Quick answer
Cracks in slab foundations are breaks in the concrete slab that forms a house floor and foundation, caused by shrinkage as the concrete dried, by temperature, by loads, or by movement of the soil beneath, and read as clues, not as a diagnosis.
Most concrete cracks somewhere. What matters is the pattern, whether the crack changes, whether the floor has moved up or down across it, and what the soil and water are doing around the house.
Quick facts
The key facts about Cracks in Slab Foundations, with sources:
- Expected cracking
- Most normal concrete develops narrow cracks from shrinkage, temperature change and predictable structural behavior 1
- Most common cause in residential concrete
- Drying shrinkage and temperature effects 2
- Early shrinkage cracks
- Usually form within about 60 days of the pour, as the concrete dries 1
- Control joints
- Grooves or saw cuts that give shrinkage cracks a planned place to form 2
- Curling
- Slab edges and corners lift when the top dries faster than the bottom 3
- No width rule here
- This page gives no safe-or-unsafe crack width, because none applies to every house 6
Key takeaways
- Most concrete slabs crack. A crack alone does not show what caused it or whether it matters.
- Common causes are drying shrinkage, temperature, curling and loss of soil support. Each leaves clues, but clues are not a diagnosis.
- What matters is change over time, vertical offset across the crack, and what else is happening, such as sticking doors or gaps at walls.
- Photograph and date cracks, and measure them the same way each time. A dated record is the most useful thing to hand an engineer.
- Call a licensed structural engineer for significant, widening or changing movement, any height difference across a crack, or any safety doubt.
What cracks in a slab foundation are
A slab-on-grade foundation is a concrete slab, usually thickened at the edges, that serves as both floor and foundation. See slab-on-grade foundation for how it is built.
Because the slab is one big piece of concrete sitting on the ground, it cracks for the same reasons any concrete cracks, plus a few that come from the soil under it.
General information, not a diagnosis of your house. FEMA’s collapse-training materials put the baseline plainly: most normal concrete develops cracks that are narrow, or hairline, as a result of shrinkage, temperature change and predictable structural behavior 1.
Penn State engineers say the most common cause of cracking in residential concrete is drying shrinkage and temperature effects, and that other causes include soil settlement and structural overloading 2.
So a crack tells you that concrete moved a little. It does not tell you why, how much, or whether it is still moving. This page explains the usual suspects and what to record. The foundation cracks guide covers wall cracks and the crack-pattern basics.
A pattern is a clue, not a diagnosis
No crack pattern, width or photo can tell you whether your slab is safe. Movement history, displacement, water, construction details and site conditions all affect interpretation. Never assume a crack is harmless or that every crack needs repair.
How slab cracks form
Concrete is strong when squeezed and weak when pulled. A crack forms where something pulls the slab apart. The common pulls are:
- Shrinkage: Concrete shrinks as it dries. Penn State engineers explain that cracks may develop when shrinkage is restrained, for example by support conditions or reinforcing, and that concrete has low tensile strength 2.
- Early surface loss of water: FHWA describes plastic shrinkage cracking as caused by excessive evaporation at the surface, wider at the top and tapering with depth 4.
- Temperature: Heating and cooling change slab size. FEMA notes temperature cracks occur in roughly the same pattern as shrinkage cracks and are hard to tell apart 1.
- Curling: NRMCA says curling comes from differences in moisture or temperature between top and bottom, and lifts edges and corners at joints and cracks 3.
- Loss of support: NDSU says clay under a slab can swell and shrink, and uneven shrinkage can leave gaps so the slab loses support and cracks under load 6.
- Overload or movement of the foundation: Settlement of soil or heavy loads can crack a slab. Penn State lists soil settlement and structural overloading among the other causes 2.
Four wall sketches showing a horizontal crack, a vertical crack, a diagonal crack and a stair-step crack in block, each with a short note on what the pattern can be consistent with.
Illustrative sketches of common crack shapes. A shape is a clue to look closer, never a diagnosis, and the drawing is not to scale or a design for any house.
