When to worry about a foundation crack
Almost all concrete cracks. The useful questions are which way it runs, whether it is moving, and whether water comes through it.
Finding a crack in a foundation wall is alarming in a way that is out of proportion to how common it is. Concrete shrinks as it cures and shrinkage cracking is close to universal. The job is not to be reassured or frightened but to work out which kind you are looking at.
Direction is the most informative thing
Vertical, narrow, in a poured wall. The typical shrinkage pattern, frequently appearing within the first year or two of a pour. Generally the least concerning family — though it can still be a route for water.
Diagonal, often from the corner of an opening. Windows and doors are stress concentrators. A diagonal crack running away from a corner commonly reflects differential settlement: part of the foundation has moved relative to another part.
Stair-step, following mortar joints in a block wall. The block-wall equivalent of a diagonal crack, and read similarly.
Horizontal. This is the one that changes the conversation. A horizontal crack running along a block wall, often around mid-height, is the signature of lateral pressure pushing the wall inward. It belongs with bowing walls rather than with cosmetic cracking, and it is a reason to get an engineer rather than a contractor.
Width matters less than people assume
Width is easy to measure and therefore gets over-weighted. A wide, stable, long-standing shrinkage crack can be less significant than a narrow one that has opened in the last six months. Width is one input. Movement is the important one.
Measuring movement, which almost nobody does
This is the single most useful thing a homeowner can contribute, it costs nothing, and it converts speculation into evidence.
Mark the crack at a couple of points. Draw a short line across it, and write the date. Measure the width at those marked points and write that down too. Then photograph it, with something for scale, and do it again in three months, and again after a wet season.
If, six months later, the marks still line up and the width is unchanged, you have real evidence of a stable crack. If it has opened, or the marks have offset, you have something concrete to hand to a professional — which is far better than an opinion formed on a single visit.
Purpose-made crack monitors do the same job with a printed scale. The principle is identical: record and date it.
Water is a separate axis
A crack can be structurally uninteresting and still be the reason your basement floods. Sealing or injecting a crack addresses the leak; it does not address structure, and treating a leak repair as a structural repair is a misunderstanding worth avoiding.
What sends you to an engineer, not a contractor
Horizontal cracking. A wall that looks bowed when you sight along it. Cracks that are visibly widening, or where one side has moved relative to the other. Doors and windows upstairs that have started binding. Sloping floors that were not sloping before. Any of these, and the right next call is a licensed structural engineer — someone paid to assess, not to sell a repair.
That independence matters more here than anywhere else on this site. An engineer's report gives you a specification, and a specification is what lets you compare quotes for the same defined work instead of comparing sales pitches.
We do not assess foundations here and we never will. What this page is for is making sure that when you describe yours, you are describing the things that actually carry information.
Frequently asked questions
Are vertical cracks always harmless?
Not always, but narrow vertical cracks in poured walls are the most common and least alarming pattern. Direction is a strong clue rather than a verdict, which is why movement over time matters so much.
How wide does a crack have to be before it's serious?
There is no single threshold that is meaningful on its own, and width gets over-weighted because it is easy to measure. A stable wide crack can matter less than a narrow one that is actively opening.
What does a horizontal crack mean?
Usually lateral pressure from soil pushing the wall inward. It belongs in a different category from shrinkage cracking, and it is a reason to get an independent structural engineer involved rather than to seal it.
Should I call the waterproofing company that offers free inspections?
For a leak, that is a reasonable conversation. For anything structural, an assessment from someone who is not selling the repair is worth paying for, because it produces a specification you can then get quotes against.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.