom of the slab where it does almost nothing). For 5-inch or thicker slabs, or for heavier use applications, #10M rebar on 16-inch centres is a better choice. Reinforcement doesn't prevent cracking — nothing does — but it holds the slab together after cracks form and prevents sections from separating and lifting.

Control joints should be cut every 10–12 feet in both directions, to a depth of 1/4 of the slab thickness (1 inch for a 4-inch slab). These joints guide where cracking will occur, keeping it in straight lines that are easy to fill rather than random diagonal cracks across the slab.

For the concrete mix, specify 30 MPa (4,000 PSI) or higher with fibre reinforcement for a garage slab. Garage floors face vehicle traffic, oil and chemical exposure, and NB freeze-thaw cycling near the door. An air-entrained mix is important for the portion of the slab near the garage door where freeze-thaw exposure occurs.

Typical cost for a two-car garage floor (400–500 sq ft) in NB runs $3,000–$5,000 fully installed including gravel base, forming, concrete, and finishing. The slab alone is a small portion of that — the gravel base, prep work, and labour are the dominant costs. New Brunswick Concrete can connect you with experienced garage floor contractors across Moncton, Fredericton, Saint John, and surrounding areas.