Garage Floors

A garage floor lives a harder life than most interior concrete. It takes point loads from jacks and vehicle wheels, it gets road salt carried in on tyres through the winter, and it is subject to moisture from below because it usually sits on grade with limited or no vapour barrier in older construction. The typical failure sequence is surface spalling from chloride attack, then cracking, then a section that settles near the door where the base is shallowest and water gets in.

A garage slab with surface damage but sound structure can take an overlay or a grind and coating. A slab that has cracked and moved needs replacing. The awkward middle case is a floor that is sound everywhere except at the door threshold, which is extremely common — that section can often be saw-cut out and repoured as a strip, tying into the existing floor with dowels so the two move together.

A garage floor should drain toward the door, at a gentle pitch of around an eighth to a quarter inch per foot. Getting this wrong is expensive to live with, because water carried in on a car in winter then sits against the back wall. The threshold detail at the door matters too: the slab edge there is exposed, takes every vehicle wheel, and is where the door seal has to land. It gets a thickened edge.

Moisture moving up through a slab is what makes coatings fail, and it is why a floor coating that was applied over a slab with no vapour barrier peels within a year or two regardless of how well the surface was prepared. On a replacement we place a vapour barrier under the slab as standard. On an existing floor, a moisture test before committing to any coating is a small cost that avoids a large disappointment.

Four inches is standard for passenger vehicles. Where a garage will hold a truck, a lift, or heavy equipment, five to six inches with steel reinforcement is the right call. A two-post lift in particular concentrates enormous load on four small baseplates, and lift manufacturers publish minimum slab specs — those specs are worth reading before the concrete goes in rather than after.