Guide
Are Foundation Cracks Serious?
How do I know whether my foundation crack is a real problem?
Four observations settle it in most cases. Which direction does the crack run, are the two faces still flush with each other, how wide is it, and is it changing. Horizontal direction and displacement are the two that carry real weight; width and growth refine the urgency. A narrow vertical crack that is flush, stable and dry is one of the most common features of a sound Calgary basement.
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Published by Calgary Foundation Pros. Last updated August 16, 2026. This is general guidance for Calgary homeowners, not an engineering assessment of any specific property, and it is not a substitute for a professional engineer where one is warranted.
Most foundation crack anxiety comes from not knowing which questions to ask. The good news is that the questions are few, they need no equipment, and you can answer them yourself in about ten minutes.
Run these four tests in order. The first two do most of the work.
Test 1: which way does it run?
Direction is the single most informative thing about a foundation crack, and it costs nothing to observe. Vertical and near-vertical cracks are usually shrinkage from the concrete curing, and they are extremely common. Horizontal cracks are a bending signature: they indicate the wall is resisting sideways pressure from the soil and has cracked where the bending is greatest.
Diagonal cracks sit between the two. A crack that leans and is noticeably wider at one end often points to differential settlement, where one part of the foundation has moved down relative to another.
| Direction | Usual meaning | Typical response |
|---|---|---|
| Vertical | Concrete shrinkage during curing | Monitor; seal only if it leaks |
| Diagonal | Differential settlement | Survey floor levels; check for other symptoms |
| Horizontal | Lateral soil pressure on the wall | Assess promptly, regardless of width |
| Stair-step in block | Movement following mortar joints | Assess; check for displacement |
Test 2: are the two faces still flush?
Run a finger across the crack. If one side sits proud of the other, the wall has moved out of plane rather than simply opening, and that is the clearest single indicator of a structural problem. A crack can be wide and still be flush, which is far less concerning than a narrow crack with displacement.
This test is worth doing at several points along the crack, because displacement is often greater at one end.
Test 3: how wide is it?
Roughly 6mm, about the width of a pencil, is the figure commonly used as a prompt to get a professional opinion. Treat it as a prompt rather than a safety threshold. Width is a weaker signal than direction or displacement, and a narrow crack that is growing matters more than a wide one that has been static for twenty years.
Measure at the widest point and write the number down with the date. That written number is what makes the fourth test possible.
Test 4: is it changing?
This is the test people skip, and it is the one that most often changes the answer. A crack that has not moved in five years is behaving completely differently from one that has widened since spring, even if they look identical today.
Mark both ends of the crack with a pencil line and write the date on the wall. Photograph it with a coin or ruler in frame. Then check again in six months. If nothing has changed, you have learned something genuinely valuable for the cost of a pencil mark.
- Draw a pencil line across the crack at two or three points
- Write today's date beside each line
- Photograph each mark with a coin for scale
- Measure the widest point and record it
- Repeat in six months, and again after a spring melt
When the four tests say get someone in
- Any horizontal crack, at any width
- Any crack where the two faces are no longer flush
- Any crack wider than roughly 6mm
- Any crack that has measurably grown
- Any crack accompanied by sticking doors, sloping floors or cracking upstairs
- Any crack that appeared suddenly after flooding, a plumbing leak or nearby excavation
What genuinely does not need action
It is worth stating the negative case plainly, because a lot of foundation content avoids it. A narrow vertical crack that is flush, stable and dry does not need repairing. Fine surface crazing does not need repairing. A hairline crack at a basement window corner in a house that is otherwise behaving normally does not need repairing.
None of these become more serious by being left, and sealing them removes your ability to notice if something does change.
Questions people actually ask
When is a foundation crack serious?
When it runs horizontally, when the two faces have moved out of line with each other, when it is wider than roughly 6mm, or when it is measurably growing. Any one of those justifies a professional assessment. Cracks failing all four tests are usually shrinkage and are very common in sound foundations.
How wide does a foundation crack have to be before it matters?
About 6mm is the commonly used prompt for getting an opinion, but width is a weaker indicator than direction and displacement. A 3mm horizontal crack with displacement is a more significant finding than a static 8mm vertical crack. Use width to refine urgency, not to decide whether there is a problem.
How long should I monitor a crack before deciding?
Six months covers a meaningful part of the seasonal cycle, and a full year including one spring melt is better, because melt is when Calgary foundations see their heaviest water loading. If a crack shows any of the serious markers, do not monitor: get it assessed now and let the monitoring inform the repair rather than delay it.
Can I tell anything from a photo alone?
Photographs establish direction and rough width, which is genuinely useful, and dated photos establish change, which is more useful still. What a photo cannot show is displacement, since that needs a finger run across the crack, or whether the wall is out of plumb, which needs a string line. Photos are a good starting point rather than a substitute for the physical checks.
Want someone to look at your specific case?
General guidance only goes so far. Send a description and a postal code and it goes to an independent Calgary foundation contractor.
We are a referral service, not a contractor. Your request goes to an independent Calgary foundation company.