Slab crack patterns: possible contributing factors and what to record
| Pattern or kind | Possible contributing factors | What to record |
|---|---|---|
| Fine, short or map-like cracks near the surface | Early drying or plastic shrinkage 45 | Location, length, date, whether it has changed |
| Straight cracks across the slab at joint-like spacing | Drying shrinkage where no joint relieved it 21 | Where it runs, spacing to the next, any offset in height |
| Diagonal cracks from corners of openings | Stress at re-entrant corners such as door and floor openings 1 | Which corner, direction, width marks, dated photos |
| Cracks near slab edges or corners with lifted edges | Curling from top-to-bottom moisture or temperature differences 3 | Rocking or hollow sound at edges, any height step |
| Cracks with one side higher than the other | Movement of soil support, such as settlement or heave | Height difference, whether it is growing, floor levels |
| Cracks with a nearby sunken or lifted area | Soil moisture changes or lost support 67 | Area size, season, nearby water sources, doors and windows |
Engineer first for height differences
A crack with one side higher than the other, a crack that keeps widening, or cracks along with sticking doors and gaps at walls point toward movement, and a licensed structural engineer is the right first call.
Example: a record that helps an engineer
An example record for one invented crack, kept the same way each time. Dates and numbers are illustrative.
| Date | Location | Width (same spot) | Height step | Notes |
|---|---|---|---|---|
| Month 1 | Living room, near door | Measured at a marked spot | None felt | Dry weather; photo with ruler |
| Month 3 | Same | Same spot, same method | None felt | After heavy rain; no change seen |
| Month 6 | Same | Same spot, same method | Slight step felt | Door sticks; photo; call an engineer |
The table shows the habit, not a threshold. A step appearing or a crack changing is the kind of change worth reporting. See foundation crack width and monitoring for how to measure.
Where slab cracks tend to show up
Look at joints and openings first. FEMA notes that diagonal cracks often start at re-entrant corners, such as openings in slabs and walls, and that straight shrinkage cracks often occur at spacings of about five to twenty feet in long floor or wall surfaces, depending on reinforcing and curing 1.
Edges and corners are next. NRMCA says curling is noticeable at joints, edges and corners and lifts them from the base, and that interior slabs are more commonly affected because the top is exposed to a dry environment while the bottom is in contact with moist soil 3.
Then look at where water and soil act. Slabs near downspouts, leaking pipes or large trees sit on soil whose moisture changes, which matters most on clay. See expansive clay soil and plumbing leaks and foundation damage.
What cracks can tell you, and what they cannot
What a record of cracks gives you
- A pattern points to likely contributors, such as shrinkage or lost support 16.
- Dated readings show whether a crack is changing, which one visit cannot 9.
- A height step across a crack is a clue that parts of the slab have moved relative to each other.
- A clear record saves an engineer time and lets them read the history.
What a crack cannot tell you
- No pattern proves a cause. Many causes look alike 1.
- A crack’s width alone does not say whether it is safe. NDSU gives no width threshold for slab or wall cracks 6.
- Early shrinkage cracks and later movement cracks can sit side by side.
- A crack says nothing about the soil depth, the slab design or the loads.
What slab cracks do not tell you
We cannot say from a description or a photo whether a slab crack is structural.
Penn State notes that plastic shrinkage cracks usually do not impair the structural performance of the slab, though cracks in some building floors have been blamed for leakage 5.
That is a general statement about one cracking type, not a verdict on yours.
Fill under a slab, tree roots, plumbing leaks, curing practices and joint layout all change what a crack means. Soil behavior depends on mineralogy, moisture, stress and drainage, and one soil type is not proof of a problem.
Repair is also a separate question. A crack may be sealed, injected or left alone, and a slab that has dropped may be lifted. The method is chosen for a specific house by an engineer from loads, soil and site, never from a crack picture. The related terms below explain slab lifting methods.
What drives the cost of slab crack work
We print no prices, because fair numbers need dated local quotes.
The cost drivers are how many cracks and how long, whether cracks are only surface or tied to movement, whether the slab has dropped or lifted, access and floor finishes to remove and replace, whether a geotechnical review is needed, and whether the cause, such as a leak or drainage, must be fixed first.
Cosmetic sealing sits at one end, and structural work at the other. The foundation cost guide and cost calculator explain how quotes are built, and the slab leveling page covers lifting services.
How to record a slab crack safely
Recording a slab crack stays on the surface and inside the house. No digging or jacking is involved.
- Pick the cracks: Choose the main ones and give each a number or a letter.
- Mark the spot: Put a small pencil mark or tape tick on each side at a place where the edges are straight and parallel.
- Photograph with scale: Include a ruler or coin and a date card, from the same angle each time.
- Run a hand across: Note any height step. Do not place heavy objects or pry at the crack.
