A crack can mean two completely different things: the concrete surface shrank a little, or the ground underneath it moved. Sealing fixes the first. Only lifting fixes the second. Here's how to tell which crack you have — and the red flags that mean neither, and you need an engineer.
Homeowners tend to treat every concrete crack the same way: something broke, seal it shut. Contractors know better, because concrete cracks for a dozen reasons and only some of them live in the concrete itself. A hairline crack in a year-old driveway is usually the concrete doing exactly what concrete does — shrinking slightly as it cures, and cracking at the weakest point because it was designed to. A crack that appears in a ten-year-old patio alongside a visible tilt is the ground underneath giving way. Seal the first and you are done. Seal the second and you have hidden the evidence while the slab keeps sinking.
The distinction that matters is surface versus structure. Surface cracks live in the top of the slab: shrinkage cracks, crazing (a network of fine surface lines), and control-joint cracks that opened where the saw cuts told them to. They look dramatic, especially when they darken with dirt, but the slab underneath is still supported and still level. Settlement cracks live through the slab and reflect what is happening below: soil consolidating, washing out, or shrinking away from the concrete. They are usually accompanied by other evidence — a drop, a tilt, a hollow sound when you tap, water pooling where it never used to. That company of symptoms is the tell. A crack alone is ambiguous; a crack plus movement is a settlement story.
Utah adds its own chapter to this story. Freeze–thaw cycles open and widen cracks every winter as water gets in, freezes, and expands — which is why an unsealed crack in Salt Lake City is a crack with a deadline. Expansive clay soils on the west side of the valley swell when wet and shrink when dry, moving slabs up and down with the seasons and cracking them at the stress points. And the valley's newer subdivisions, built on recompacted hillside, sometimes hide loosely compacted fill under a slab that looks perfect for its first few years. The why Utah concrete settles guide covers the local geology; this article is about reading the crack itself.
You can run most of this diagnosis yourself with a ruler, a pencil, and a month of patience:
Measure the crack at several points along its length. Hairline cracks — thinner than a credit card edge — are almost always shrinkage or surface crazing. Cracks around an eighth of an inch or wider deserve attention, and cracks that taper (wide at one end, hairline at the other) are a settlement signature: the slab is rotating or dropping more on one side, so the crack opens where the movement is greatest.
Run a finger, a ruler, or a short straightedge across the crack. If both sides are flush, the slab is still sitting level and the crack is likely surface-only. If one side sits higher than the other — even by an eighth of an inch — the slab has moved vertically, and that is settlement until proven otherwise. Vertical displacement is the single most reliable crack-reading test a homeowner can do, and it is the one contractors check first.
Mark both ends of the crack with a pencil line and a date, or photograph it with a ruler laid beside it. Check again in a month, and again after a wet spell or a freeze. A shrinkage crack stays exactly where it was. A settlement crack grows — wider, longer, or more offset. Movement is the difference between "seal it and forget it" and "find out why the ground is moving," and no single visit, yours or a contractor's, can substitute for watching it.
Look around the crack, not just at it. Is the slab tilted? Does water pool nearby? Does the slab sound hollow when tapped? Is there a gap where it meets the house, the garage floor, or a step? Is a downspout discharging at the slab edge? Each companion symptom strengthens the settlement case, and each one is also part of the fix — lifting a slab without fixing the downspout that undermined it is a repair with an expiration date. Our drainage guide covers the water side of this.
When did the crack appear? A crack in the first year after a pour is usually curing shrinkage, especially if it follows a control joint or a straight line. A crack that showed up years later, especially after a wet spring or a winter of freeze–thaw, is more likely movement. And a crack that reappears after a previous repair — sealed once, opened again — is telling you the first repair treated the symptom. Believe it the second time.
| What you see | What it usually is | The fix |
|---|---|---|
| Hairline, even width, level both sides, stable over months | Shrinkage or surface crazing | Seal it; monitor |
| Crack along a control joint, level, stable | Concrete cracking where it was designed to | Clean and seal the joint |
| Tapered width, one side higher, or widening over time | Settlement | Lift the slab, then seal |
| Crack plus tilt, drop, hollow sound, or pooling water | Settlement with an active cause | Lift + fix the cause (usually water) |
| Shattered slab, pieces moving independently | Structural failure of the slab | Replacement, not lifting |
| Foundation wall cracks, stair-step brick cracks, sticking doors | Foundation movement | Structural engineer first |
Describe the crack — width, offset, whether it's growing — and we'll tell you honestly whether it's a seal, a lift, or something that needs an engineer before anyone touches it.
When the diagnosis is a surface crack, sealing is a genuinely good repair — but only if it is done with the right material and at the right time. For exterior concrete in Utah, polyurethane or silicone sealants outperform rigid epoxy for most cracks, because they stay flexible through freeze–thaw cycles instead of cracking again when the slab moves a hair with the seasons. Clean the crack first — wire brush, compressed air or a leaf blower, and dry conditions — because sealant bonded to dirt and dust is sealant waiting to peel. And backer rod (a foam strip pushed into wider cracks) keeps the sealant at the right depth so it stretches instead of tearing.
The timing rule most homeowners get backwards: seal after lifting, not before. Lifting moves the slab, and that movement can crack a fresh seal or open new hairlines next to it, so sealing first usually means sealing twice. The one exception is a crack actively funneling water under the slab — a downspout discharging straight into it, for example — where a temporary seal or, better, redirecting the water is worth doing immediately to stop the void from growing before the lift. After the lift, sealing is not optional: an unsealed crack is a water highway into the base, and water in the base is what caused the settlement you just paid to fix. The full routine is in our sealing after lifting guide.
