What seals a New Brunswick garage slab?

A New Brunswick garage slab needs a 6-mil polyethylene vapour barrier laid directly on the compacted granular base before the concrete is poured — this is the single most important detail for keeping your garage floor dry and your floor coating intact.

Ground moisture in NB is relentless. Even in summer, the soil beneath a concrete slab stays cold and damp, and water vapour migrates upward through concrete by capillary action. Without a vapour barrier, that moisture reaches the surface as a fine, persistent dampness that causes epoxy and polyaspartic coatings to bubble and peel, makes stored cardboard boxes and tools go mouldy, and leaves the garage feeling perpetually clammy no matter the season. A roll of 6-mil poly costs $50-$150 and is physically impossible to add after the slab is poured — it is the cheapest insurance in the entire garage project.

The vapour barrier works as part of a layered system. Below it sits a minimum 4-inch bed of compacted granular fill (crushed stone or gravel), which provides drainage and a stable, uniform base for the concrete. The poly goes on top of that granular layer, lapped a minimum of 6 inches at any seams and turned up at the edges against the frost walls or forms. The concrete — typically a 4-inch slab with a minimum 25 MPa mix for NB residential garages — is then poured directly on top. Some contractors place a thin layer of sand between the poly and the concrete to protect the membrane during the pour, though this practice is debated; what matters most is that the barrier stays intact and continuous.

The perimeter seal matters just as much as the field. Where the slab meets the frost wall or the thickened edge, the vapour barrier must lap up and be sealed or mechanically fastened so ground moisture cannot enter at the joint. Over time, the concrete-to-wall joint often develops a hairline crack as the slab cures and settles slightly — this is normal, but it is also a moisture entry point. A bead of polyurethane caulk along the perimeter joint, applied after the concrete has fully cured (28 days minimum), closes this gap and is worth doing before any floor coating is applied.

Reinforcement keeps the slab from cracking and separating. A 6-mil vapour barrier seals moisture from below, but a cracked slab creates pathways for moisture from above — snowmelt, road salt, and wash water. Standard NB practice is either 6x6 welded wire mesh or 10M rebar on 16-inch centres, placed on chairs to keep the steel at mid-depth in the slab. Fibre reinforcement (polypropylene fibres mixed into the concrete) is increasingly common as a supplement or replacement for mesh in residential garage slabs and does an excellent job of controlling plastic shrinkage cracking.

After the slab cures, the surface itself can be sealed. A penetrating concrete sealer (silane-siloxane type) applied to a bare slab repels water and road salt, reducing dusting and extending the life of the concrete. If you plan to apply an epoxy or polyaspartic floor coating, the slab must be mechanically ground or acid-etched first to open the surface profile — coatings applied to a smooth, unprepped surface will peel within one to two seasons regardless of how good the product is. Polyaspartic coatings are particularly well-suited to NB garages because they cure in cold temperatures and tolerate the freeze-thaw cycling that causes standard epoxy to fail at the edges.

Practical tips before your slab is poured: Confirm with your contractor that 6-mil poly is on the material list — it sometimes gets omitted on rushed jobs. Ask to see the granular base compacted and inspected before the poly goes down. Make sure the slab is sloped slightly (1/8 inch per foot) toward the garage door opening so meltwater drains out rather than pooling. And plan your floor coating or sealer for late spring or early summer when temperatures are consistently above 10°C for proper curing.

Need help finding a garage contractor in your area? New Brunswick Garages can match you with local builders at no cost — and you can also browse general contractors through the New Brunswick Construction Network directory at newbrunswickconstructionnetwork.com/directory?category=general-contracting.