Foundation cracks: types and when to worry
Short answer
Not every foundation crack means your house is falling apart. A hairline vertical crack in a poured concrete wall is often shrinkage from when the concrete cured. What matters is the crack’s direction, its shape, whether one side has shifted past the other and, most of all, whether it’s moving. If you have a horizontal crack, a crack that’s growing or one with displacement, have a structural engineer take a look.
On this page
How to read a crack
Our structural contributor, Daniel Mercer, looks at five things, in this order:
- Direction: vertical, horizontal, diagonal or stair-step.
- Shape: the same width all along, or wider at one end, like a wedge.
- Displacement: not just how wide it is, but whether one face sits farther out than the other.
- Location: at a corner, near a window, in the middle of the wall or at a point where a beam bears on the wall.
- Change over time.
That last point often decides it. A 1/8-inch crack that has been there for 20 years may concern Daniel less than a 1/16-inch crack that appeared three months ago and is growing.
- Hairline vertical crack, even width
- Vertical crack wider at one end (wedge-shaped)
- Diagonal crack from a window corner
- Horizontal crack
- Stair-step crack along the mortar joints
- Horizontal crack along a mortar joint, near mid-height
Common foundation crack types
| What you see | What it can mean | What to do |
|---|---|---|
| Hairline vertical crack in poured concrete, even width, no displacement | Often shrinkage from when the concrete cured | Photograph and date it, then watch for changes and water |
| Crack around 1/8 inch wide | Could be shrinkage or movement | Find out more: monitor it and note its shape and location |
| Crack around 1/4 inch, especially if new, growing, wedge-shaped or displaced | Movement | Have a structural engineer evaluate it |
| Horizontal crack, especially with the wall bowing in | Soil pressing on the wall | Have it evaluated; see bowing walls |
| Stair-step crack in a block wall | Settlement, or pressure on the wall | Check where it starts, whether it widens toward one end and whether the wall bows |
| Any crack letting water in | A path for water; it may or may not be structural | Ask why it opened and why it leaks (see below) |
Widths are Daniel Mercer’s working criteria, not a code or a standard.
Width isn’t the whole story
Daniel doesn’t have a magic number that tells him whether a crack is OK. In his experience, a fairly uniform vertical crack in poured concrete is often shrinkage, especially if it looks old and has no displacement. Around 1/8 inch, he wants to know more about it. Around 1/4 inch, he wants it looked at, especially if it’s new, growing, wedge-shaped or displaced. A smaller horizontal crack can concern him more. And a 1/16-inch stair-step crack that’s growing can matter more than a 1/4-inch vertical crack that isn’t moving. It depends on the pattern and on whether it’s moving.
Stair-step cracks in block walls
A stair-step crack in a block wall doesn’t automatically mean your foundation is failing. Daniel looks at:
- Where it starts
- Whether it ends at a window or door
- Whether it’s at a corner
- Whether it opens wider toward one end
- Whether one side has dropped relative to the other
- Whether the wall is bowing
- What’s happening upstairs in the house
A diagonal crack that starts at a corner can go along with settlement. If you have a horizontal crack and the wall is bowing in, soil pressure on the wall is the more likely explanation. See horizontal cracks and bowing walls.
How to monitor a crack
- Take photos from the same spot every time.
- Put a ruler or scale in the photo.
- Write the date.
- Make small pencil marks at each end of the crack.
- For a crack worth watching closely, use a crack monitor, a small gauge fixed across the crack that shows movement.
If there are no other warning signs, Daniel likes to see a few months of readings, preferably through a wet season and a dry one. For a stable crack, monitoring it for 6 to 12 months can be useful. That’s his own judgment, not a code requirement. If you see movement, bowing or displacement, don’t wait a year.
Cracks that let water in
Why did the crack open, and why is it leaking? Daniel treats those as two different questions. A structural crack can leak. A shrinkage crack that isn’t moving can leak too.
To tell a leak from condensation, I dry the area if I can and then see whether the water lines up with rain. If it comes back after rain and starts right at the crack, that counts for a lot. If a whole cold surface is wet on a humid day, it’s more likely condensation. I also check for efflorescence, staining and mineral tracks, the joint where the floor meets the wall, the downspouts and the grading. The taped plastic sheet test can help, but I wouldn’t base a repair on it alone. See dew point and condensation.
Polyurethane injection is used mainly to stop water from coming through a crack. It expands and can handle some movement. Epoxy injection can structurally bond a concrete crack back together when the situation calls for it, but it won’t stop a foundation that’s still moving. Neither one corrects poor grading, water pressure in the soil or settlement. And in a hollow concrete block wall, Daniel wouldn’t expect an injection to work the same way it does in poured concrete. For water coming into the basement, see basement water.
Mistakes to avoid
- Covering the crack before photographing it
- Painting over it
- Measuring it only once
- Assuming a horizontal crack means the house is collapsing
- Assuming a hairline crack is harmless
- Asking only a company that sells piers, anchors or waterproofing
- Repairing the crack without asking why it appeared
When to call a structural engineer
- A horizontal crack, or a wall bowing inward
- A crack that’s new and growing, wedge-shaped or displaced
- A stair-step crack that keeps widening
- Settlement or other foundation movement, or any time it isn’t clear why the house is moving
- Posts or piers that are moving
- Before anyone digs near or under the foundation
A home inspector can identify a crack. A structural engineer can determine why the foundation is moving, whether it needs a repair and how that repair should be designed. A repair company installs it. If you can, get an engineer’s opinion before you get quotes, especially from companies that offer only one type of repair.
Crack widths, monitoring periods and repair advice come from Daniel Mercer’s residential work in Virginia. They are his working criteria, not a code or a standard.
Structural information reviewed for technical accuracy by Daniel Mercer. This content is educational and is not a substitute for an on-site evaluation by a licensed professional engineer.