Some cracks are outside a lifting contractor's lane entirely, and the costliest mistake in this whole decision is letting the wrong specialist work on the wrong problem. Call a structural engineer first if you see any of these: stair-step cracking in brick or block walls; horizontal cracks in a foundation wall; cracks that run through the foundation itself rather than just the flatwork; doors or windows that suddenly stick, won't latch, or show gaps in their frames; interior floors that slope or feel springy; or a basement floor slab moving in step with the walls. These are foundation-movement signs, and they need a stamped engineering opinion on what is moving and why before anyone injects material, patches a wall, or lifts anything. Lifting flatwork under a foundation problem can mask the evidence and complicate the real repair.
The same rule applies in reverse: a foundation contractor is the wrong call for a sunken driveway. The boundary is the house itself. Flatwork outside the walls — driveways, sidewalks, patios, porches, garage floors — is a lifting contractor's job. Movement in the walls, footings, or foundation is an engineer's diagnosis first. When the two overlap — say, a porch slab dropping away from the house wall — start with the engineer if the wall shows any cracking, and with the lifting contractor if the wall is clean and only the slab moved. Our foundation vs. slab settlement guide works through the borderline cases.
Seal the cracks that are stable, level, and surface-only. Lift the slabs that are dropping, tilting, or offset — and fix the water or soil cause at the same time, or the crack will be back. Replace slabs that are shattered beyond holding together. And hand the house itself to an engineer the moment the cracking touches walls, footings, or door frames. If you are unsure which category your crack falls into, the measurement routine in our how to measure sunken concrete guide gives you the numbers, and an in-person evaluation settles the rest. Lifting, when it is the right call, is priced from the published ranges — mudjacking $3–$8 per square foot, foam $5–$25 per square foot, most residential jobs $500–$3,000 total — and the lifting vs. replacement comparison covers the case where the slab is past both sealing and lifting.
Sometimes — and when the crack is the only problem, sealing is the right fix. Hairline shrinkage cracks and stable control-joint cracks with no vertical offset and no ongoing movement seal well and stay sealed. Sealing fails when the crack is a symptom of settlement: if the slab is still dropping, the crack will reopen or a new one will appear next to the seal, because the seal treats the symptom while the soil underneath keeps moving. The test is movement: stable crack, seal it; moving or offset crack, diagnose the settlement first.
The settlement signatures are a crack that is wider at one end than the other, a crack with one side higher than the other (vertical displacement you can feel with a finger or a ruler), a crack that keeps widening month to month, or a crack paired with a visible drop or tilt in the slab. Any of those says the ground underneath moved, not just the concrete surface. Surface-only cracks — thin, even-width, level on both sides — are usually shrinkage or curing cracks and rarely need lifting. See can cracked concrete be lifted for the full diagnosis.
When the cracking involves the house structure, not just flatwork: stair-step cracks in brick or block walls, horizontal cracks in a foundation wall, cracks that run through the foundation itself, doors or windows that stick or won't latch, or floors inside the house that slope. Those are foundation-movement signs, and they need an engineer's diagnosis before anyone injects material or patches anything. A lifting contractor handles driveways, sidewalks, patios, and garage floors; an engineer handles the house. Our foundation vs. slab guide explains the boundary.
After, with one exception. Lifting moves the slab, and movement can crack a fresh seal or open new hairlines, so sealing first usually means sealing twice. The exception is a crack actively funneling water under the slab — a downspout discharging straight into it, for example — where a temporary seal or, better, redirecting the water is worth doing immediately to stop the void from growing before the lift. After the lift, sealing is part of the job: unsealed cracks let water back into the base, which is what caused the settlement in the first place. See sealing concrete after lifting.
Three checks. Width pattern: shrinkage cracks are thin and fairly even along their length; settlement cracks often taper, wider at one end. Level: run a finger or a straightedge across it — shrinkage cracks are level on both sides, settlement cracks are often offset. Movement: mark the crack ends with a pencil line and a date, or photograph it with a ruler beside it, and check again in a month. Shrinkage cracks stay put; settlement cracks grow. Age matters too: a crack that appeared in the first year after a pour is usually curing-related; a crack that appeared years later, especially alongside a drop or a tilt, is usually movement-related.
Stitching — stapling steel bars or carbon-fiber strips across a crack — holds a cracked slab together, but it does not lift it and does not fix the soil underneath. It makes sense as a companion to a lift: lift the slab back to grade, then stitch the crack so it stays closed. Sold alone on a settling slab, stitching is a brace on a sinking foundation — the slab keeps dropping and the stitch just rides along. Ask any contractor proposing stitching what they are doing about the settlement underneath, and be skeptical if the answer is nothing.
Sealing is the cheap end of the spectrum — a tube of polyurethane sealant and an afternoon for a homeowner, or a modest line item on a contractor's visit. Lifting is priced like any flatwork job: mudjacking at $3–$8 per square foot, polyurethane foam at $5–$25 per square foot, most residential jobs between $500 and $3,000 total. The expensive mistake is paying for the cheap fix twice — sealing a settlement crack, watching it reopen, sealing it again — when the lift would have solved it once. The cost guide has the full tables.