- Re-check on a schedule: Repeat every month or season, and after big rain or drought, and note the weather 9.
- Note other clues: Sticking doors, gaps at baseboards, new wall cracks or sloping floors.
- Share the log: Hand the record to a licensed engineer if anything changes.
Use the tools on this site
The crack identification tool helps describe what you see, and the elevation log helps track floor levels over time.
What a homeowner can do to limit slab movement
- Keep water away from the slab edge: clean gutters, downspout extensions and a slope away from the house.
- Fix plumbing leaks and irrigation leaks quickly.
- Keep soil moisture even around the house in dry spells, as NDSU describes for dry northern soils: slow, even watering a foot or two from the wall. Local soils and rules differ, so ask a local engineer first 6.
- Avoid heavy watering at one spot near the slab edge, and ask about tree distance for your region before planting.
- Do not fill gaps between soil and wall with loose soil, since wet soil expands 6.
- Seal only small surface cracks, and only after an engineer says movement is not active, since repairs made during movement can fail.
See drainage and grading for water at the wall.
Warning signs and when to call a structural engineer
Call a licensed structural engineer when you see any of these, and do not wait for a set width:
- Significant, widening or changing movement in a slab crack.
- A height difference across a crack, or a floor that now slopes.
- Sticking doors or windows together with new cracks.
- Separation at corners, or a gap between the slab and a wall.
- A wall that bows, leans or shows a horizontal crack.
- Any safety doubt, including damp, musty or wet areas under the floor.
The structural engineer guide and the engineer or contractor tool help choose who to call first.
What the model code says that bears on slab cracks
IRC R403.1.8 says foundations and floor slabs on expansive soils shall be designed under Section 1808.6 of the International Building Code, with an exception for slab-on-ground and other systems that have performed adequately in similar soil, subject to the building official’s approval 87.
IRC R401.4 lets the building official require a soil test where expansive, compressible, shifting or other questionable soils are likely 8.
The code does not set a crack width for a slab, and neither does this page.
Concrete mix, slab thickness, joint layout and curing come from the concrete chapters and from industry guides such as ACI 302.1R and ACI 360R, which NRMCA names in its guidance on curling 3.
Seattle’s code reproduces the 2021 IRC with local amendments, and Texas’s bulletin covers the 2015 IRC, so check the edition and amendments your jurisdiction adopted.
Slab cracks vs. wall cracks, and what each points to
| Feature | Cracks in slabs | Cracks in foundation walls |
|---|---|---|
| Common early causes | Drying shrinkage, plastic shrinkage, curling 243 | Shrinkage and temperature, plus soil and water loads 1 |
| Soil clue | Loss of support under the slab, such as from clay 6 | Soil pressure, water pressure and movement |
| Where it shows | Floors, often with height steps | Basement and crawl space walls, often with pattern clues |
| Read more | This page | horizontal, vertical and diagonal cracks |
Common myths about slab foundation cracks
- Myth All slab cracks are harmless shrinkage.
- Reality Many are, but no crack is harmless by description alone. Movement history and site conditions decide.
- Myth A crack over a set width means the slab is failing.
- Reality No universal width applies. NDSU names no numeric threshold for crack size or alignment 6.
- Myth Every slab crack needs underpinning.
- Reality Not every slab crack calls for underpinning. Whether a crack needs a record, a seal or something more is for an engineer to decide for a specific house.
- Myth Filling the crack stops the movement.
- Reality A seal hides a symptom. If the cause continues, the crack can reopen.
- Myth Cracks mean the builder did a bad job.
- Reality Most normal concrete cracks narrowly 1. Quality is judged by pattern, change and design, not by a crack’s presence.
Where soils and climate change what a slab does
Slab behavior is regional. Clay-rich regions such as parts of Texas and Colorado see slabs rise and fall with soil moisture, and Texas guidance sends expansive soil foundations to the IRC provisions 7.
Dry northern regions see soil shrink away from edges in drought, as NDSU describes 6. Frost depth, groundwater and local fill practices also vary. Ask your building department what soil reports and slab designs exist for your area.
When slab cracks matter, and what to do next
Slab cracks matter when they change, when the floor steps up or down across them, and when they come with other signs of movement. For a crack with no other signs, a dated record is a common first step, and an engineer can say whether more is needed.
If you are unsure, bring your log to a licensed engineer. The foundation inspection page explains what a visit covers, and the inspection checklist lists what to prepare.
