Foundation cracks matter by direction, width and change over time. A thin vertical crack in poured concrete is usually harmless shrinkage. A horizontal crack, a stair-step crack in a block wall, or any crack wider than about 6 mm that keeps growing points to soil pressure or settlement. Those need a structural engineer. Measure each crack, date it, and check it again in a few months. A crack that does not change is a different problem from one that does.
The quick answer: which cracks matter
Use this table as a first sort. It is a rule of thumb, not a diagnosis. The width figures vary between sources and are not a building code limit.
| Crack type | Where you see it | Usual cause | Level of concern | Next step |
|---|---|---|---|---|
| Hairline vertical | Poured concrete wall | Shrinkage while the concrete cured | Low | Monitor, seal if it leaks |
| Vertical, over 6 mm or growing | Poured or block wall | Settlement or movement | Medium to high | Measure, then get an assessment |
| Diagonal | Near corners, windows, beams | Differential settlement | Medium to high | Engineer review |
| Stair-step | Mortar joints in block wall | Uneven settlement or lateral pressure | High | Engineer review |
| Horizontal | Mid-height on the wall | Soil or water pressure pushing in | High | Engineer, soon |
| Horizontal with bowing | Wall leans into the room | Wall is failing under load | Urgent | Engineer now |
Concrete cracks. That is normal. The question is why it cracked and whether it is still moving.
Vertical cracks: usually shrinkage
Concrete shrinks as it dries. A poured foundation wall cannot shrink freely because the footing holds the base in place. So it relieves the stress by cracking, often in a straight vertical line, and often near the middle of a long wall or near a window corner.
These cracks are common in Ontario homes. They are often thin and the two faces of the crack sit flush. In that case the crack is usually not a sign that the house is failing.
When a vertical crack needs a second look
Check for these signs:
- The crack is wider at the top than at the bottom, or the reverse. A wedge shape suggests one side is moving.
- One side of the crack sits proud of the other.
- The width is more than about 6 mm (1/4 inch). US guidance such as HouseLogic uses this kind of figure as the point to get a professional evaluation. It is a US source, so treat it as a guide only.
- The crack grows over weeks or months.
- The crack leaks after rain or during spring thaw.
A leaking vertical crack is common and usually fixable from the inside. The water is the main problem. A repair company can inject the crack with polyurethane or epoxy, depending on the cause. Ask which material they plan to use and why. Epoxy is typically used to bond a crack that is stable. Polyurethane foam is typically used to stop water and allows a little movement. Ask the company to explain the choice for your wall. Water that enters through a crack can also raise indoor moisture. Health Canada explains how to control moisture and mould in your home.
Horizontal cracks: take these seriously
A horizontal crack usually runs along the wall at about the height of the soil outside. It often shows up around mid-height. The cause is lateral pressure. Wet soil is heavy. Clay soil swells when wet and pushes on the wall. In winter, frozen soil and frost can add to the load. A basement wall is designed to resist some of this, but a wall that has cracked horizontally has already been overloaded.
Block walls often crack horizontally along a mortar bed joint. Poured walls can crack too, but it is less common.
Signs the wall is moving
- The wall bows or leans inward.
- The crack is wider in the middle than at the ends.
- The upper part of the wall has moved in relative to the lower part.
- Floor joists or the sill plate show signs of shifting.
- The crack gets wider after wet weather.
Horizontal cracks with bowing are the case where waiting costs the most. Contact a structural engineer. Some US sources, such as US Waterproofing, say a bow of about 1 inch needs prompt stabilization. That figure comes from a contractor in the US. An Ontario engineer will set their own limits for your wall.
Stair-step and diagonal cracks
A stair-step crack follows the mortar joints in a concrete block wall. It looks like a staircase. A diagonal crack runs at an angle across a poured wall, often from a corner or a window.
Both usually point to differential movement. One part of the foundation has settled more than the rest. Causes include:
- Soil that was not compacted well under part of the footing.
- Clay soil that shrinks in dry summers and swells in wet seasons.
- A downspout or a grading problem that sends water to one corner.
- A tree close to the house that dries the soil.
- A leaking pipe or a broken weeping tile.
Soil matters here. Much of Ottawa sits on marine clay left by the Champlain Sea, and clay can move with moisture. Toronto has a mix of clay till and other soils. The Windsor area has a lot of clay-rich soil, and a flat landscape that drains slowly. These facts vary lot by lot. Do not assume your soil from the town name. A soils report or an engineer’s site visit gives you the facts for your property.
Width thresholds and how to measure
Width is useful, but direction and movement matter more. A 2 mm horizontal crack can be worse than a 5 mm vertical shrinkage crack.
| Width | Common description | What to do |
|---|---|---|
| Under about 1 mm | Hairline | Monitor, usually cosmetic |
| About 1 to 3 mm | Thin | Monitor, seal if it leaks |
| About 3 to 6 mm | Moderate | Measure often, consider an assessment |
| Over about 6 mm | Wide | Get an assessment |
| Any width, horizontal | Direction is the issue | Engineer |
These bands are rules of thumb that appear in many guides. They are not an Ontario Building Code limit, and I could not find an official Ontario figure for them. Different engineers use different cut-offs.
