Why concrete cracks at all
Concrete shrinks as it cures, and a poured foundation wall or slab is rarely free of cracks. This Old House notes that hairline cracks or widely spaced vertical cracks less than about 1/8 inch wide in a poured foundation generally mean the concrete shrank as it cured, and unless they grow wider or leak, they can usually be left alone. Cracks that are wider, that change over time or that run in certain directions are a different story.
Crack types at a glance
| Crack | What it usually means | What to do |
|---|---|---|
| Hairline vertical crack in a poured wall | Shrinkage while curing | Monitor; seal if it leaks |
| Vertical crack wider than about 1/8 inch, especially near a corner | Possible settlement of one part of the foundation | Have it inspected |
| Diagonal crack from a corner of the wall | Differential settlement | Have it inspected |
| Horizontal crack in a basement wall | Soil or frost pressure pushing the wall in | Have it inspected soon |
| Stair-step crack in block or brick | Settlement or lateral pressure | Have it inspected |
| Crack with one side offset or displaced | Active movement | Have it inspected soon |
| Hairline cracks in a slab floor | Shrinkage | Usually cosmetic |
Horizontal cracks: the one not to ignore
A long horizontal crack partway up a basement wall is the classic sign that the soil outside is pushing harder than the wall can resist. Wet clay, frozen soil and water pressure all add load. Horizontal cracks are often accompanied by diagonal cracks in poured walls or stair-step cracks in block walls, and the wall may start to lean or bulge. At that point sealing the crack is not enough; the wall needs to be stabilized. Our guide Bowing basement walls: causes and fixes explains the options.
Stair-step cracks in block and brick
Concrete-block walls and brick veneer crack along the mortar joints, so movement shows up as a staircase pattern. A stair-step crack near a corner often means that part of the foundation has settled. A stair-step crack in the middle of a block basement wall, together with a horizontal crack, often means the wall is being pushed in. In Texas and other slab regions, stair-step cracks in brick veneer near garage corners are one of the most common signs of slab settlement.
Signs inside the house that go with foundation cracks
- Doors that stick, swing on their own or no longer latch
- Drywall cracks running diagonally from door and window corners
- Gaps between walls and the ceiling or floor, or baseboards separating
- Floors that slope, sag or feel bouncy
- Cracked tile in a line across a room
- Windows that are hard to open or show gaps at the frame
One of these alone is common in any house. Several together, or signs that keep getting worse, point to foundation movement.
How to track a crack yourself
- Mark both ends of the crack with a pencil and write the date next to it.
- Measure the widest point with a ruler or a crack gauge and photograph it next to a coin or tape measure.
- Check again every few months, especially after a dry summer, a wet season or a freeze.
- Note doors that begin to stick and any new cracks nearby.
A crack that grows past its pencil marks, gets wider, or reopens after being patched is active and should be inspected.
Why cracks change with the seasons
Many cracks open and close during the year. In clay regions such as San Antonio and the rest of Central Texas, soil shrinks in late summer and swells after heavy rain, so a crack can be widest in August and nearly closed in winter. In cold climates such as North Jersey, frost pushes on basement walls in winter and saturated soil adds pressure during spring thaw. That is why a single measurement says little. A crack that gets a little wider every year, rather than just breathing with the seasons, is the one to worry about.
When to call a foundation professional
- Any horizontal crack in a basement or crawl space wall
- Cracks wider than about 1/4 inch, or wider at one end than the other
- Cracks where one side is offset from the other
- Cracks that keep growing or reopen after patching
- Cracks together with sticking doors, sloping floors or a bowing wall
- A sudden depression in the yard near the house, especially in limestone areas
In karst regions such as parts of Montgomery and Chester County, PA, Middle Tennessee and Loudoun County in Northern Virginia, the Pennsylvania DEP advises watching for new depressions, cracks and doors that suddenly stick, because they can be early signs of a sinkhole.
Slab cracks, wall cracks and brick cracks are not the same
Where a crack appears changes what it means. Slab floor cracks under carpet or tile are very common and usually come from shrinkage; they matter more if one side is higher than the other or if the crack lines up with drywall cracks and sticking doors above. Basement wall cracks tell you how the soil is acting on the wall: vertical cracks usually point to shrinkage or settlement, horizontal cracks to pressure. Brick veneer cracks on the outside of a house reflect movement of the foundation below, because the brick sits on the foundation ledge; stair-step cracks near corners are the typical pattern.
What a professional inspection looks like
A foundation contractor or engineer will usually walk the outside of the house to look at grading, gutters and cracks in brick or block, then check doors, windows and drywall inside. Many measure floor elevations with a digital level to map which parts of the house are high or low, and they may look at the crawl space or basement walls with a level or string line. The result should be a written explanation of what is moving, why, and what repair is recommended, not just a price.
How cracks are repaired
Stable, non-structural cracks in poured walls are usually sealed by injecting epoxy, which bonds the crack, or polyurethane foam, which stays flexible and stops water. If the crack comes from movement, the cause has to be fixed first: piers for settlement, anchors, braces or straps for a bowing wall, and better drainage in almost every case. See foundation crack repair for methods and typical prices.