Guide
What Causes Foundation Cracks
Why did my foundation crack in the first place?
Five mechanisms account for nearly all foundation cracking: concrete shrinking as it cures, soil pushing sideways against the wall, uneven settlement removing support beneath it, frost and freeze-thaw action, and deterioration of the foundation material itself. Each leaves a recognisable signature, which is why the pattern of a crack tells you more than its size.
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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.
Understanding the mechanism matters practically, not just academically. A repair that addresses the crack without addressing the mechanism is how people end up paying twice.
1. Shrinkage as concrete cures
Concrete contains more water than the cement needs to hydrate, and the excess leaves over time. The result is a small loss of volume, mostly in the first year but continuing at a decreasing rate for years afterward.
Because the wall is restrained at the footing and at the corners, it cannot shrink freely, so it relieves the tension by cracking. The signature is a vertical crack, often starting at a window corner or a service penetration where stress concentrates, roughly consistent in width along its length, and stable once curing is largely complete.
This mechanism produces the majority of the cracks in Calgary basements and indicates nothing about the strength of the foundation.
2. Lateral soil pressure
Soil against a basement wall pushes sideways, and that push increases substantially when the soil is saturated, because water adds hydrostatic pressure to the soil's own weight. Expansive clay adds a third component by increasing in volume as it wets, generating pressure rather than merely transmitting it.
The signature is a horizontal crack, typically around mid-height where the bending moment peaks, often accompanied by visible inward bowing. In block walls it appears as a stepped crack running roughly along a mortar course.
Surcharge loads intensify it: driveways, parked vehicles, patios, fill and retaining walls all add load to the soil bearing on the wall.
3. Differential settlement
If the ground beneath one part of a footing loses bearing capacity while the rest holds, that part moves down and the structure above is effectively bent. Causes include clay shrinking as it dries, fill consolidating, water softening the soil, and mature trees drawing moisture out of clay through a dry summer.
The signature is diagonal cracking that tapers, usually widest at the end nearer the settled area, often accompanied by sticking doors, sloping floors and cracks at window and door corners upstairs. The symptoms frequently appear inside the house before anyone notices anything in the basement.
4. Frost and freeze-thaw action
Two related effects. Frost heave occurs when soil freezes and expands, and it can lift or push a foundation where a footing sits above the frost line, though modern Calgary footings are placed well below it precisely to avoid this.
Freeze-thaw cycling is the more common issue here. Water sitting in an existing crack freezes, expands by roughly nine percent, and levers the two faces apart a little further. Repeated across a winter, this widens cracks that other mechanisms created.
5. Deterioration of the material
Older foundations can lose material integrity independently of any movement. Concrete carbonates and loses alkalinity over decades, reinforcement corrodes and expands, mortar in block and rubble foundations erodes, and repeated freeze-thaw cycling degrades the concrete surface itself.
The signature is crumbling, flaking or spalling concrete, exposed or rusting reinforcement, and mortar that can be raked out by hand, rather than a discrete crack. This matters in Calgary's older inner-city housing stock, where rubble and unreinforced foundations are common.
Reading the signature
| What you see | Likely mechanism | Is it ongoing? |
|---|---|---|
| Vertical, consistent width, at a window corner | Shrinkage | No; stabilises after curing |
| Horizontal at mid-height, wall bowed | Lateral soil pressure | Yes; pressure remains |
| Diagonal, tapered, with sticking doors | Differential settlement | Depends on the cause |
| Existing crack widening each winter | Freeze-thaw | Yes; recurs seasonally |
| Crumbling or flaking rather than a clean crack | Material deterioration | Yes; progressive |
Sources
- Freeze-thaw and the long-term climate impacts on Calgary infrastructure — LiveWire Calgary, reporting City of Calgary Climate Projections (January 2022) (Calgary, Alberta), published 2023-02-24, retrieved 2026-08-16.
Questions people actually ask
What causes basement foundation cracks?
Five mechanisms cover nearly all of them: concrete shrinking as it cures, soil pushing sideways against the wall, differential settlement removing support beneath the footing, freeze-thaw action widening existing cracks, and deterioration of the foundation material itself. Shrinkage is the most common by a wide margin and the least concerning.
Are foundation cracks caused by poor construction?
Occasionally, but far less often than people assume. Shrinkage cracking happens in correctly built foundations because it is a property of concrete rather than a defect. Genuine construction issues do exist, such as inadequate reinforcement, poor compaction of backfill or drainage that was never installed properly, and those tend to show as premature or unusually severe versions of the normal patterns.
Does freeze-thaw cause foundation cracks in Calgary?
It more often widens existing cracks than creates new ones. Calgary sees roughly 16 freeze-thaw cycles across December, January and February, driven largely by chinooks, and each cycle moves water in and around the foundation. Water in an existing crack freezes, expands and levers it wider, which is why a crack that leaked slightly one spring can leak more the next.
Can tree roots crack a foundation?
Roots rarely crack a foundation by pushing on it directly. The more common mechanism on clay soils is indirect: a mature tree draws a large volume of water from the ground over a dry summer, the clay shrinks, and bearing support under the footing is reduced. That produces settlement cracking rather than root-pressure cracking.
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.