Foundation wall cracks are common, and most of them are harmless shrinkage. A few are not. The difference is in the shape, the width, and whether it grows — and knowing which kind you have decides whether you fix it with a $30 kit or call an engineer. A basement floor crack is a different job — the basement floor crack guide covers that one.
A hairline vertical crack is usually shrinkage: the concrete dried and shrank, and the crack is thin and straight. A horizontal crack is pressure from the soil outside pushing the wall in — that one is serious. A stair-step crack follows the mortar joints and usually means settling. And any crack wider than 1/4 inch, or one that keeps growing, is a crack for an engineer, not a kit.
Measure and photograph. Measure the width and the length of the crack and photograph it with a coin for scale. Write the date on the photo. You need the baseline to know whether the crack grows.
Clean the crack. Brush the loose material out of the crack with a wire brush and let it dry. Epoxy and foam do not stick to dust or wet concrete.
Seal a hairline crack. For a hairline vertical crack, use an epoxy injection kit or polyurethane foam, following the kit's instructions. Inject from the bottom up, let it cure, and wipe the excess. A hairline crack sealed this way stays sealed.
Monitor it. Mark the ends of the crack with a pencil and check it monthly for six months. If the crack grows past the marks, or a new crack appears, stop and call an engineer — a growing crack is a moving foundation.
Fix the water. Most wall cracks come from water pressure: gutters that dump next to the wall, soil that slopes toward the house, or a downspout that ends at the foundation. Extend the downspouts, grade the soil away from the wall, and the pressure drops.
Call an engineer. A horizontal crack, a crack wider than 1/4 inch, a crack that grows, or a wall that bows — all four need a structural engineer. Do not seal a serious crack and hope: sealing hides the symptom and the wall keeps moving. The foundation crack repair cost guide covers what the repair costs and when it is not just a crack.
Soil pressure causes most wall cracks: wet soil expands and pushes the wall in, dry soil shrinks and lets the wall settle. Water is the driver — a gutter that overflows, a downspout that ends at the wall, or a grade that slopes toward the house all push water against the foundation. Freeze-thaw cycles widen small cracks year by year. And settling causes the stair-step cracks: the house sinks unevenly as the soil under it compacts. Sloping floors are the other sign of the same problem — the sloping floors guide covers when a slope is fine and when to worry.
Hairline vertical cracks are normal — most concrete shrinks as it cures. Horizontal cracks, stair-step cracks, and cracks wider than 1/4 inch are not normal and need an engineer.
A hairline vertical crack, yes — with an epoxy injection kit. A horizontal crack, a wide crack, or a growing crack, no — those need a structural engineer.
Measure it: wider than 1/4 inch is serious. Watch it: a crack that grows is serious. Look at the shape: horizontal is serious. When in doubt, an engineer costs a few hundred dollars and a wrong guess costs far more.
Maybe. A shrinkage crack usually stays put. A crack caused by water pressure or settling grows as long as the cause stays. Fix the water, monitor the crack, and call an engineer if it moves.