A crack that stays put versus one that moves
The National Park Service's Preservation Brief 15, a guide to concrete repair, sorts cracks into two kinds: dormant cracks that have stopped changing and active cracks that are still opening, closing or growing. It says early shrinkage cracks are not a structural concern on their own, though they can let water into the concrete. A fine web of surface cracks, called map cracking, is also common on older slabs.
The brief also says structural cracks can come from uneven settling, and that they stay active while the settling continues. Bending or sagging in concrete points to a problem with its strength or the support under it. Hairline cracks that are not getting bigger may be left alone. Its advice when in doubt is to assume a crack is moving until you know otherwise.
One thing to rule out first: flaking or pitting on the surface, called scaling or spalling, is damage to the concrete itself from water freezing and thawing inside it. It isn't the same as the slab lifting, and the repair is different.
Why the ground under a garage moves
Water is behind most of it. The Building America Solution Center, a U.S. Department of Energy guide, says expansive soils swell when wet and can crack or lift foundations, while collapsible soils lose volume when they get wet, letting things settle. That means a raised edge doesn't tell you which side moved: one section may have risen, or the one next to it may have sunk.
Cold adds to it. The same guide's page on slab edges says that in cold climates the ground should slope away and backfill should drain freely, with a footing drain, to avoid frost heave. It also notes a slab can be built separate from the foundation wall, with a break between them, so a cracked garage floor doesn't automatically mean the foundation has failed.
What to check and record yourself
Start a dated photo log before you cover the floor with mats or shelving. Shoot the same spots each time with a ruler or coin for scale, and measure two things: how wide the crack is and how much higher one edge sits than the other. Note any new cracks and whether old ones get longer. Changes that show up after a freeze, a thaw or a long rain are useful clues for whoever evaluates it.
Then walk the outside of the garage from ground level. Look for overflowing gutters, disconnected downspouts, puddles along the walls and ground that slopes toward the building. The Solution Center says downspouts should empty at least 5 feet from the foundation, or into an underground pipe that discharges at least 10 feet away, and that gutter drains should not be tied into a perforated foundation drain. Our guide to interior drainage versus exterior waterproofing explains how those two kinds of fixes differ.
Don't drill into the slab, dig under it or try to jack it up to see what's going on. If a section is moving quickly or looks unstable, keep cars and people off it until someone qualified has looked.
When to bring in an engineer
Get an evaluation if a crack keeps widening, one side keeps rising or dropping, or the floor visibly sags. The Solution Center says soil problems should be assessed by a hydrologist, soil scientist or engineer. For a garage, a structural engineer who works on residential slabs is a sensible first call. Ask whether soil testing is needed and whether the movement involves only the slab or also the walls and footings.
What a repair quote should explain
Filling or sealing a crack won't stop soil from swelling or freezing under it. Preservation Brief 15 says a crack that is still moving needs a flexible sealant, such as an elastomeric product in a routed groove, because a rigid fill will just crack again. Before you sign, the written scope should say what was measured, including the height difference across each crack; what the contractor thinks is causing it and what is still uncertain; whether drainage, grading or soil work is included or only the concrete; how the finished floor will meet the garage door and the house; and what should be watched afterward.
If the job grows into removing and replacing the slab, have the contractor confirm permit and inspection requirements with Philadelphia L&I or your township's building office before work starts. Our contractor hiring checklist covers what to verify first, and our water documentation guide shows how to keep a record that helps if moisture is part of the problem.