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Foundation cracks: which ones matter and which ones don't

Most concrete cracks. The question worth answering is not whether there's a crack, but what shape it is, whether it's moving, and whether it's letting water in.

A concrete foundation wall with four numbered crack patterns drawn on it: a vertical straight crack, which is the most common and usually just shrinkage; a horizontal crack, caused by soil pushing on the wall from outside and treated as a structural issue rather than a moisture one; a diagonal crack running from a corner, often a sign of uneven settling; and a stair-step crack that follows the mortar joints of a block or brick wall. A side panel explains each pattern, and a second panel explains that whether a crack is urgent depends on whether it is still changing width and whether it is letting water in, not on its length.
Most concrete cracks. What decides whether it matters is the shape, and whether it's still moving, not whether one exists at all.

Direction tells you more than width

Concrete shrinks as it cures, and shrinkage cracks are close to universal in poured foundations. That is why "there's a crack in my foundation" is not, by itself, alarming: and also why the internet is full of people being alarmed about it. Useful triage looks at three things: direction, movement over time, and whether water is coming through.

Direction is the most informative and the most overlooked. A narrow vertical crack in a poured wall is the common shrinkage pattern. A diagonal crack running from a corner of a window or door opening usually reflects differential settlement: one part of the foundation has moved relative to another. A stair-step crack following the mortar joints in a block wall says the same thing in block. And a horizontal crack running along a block wall is the one that belongs in a different category altogether: that is the signature of lateral pressure from soil pushing inward. And it connects directly to the bowing walls section.

Movement is the thing almost nobody measures and everybody speculates about. A crack that has been the same width for fifteen years is a different object from one that opened last spring, even if they look identical today. Marking, dating and monitoring a crack costs nothing and turns an argument into evidence: and it is exactly the evidence an engineer wants.

Water is the third axis, and it is largely independent of the first two. A hairline shrinkage crack that is structurally irrelevant can still be the route a basement floods through, which is why crack injection exists as a repair. It addresses the leak, not the structure.

Where we stop is deliberate and absolute: this site does not assess your foundation. We explain what patterns generally indicate so that you can describe yours accurately. Anything involving movement, horizontal cracking or structural repair is the domain of a licensed structural engineer: an independent one, ideally not the company that would also sell you the repair.

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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.