- If A crack with no change over several months and no other signs
- Then Keep recording and keep water away from the slab
- If A crack with a height step, growth, or sticking doors
- Then Call a licensed structural engineer
- If A slab that has dropped or lifted
- Then Do not jack or dig; ask an engineer to assess the cause first
Questions about Cracks in Slab Foundations
Are cracks in a concrete slab foundation normal?
Some cracking is common. FEMA training materials note most normal concrete develops narrow cracks from shrinkage, temperature change and predictable structural behavior. Normal does not mean harmless, since cracks can also come from soil movement. Record changes and ask an engineer if they continue.
What causes slab foundation cracks?
Common causes are drying shrinkage and temperature effects, plastic shrinkage while the concrete cures, curling of edges, and loss of soil support. Penn State engineers list soil settlement and overloading among other causes. The cause of one crack needs a review of history and site.
How wide is too wide for a slab crack?
No width applies to every house, and this site does not give one. NDSU extension says crack size helps indicate whether a professional is needed but gives no number. Change over time, height steps and other signs matter more than width alone.
Are hairline cracks in a slab a problem?
Often they are shrinkage cracks that formed as the concrete dried, but a hairline crack alone cannot be called harmless. Mark it, photograph it with scale, and re-check. If it widens or the floor steps across it, call a licensed structural engineer.
Do slab cracks mean my foundation is failing?
Not by themselves. A crack is a clue, and its meaning depends on movement history, displacement, water, construction details and site conditions. Signs such as height steps, sticking doors and widening cracks point toward movement and justify an engineer’s review.
Can I seal a slab crack myself?
Small surface cracks are sometimes sealed, but only after movement is understood, because a sealed crack can reopen if the cause continues. Sealing also hides the crack from your monitoring. Photograph and measure first, and ask an engineer if you see a change.
What is slab curling?
Curling is the lifting of slab edges and corners when the top dries or warms differently from the bottom. NRMCA says it is mostly drying shrinkage and moisture or temperature differences, is most common in interior slabs, and can leave an unsupported edge. Fixes such as grinding or grout belong to professionals.
Can tree roots or leaks crack a slab?
They can contribute, by changing soil moisture under part of the slab. Clay under a slab can swell or shrink unevenly and leave gaps, as NDSU describes. Fix leaks quickly, keep large trees back, and ask an engineer if cracks and floor steps appear.
When should I call a structural engineer about slab cracks?
Call for significant, widening or changing cracks, any height difference across a crack, sticking doors or windows with new cracks, gaps at walls, a bowing wall, or any safety doubt. Bring your dated photos and measurements.
Sources
- FEMA National US&R Response System, Structural Collapse Technician, Module 1C Part 3: Hazard Identification and Building Monitoring (cracks in reinforced concrete and masonry), retrieved .
- Penn State, Singh and Scanlon, Concrete in Residential Construction, 1st Residential Building Design and Construction Conference (2013), retrieved .
- NRMCA, CIP 19 Curling of Concrete Slabs (2018 edition), retrieved .
- FHWA, Petrographic Methods of Examining Hardened Concrete, Chapter 4 Cracks, FHWA-HRT-04-150 (July 2006), retrieved .
- Penn State Civil Engineering course library, Plastic Shrinkage cracking (compiled by C. J. McBryde), retrieved .
- NDSU Extension, Ken Hellevang, Dry Soil May Cause Building Problems (June 30, 2017), retrieved .
- Texas Dept. of Licensing and Regulation, IHB Technical Bulletin TB 10-01, Foundations on Expansive Soils: Residential (2010, rev. 2018), retrieved .
- City of Seattle, 2021 Seattle Residential Code, Chapter 4 Foundations (2021 IRC text with Seattle amendments), retrieved .
- Specification Online trade article, Monitoring structural movement: from visual observation to measured record (September 3, 2026; carries a gauge maker’s sponsorship line, used only for general monitoring practice), retrieved .
Expert review
Written by the HyreFoundation Research team. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Concrete cracking facts come from FEMA structural collapse training materials, a Penn State paper and course page, FHWA’s petrographic manual and NRMCA’s curling bulletin. Soil-support points come from NDSU extension and a Texas agency bulletin.
Code text comes from Seattle’s 2021 residential code, which reproduces the 2021 IRC with city amendments. The recording advice reflects general monitoring practice from one trade article that carries a sponsor line. No crack width threshold is given, by design. No prices.
Suggest a correction. We fix errors and say what changed.