The direction of the crack is the useful information
Concrete cracks at right angles to whatever is pulling it apart. A wall shrinking as it cures is pulling along its own length, so the cracks it makes stand up vertically, often near an opening or where the wall changes thickness. Those are common, and most of them are a shrinkage crack doing nothing more interesting than that.
A crack that runs horizontally, along the wall rather than up it, was made by something bending the wall inward. The only thing pushing a basement wall from that direction is the ground outside, plus whatever water is sitting in that ground. So a horizontal crack is not the wall shrinking. It is the wall losing an argument with the dirt.
Where it sits tells you a little more. On a poured wall the bending concentrates somewhere around mid height, so that is where the crack usually turns up. On a block wall it follows a mortar joint, because that is the weak line available to it. The mechanism behind both is the same.
Ten minutes of looking before you call anybody
None of this is a diagnosis. It is the information anyone you call is going to ask you for, so it is worth having before you dial.
- Hold a string line or a long straightedge tight to the wall from the slab up to the joists, and see whether it touches the whole way or whether the middle stands proud of it
- Run your hand across the crack. A step you can feel, where the upper side sits inboard of the lower, is displacement rather than a hairline
- Follow it end to end. Does it run most of the length of the wall, or does it die out after a few feet
- Look for water, a damp line tracking the crack, or the white powdery bloom that is efflorescence coming through it
- Go outside and look at the same wall from that side. Downspouts emptying at the corner, ground sloping back toward the house, new fill that raised the grade, heavy equipment parked in close
- Photograph it with a tape or a coin in frame for scale, then photograph it again in a month from the same spot
What pushes a wall in, and what makes it worse
Earth pressure against a basement wall is normal and every wall is built to take some of it. It stops being normal when the ground behind gets loaded, or gets wet, in ways nobody planned the wall around.
Backfill is the most common cause and it almost always happens with good intentions. A free standing wall picks up much of its strength from the floor system framed on top of it and the slab poured below it. Push a pile of wet dirt against one side before either of those exists and you have loaded the wall exactly where it is weakest. The same goes for an excavator or a loaded truck working in close to a green wall.
Water is the slower version of it. Clay holds water and swells when it takes it up, and red clay over rock is the common ground here. A drain at the footer that is clogged or was never installed, a downspout emptying at the corner of the house, or grade that has settled over the years and now runs toward the wall all mean the soil behind it stays saturated. Freeze and thaw at this elevation adds to that, because water in the soil expands when it freezes.
None of that means the wall was built wrong. It usually means the conditions around it changed, and the crack is the wall telling you so.
The people who fix a bowing wall, and what we do instead
Here is the honest part. If a wall has actually moved, the fix gets designed rather than guessed at. A structural engineer looks at the wall, the soil and the loads and specifies what it takes. A foundation repair contractor installs whatever that turns out to be, and relieving the pressure by fixing the drainage and the grade outside is usually part of it. We are not that outfit and we are not going to pretend to be.
Patching the crack is not the fix either. Filling it changes what you can see and nothing about what is pushing, and it takes away the one thing you were using to tell whether the wall is still moving.
What we do is pour walls and footers. If your wall has reached the point where it comes out and a new one goes in, that is our work: forming, steel placed to the engineer's detail, and the pour, with our own line pump because a basement wall is almost never somewhere a chute reaches. Describe what you are looking at on the phone and we will tell you plainly whether it is work we take on, or whether you want an engineer to look before anybody prices anything.