Garage Floor Slab: Repair or Replace? 6 Signs to Check
Updated October 2026
A garage floor slab may need replacing, not repairing, when the problem sits under or through the concrete: panels that have sunk or heaved out of level, or wide cracks that run through the full depth and keep reopening. A hollow sound when you tap it points the same way. Hairline cracks and light surface wear on a flat, solid slab are repair or sealing jobs. A free site visit and a written quote will tell you which one you have.
A garage floor takes more punishment than most concrete. It carries the weight of vehicles, it sees road salt off the car all winter, and it often sits on the same Southern Ontario clay that moves with every frost. Some damage is cosmetic. Some means the slab has failed. The difference is whether the problem is on top of the concrete or under it.
Sign 1: panels that have sunk or heaved
A floor that sat flat when it was poured should still sit flat. If one section has dropped, tilted or risen out of line with the next, the ground or base under it has moved. Much of Southern Ontario sits on clay, which holds water, and frost in Ontario reaches down past a metre (as we explain for driveways). Water that freezes under a slab pushes it up. Settled fill or washed-out soil lets it drop. You cannot fix movement from the top.
Sign 2: wide cracks that keep reopening
Hairline cracks are thin and shallow. They come from normal curing and seasonal movement. Structural cracks are wide enough to slip a coin into, run through the full depth of the slab, or leave one side higher than the other. If a crack reopens after you fill it, the force that opened it is still there.
Sign 3: a hollow sound
Tap a low spot or a cracked area with a hammer or the handle of a tool. A hollow sound over a low spot is a sign the base under the slab may have failed. Take it together with the other signs, and have the floor looked at before you decide.
Sign 4: a floor that no longer drains
A garage floor should slope gently toward the door so snowmelt off the car finds its way out. If water now pools against the walls or in the middle of the floor, the slab may have settled or may never have been sloped right. Pooling water feeds the freeze-and-thaw damage above, so ask for a site check.
Sign 5: surface damage on a slab that is also moving
Surface flaking from winter salt is a surface problem. Resurfacing can bring back a worn but solid slab, and we explain the cause in our guide to spalling after winter. When the surface damage sits on top of a slab that is also sinking or heaving, resurfacing will not hold, because the movement is underneath.
Sign 6: the floor was never built for the load
Some garage floors fail because they were poured too thin, with light mesh, on a base that was not compacted. A floor that carries vehicles needs more. We pour garage floors at 125 to 150 mm thick, with 10M rebar on a 400 mm grid, over 150 to 200 mm of compacted Granular A.
Repair signs versus replace signs
| Lean toward repair | Lean toward replace |
|---|---|
| Hairline cracks | Wide cracks through the full depth |
| Slab sits flat and level | Panels sunk, heaved or tilted |
| Light surface wear or salt flaking | Surface wear on a slab that is also sinking or heaving |
| Solid sound when tapped | Hollow sound over a low spot (a sign the base may have failed) |
| Floor still drains to the door | Water pools; the slope may be gone |
What replacing a garage floor slab involves
When the slab has failed, the old concrete has to come out. We remove and haul away the old concrete, take out soft or disturbed soil, and rebuild the base in compacted lifts. Then we set reinforcement for the load and pour 32 MPa air-entrained concrete, the mix we use on garage floors because of the salt. Anchor bolts, thickened edges and a slope to drain at the door go in on pour day when the building calls for them. Keep vehicles off for at least 48 hours, and wait about 7 days before framing on the new slab. Full design strength comes at 28 days.
What the work costs depends on the size of the floor, the base and soil under it, how much old concrete comes out, access, and the reinforcement the load needs. We do not quote from a price list. Our concrete slabs and garage pads service page explains the full job, and our slab cost guide shows where the money goes.
Get a straight answer for your garage floor
Photos cannot show what a hammer and a level can. We come out, look at your actual floor, and tell you whether it needs sealing, resurfacing or replacing, with the reasons in writing. Every quote is free and in writing, and our labour carries a lifetime warranty. Reach out for a free written quote.
Are cracks in my garage floor a sign it needs replacing?
Not by themselves. Hairline cracks are common in concrete and often harmless on a slab that is still flat and solid. Cracks wide enough to slip a coin into, cracks with one side higher than the other, or cracks that reopen after they are filled point to movement under the slab. That is when replacement becomes the honest answer.
How do I know if the base under my garage slab has failed?
Look for panels that have sunk or tilted, a hollow sound when you tap a low spot, and cracks that return after repair. Watch the slab, not only the cracks. If it still sits flat and level, the base is probably doing its job.
Why does my garage floor flake and crumble on the surface?
Winter road salt dripping off parked cars, working with freeze and thaw, is the usual cause on a surface that was not built for it. If the slab below is still sound and level, this is a surface problem and resurfacing may fit.
What does a new garage floor slab need to last?
A compacted granular base, reinforcement matched to the load, and the right concrete mix. We pour garage floors at 125 to 150 mm with 10M rebar on a 400 mm grid over 150 to 200 mm of compacted Granular A, in 32 MPa air-entrained concrete.
How long before I can park on a new garage slab?
Keep vehicles off for at least 48 hours. Wait about 7 days before framing on it. Full design strength arrives at 28 days.