A simple monitoring routine
- Mark both ends of the crack with a pencil and write the date.
- Measure the width with a crack gauge or a ruler with millimetre marks.
- Take a photo with a ruler beside the crack.
- Measure again each month for at least three to six months. Include a spring thaw and a wet week if you can.
- Keep the record. An engineer or a buyer will want it.
If the crack grows, or if doors and windows start to stick, or floors slope, call for help.
When to call an engineer, and when to call a repair company
Call a structural engineer first when you see any of these:
- Horizontal cracks.
- Stair-step cracks in block walls.
- A wall that bows or leans.
- Cracks that grow.
- Upstairs signs, such as cracked drywall, sticking doors or sloping floors, along with basement cracks.
- A planned purchase of a home with these signs.
In Ontario, look for a professional engineer licensed by Professional Engineers Ontario. An engineer is paid for the diagnosis. They do not sell the repair. That gives you a neutral scope of work to take to companies. HouseLogic makes the same point about getting an independent opinion before you hire a contractor.
Call a repair company directly for a thin, stable vertical crack that leaks. Many foundation repair and waterproofing companies handle crack injection. For a wall that moves, ask for a quote that follows the engineer’s scope.
Costs vary a lot with crack length, access, wall type and the method used. I did not find a reliable Ontario price source, so this guide gives no price. Ask for at least three written quotes and ask what each one includes and what the warranty covers.
What our boards show for Toronto, Ottawa and Windsor
This directory ranks companies by Google rating and review count. It does not do repair work. The numbers below come from our boards for October 2026. They describe the companies. They are not endorsements.
| Board | Companies ranked | Median rating | Median reviews | Total reviews | Under 20 reviews |
|---|---|---|---|---|---|
| Foundation repair companies in Toronto | 20 | 5.0 | 58.5 | 3,420 | 3 |
| Basement waterproofing companies in Ottawa | 20 | 5.0 | 61.0 | 1,482 | 5 |
| Foundation repair companies in Windsor | 20 | 4.9 | 66.5 | 2,816 | 2 |
For Ottawa, we use the basement waterproofing board in the table, because it lists the companies that do foundation crack work there.
What this means for your choice:
- Medians near 5.0 or 4.9 mean ratings alone do not separate companies. Almost everyone is near the top.
- Review count separates them better. On the Toronto board, the top company by reviews has 874 reviews. The median company has 58.5. A rating from 10 reviews and a rating from 800 reviews carry different weight.
- Some companies have few reviews. The Toronto foundation repair board has 3 companies under 20 reviews. The Ottawa basement waterproofing board has 5. The Windsor board has 2.
- Rating and volume do not always move together. On the Ottawa board, The Foundation Guys Inc. shows 4.2 from 223 reviews. Braden Foundation Construction shows 5.0 from 106 reviews. Read the reviews themselves to see what the lower scores say.
- A Google rating says nothing about whether a company can diagnose a structural crack. Ask whether the company works to an engineer’s drawings, and ask to see the engineer’s report requirement in the quote.
Windsor has other boards in this vertical too. See basement waterproofing companies in Windsor for a wider list, since many companies there handle both jobs. To compare other towns, use the foundation repair hub.
What to do this week
- Photograph and measure every crack you can see, inside and outside.
- Note direction: vertical, horizontal, diagonal or stair-step.
- Check the outside. Look at grading, downspouts and gutters near the cracked wall. Water that runs toward the house adds load.
- Look upstairs for sticking doors and sloping floors.
- If you see a horizontal crack, stair-step crack or bowing, book a structural engineer.
- If you see only thin stable vertical cracks, monitor them and seal any that leak.
Frequently asked questions
Are vertical foundation cracks serious?
Most thin vertical cracks in a poured concrete wall come from shrinkage as the concrete cured. They are usually not structural. Have them checked if they are wider than about 6 mm, if they widen over time, or if they leak.
Is a horizontal crack in a basement wall an emergency?
A horizontal crack is the type to take most seriously. It often means soil or water pressure is pushing the wall inward. Do not wait. Call a structural engineer, and watch for bowing or a wall that moves further in.
When should I call a structural engineer instead of a repair company?
Call an engineer for horizontal cracks, stair-step cracks in block walls, bowing, or cracks that keep growing. An engineer diagnoses the cause and does not sell the repair. You can then ask repair companies to quote the engineer’s scope.
Does a crack that leaks mean the foundation is failing?
Not always. A thin vertical crack can leak without any structural movement. A leak is a water problem first. The crack still needs a proper repair, and the cause of the water needs a look too.