What is the difference between a concrete slab and a floating slab in NB?

A standard concrete slab is structurally connected to its supporting footings or foundation walls, while a floating slab is designed to move independently — it literally floats on the soil surface, free to rise and fall slightly with frost heave without being constrained by or stressing any connected structure. This distinction matters enormously in New Brunswick, where 4 to 5 feet of frost depth means the ground moves significantly every winter.

A standard or structural slab is poured after footings and foundation walls are in place, with rebar dowels tying the slab to the walls. The footings extend below frost depth, so they are stable and do not move seasonally. The slab is locked to this stable structure. Any frost heave in the soil beneath the slab either pushes up against the tied structure (which is strong enough to resist it) or lifts the slab and cracks it where it is restrained. This is the system used for basement floor slabs, garage floors in attached structures, and slabs that are part of a full foundation system.

A floating slab — also called a freestanding slab — has no physical connection to any frost-depth footing. The slab edges rest directly on the compacted gravel base. When the soil beneath heaves in winter, the entire slab can rise and fall as a unit rather than cracking where it is tied to a stationary structure. The edges of a floating slab are thickened (typically 8 to 12 inches thick at the perimeter tapering to 4 to 5 inches in the field) to provide a footing function and some mass to resist minor heave. This is the system used for detached garage slabs, shed pads, and similar structures not attached to a frost-depth foundation.

In NB, floating slabs for detached structures work — but only when properly built. The gravel base must be well-drained and of adequate depth (typically 6 to 8 inches of compacted crusher run), and the base material must be clean granular fill that does not hold water and freeze. The premise of a floating slab is that if the soil heaves, the whole slab heaves together. Problems arise when: the base compacts unevenly (one corner heaves more than another), tree roots grow beneath the slab, or moisture under the slab creates differential heaving. A floating slab that tilts is the common failure mode.

For attached structures in NB — a garage addition attached to a house, a slab that supports a load-bearing post or column, or any structure where differential movement between the slab and an adjacent foundation wall would be a problem — a floating slab is not appropriate. You need frost-depth footings and a structurally connected slab.

If you are planning a new detached garage, workshop, or outbuilding in NB, discuss the floating versus full-footing approach with your contractor. In many NB municipalities, the permit and inspection process will specify which approach is required for your project type. New Brunswick Concrete can connect you with contractors who can advise on the right system for your site.