Foundation Cracks: What's Normal and When to Worry
Finding a crack in your foundation is unsettling, but most are harmless. Here's how to tell the normal ones from the ones that need attention.
Short answer: thin vertical cracks (under 1/8") in a poured concrete foundation are very common and usually come from the concrete shrinking as it cured. Be more concerned about horizontal cracks, stair-step cracks in block walls, cracks wider than 1/4", cracks where one side is higher than the other, and any crack that keeps growing.
Why foundations crack
- Shrinkage: concrete loses water and shrinks as it cures, which often leaves thin vertical cracks within the first year or two.
- Settlement: soil under a house compresses over time. Uneven settlement can cause diagonal and stair-step cracks.
- Soil pressure: wet, expanding or frozen soil pushes on basement walls from outside, which can cause horizontal cracks and bowing.
- Water and drainage: poor grading, clogged gutters and short downspouts saturate the soil next to the foundation and make every other cause worse.
- Tree roots and drought: clay soils shrink as they dry out, so a foundation can drop where large trees pull moisture from the ground.
Types of foundation cracks
How wide is too wide?
| Width | What it usually means | What to do |
|---|---|---|
| Hairline (under 1/16") | Shrinkage or minor settling | Mark, date and monitor |
| 1/16"–1/8" | Common, usually not structural | Seal it if water gets in, and monitor |
| 1/8"–1/4" | Possible movement | Have it inspected, especially if it's growing |
| Over 1/4" | Likely structural movement | Structural engineer evaluation |
Direction can override width. A horizontal crack in a basement wall deserves an engineer's opinion even when it's thin, and published guidance commonly uses 1/8" as the point to involve one.
Warning signs that matter more than the crack
- The crack is getting longer or wider over weeks or months
- One side of the crack sticks out further than the other (displacement)
- A basement wall bows inward or leans. Hold a long level or a taut string against it to check.
- Doors and windows stick, or new gaps appear around frames
- Floors slope noticeably, or new cracks appear in drywall upstairs
- Water regularly comes through the crack
How to monitor a foundation crack
- Pencil a short line across each end of the crack and write the date.
- Measure the widest point and write that down too.
- Photograph it with a ruler in the frame, from the same spot each time.
- Check monthly for six months, plus after heavy rain and freeze-thaw periods.
A crack that doesn't change is "dormant" and usually only needs sealing. A crack that grows is "active", and the cause should be found before it's repaired.
Who to call
For a second opinion on a crack you're unsure about, start with an independent structural engineer. Their report tells you the cause and the fix, and they aren't selling the repair. Then get quotes from foundation repair contractors based on that report. See what foundation crack repair costs.
Check yours
Use the Crack Checker for a verdict based on your crack's location, direction, width and warning signs.