Types of Foundation Cracks — and Whether Yours Is Actually a Problem.
A visual guide from a 40-year Ottawa foundation specialist.
Why crack ID matters.
Crack-type misidentification is the #1 reason homeowners overpay for foundation work.
A foundation contractor who doesn't bother to differentiate between a hairline shrinkage crack and a horizontal pressure crack will quote you the same fix for both — and that fix will be the most expensive one. The right approach is to look at the crack, identify what kind it is, understand what caused it, and only then talk about whether it needs repair.
This page is the visual reference. Each crack type below has a description, a typical cause, a severity rating, and a recommendation for what to do.
Hairline cracks
- What it looks like
- Very thin (under 1/16 inch), often single, sometimes branching. May appear shortly after a foundation is poured or up to a few years in.
- What causes it
- Concrete shrinkage during cure. Concrete naturally loses moisture and shrinks slightly as it sets; hairline cracks are often the result.
- Severity
- ★☆☆☆☆Cosmetic
- What to do
- Usually nothing. If it's not leaking and not growing, leave it alone. If it's leaking, a polyurethane injection seals it.
Vertical cracks
- What it looks like
- A roughly straight crack running vertically (or close to it) in a foundation wall. Width usually under 1/4 inch.
- What causes it
- Settlement, often early in a foundation's life. Most vertical cracks have stopped moving and are stable.
- Severity
- ★★☆☆☆Usually fine
- What to do
- Measure the width with a marker (note the date and the width). If it's not growing over 6-12 months, it's stable. If it's leaking, polyurethane injection. Otherwise leave it.
Horizontal cracks
- What it looks like
- A crack running horizontally across a foundation wall, often at or near the mid-height of the wall.
- What causes it
- Lateral pressure on the wall — usually from soil, frost, or water pressure. The wall is being pushed inward.
- Severity
- ★★★★★Worry about this
- What to do
- Get an inspection soon. Don't try to inject this yourself. The fix is usually wall stabilization, not crack repair.
Stair-step cracks (block walls)
- What it looks like
- A crack in a block wall that follows the mortar joints in a stair-step pattern (up-over-up-over).
- What causes it
- Differential settlement or wall movement. The blocks aren't broken; the mortar between them has separated as the wall has shifted.
- Severity
- ★★★★☆Needs attention
- What to do
- Get an inspection. Severity depends on how much movement has occurred and whether it's still active. Fix may be repointing, wall stabilization, or both.
Diagonal cracks
- What it looks like
- A crack running at an angle (usually 30-60 degrees from horizontal) across the wall.
- What causes it
- Typically settlement of one corner or section of the foundation. The crack tells you which direction the foundation is moving.
- Severity
- ★★★☆☆Investigate
- What to do
- Get an inspection. If movement is active, the underlying settlement needs to be addressed (sometimes piering); if it's stable, the crack itself can be sealed.
Settlement cracks
- What it looks like
- Cracks that radiate from corners or window/door openings, often forming a pattern.
- What causes it
- The foundation has settled, usually unevenly. Settlement cracks tell you the soil under your foundation has moved.
- Severity
- ★★★☆☆Investigate
- What to do
- Measure and monitor. If movement has stopped (most settlement is finished within 5-10 years of construction), the cracks can be sealed. If it's ongoing, the underlying soil issue needs to be addressed.
How to measure if a crack is growing.
The simplest way to know if a crack is active or stable: measure it.
- Mark the ends of the crack with a permanent marker. Note today's date.
- Measure the width at the widest point with a ruler or feeler gauge. Note the measurement.
- Take a photo with the date and measurement visible.
- Repeat in 6 months and again at 12 months.
If the crack hasn't grown in 12 months, it's stable. If it has, you have movement and you need an inspection.
For active cracks, you can also tape a dated pencil mark across the crack — if the pencil mark separates over time, the crack is moving.
Send us a photo of your crack — free remote assessment.
If you have a crack in front of you right now and you're not sure what it is, take a photo (ideally with a coin or ruler in the frame for scale) and send it to us. We'll respond within 24 hours on weekdays with a plain-language read on what we're seeing and whether it needs in-person attention.