Last updated: October 17, 2026. Guidance checked against Government of Canada, Health Canada, Tarion and concrete industry sources on that date.
Affiliate disclosure: True North DIY may earn a commission from some links on this page, at no extra cost to you.
Key takeaways
– Most cracks in poured concrete basement walls are not structural. Concrete shrinks as it dries and moves with temperature, so small vertical or slightly diagonal hairline cracks are common.
– Warning signs are horizontal cracks, cracks with one side offset from the other, walls that bow or lean, cracks that keep widening, stair-step cracks in block walls, and cracks combined with sticking doors or sloping floors.
– The Government of Canada recommends a professional for cracks wider than about 5 mm (1/4 in.), cracks that keep growing, large networks of cracks, and cracks where water seeps through.
– Monitor before you panic. Mark the ends of a crack, write the date, measure the width and check again over the seasons.
– Cracks can also let in radon. Test your home: Health Canada’s guideline is 200 Bq/m³.
Finding a crack in your basement wall is unsettling, but cracks in concrete are extremely common and most don’t mean your house is in trouble. The trick is reading them: direction, width, movement and water.
This guide explains what causes foundation cracks, how to tell normal cracks from structural warning signs, how to track a crack over time, and when to call a professional. It’s written for the poured concrete and concrete block foundations common in Ontario and across Canada.
Why concrete cracks at all
Concrete is strong in compression but weak in tension. When forces inside the wall pull harder than the concrete can resist, it cracks. The Concrete Foundations Association lists the usual causes: drying shrinkage as the water in the mix evaporates, thermal expansion and contraction, settlement or expansion of the soil, uneven bearing where part of the footing sits on harder soil, and applied loads such as soil pressure, water pressure or heavy equipment driven too close to the wall (CFA Tech Note: Cracking in Concrete Walls).
Concrete Saskatchewan makes the same point for Canadian basements: newly placed concrete is at the largest volume it will ever have, and it shrinks as it dries. When that shrinkage is restrained, it cracks. Their guide states plainly that some cracking is normal in basement walls, and that cracking is not necessarily a sign of poor materials, poor workmanship or a structural problem (Concrete Saskatchewan: Cracks in Residential Basement Walls).
Canadian winters add another stress. Freeze-thaw cycles, frost in poorly drained soil, and saturated backfill in spring all push on foundation walls. That’s why drainage matters so much: wet soil is heavier and can freeze and expand.
Crack types: what each one usually means
Use this as a starting point, not a diagnosis. One crack type can have several causes, and only an on-site assessment can tell you for sure.
| Crack type | What it usually means | Level of concern |
|---|---|---|
| Thin vertical hairline crack in poured concrete | Drying shrinkage or temperature movement | Usually low. Monitor and seal if it leaks |
| Slightly diagonal crack from the corner of a window or other opening | Stress concentrates at corners of openings | Usually low if thin and stable |
| Vertical crack wider at the top or bottom | Possible settlement of part of the footing | Moderate. Monitor and get advice if it grows |
| Horizontal crack partway up the wall | Soil or water pressure pushing on the wall | Higher. Get a professional assessment |
| Any crack where one side sticks out past the other | Movement or displacement of the wall | Higher. Get a professional assessment |
| Stair-step crack along mortar joints in a block wall | Settlement or lateral pressure on the blocks | Moderate to high, depending on width and bowing |
| Wall bowing or leaning, with or without cracks | Lateral pressure exceeding the wall’s strength | High. Get a professional assessment soon |
| Network of fine cracks across a slab floor | Shrinkage of the floor slab | Usually cosmetic |
Several Canadian and industry sources agree on the warning signs. Horizontal cracks, stair-step cracks in masonry, cracks with wall bowing, cracks that keep widening, cracks with water entry, and cracks combined with signs of settlement are generally more concerning than small isolated hairline cracks. The US National Association of Home Builders summary adds that cracks are more likely to be structural when they come with other signs of distress, such as doors and windows that stop working or floors that slope noticeably, and that horizontal cracks with bowing or leaning walls are cause for concern (NAHB: Misconceptions About the Common Crack).
Poured concrete vs concrete block
Poured concrete walls usually show shrinkage cracks as single vertical or slightly diagonal lines. Concrete block walls tend to crack along the mortar joints, in a stair-step pattern or a horizontal line. A horizontal crack along a block wall, especially with the wall pushing inward, is a classic sign of soil pressure and deserves attention.
Floor slab cracks
Cracks in the basement floor slab are very common. NAHB notes that shrinkage cracks in concrete floor slabs are expected and do not compromise structural integrity. The exceptions: a slab crack that lets water in, one where one side has heaved up or sunk relative to the other, or cracks that keep growing.
How wide is too wide?
There is no single national standard, and different sources use different thresholds. Here is what they say:
- Government of Canada: consult a professional for cracks larger than about 5 mm (1/4 in.), cracks that keep growing, a large network of cracks, or cracks where water is seeping through (Canada.ca: Foundation maintenance).
- Concrete Foundations Association: probably time to seek professional help when a crack exceeds 1/4 in. in width, shows 1/4 in. of lateral displacement, leaks, or when you find long horizontal cracks (CFA).
- Concrete Saskatchewan: a typical builder basement warranty requires repair only when cracks leak, have measurable vertical displacement, or exceed 3 mm in width (Concrete Saskatchewan).
Width alone doesn’t tell the whole story. A 2 mm crack that has doubled in a year is more worrying than a stable 4 mm crack that hasn’t changed in a decade. Movement, direction, displacement and water matter as much as width.
How to monitor a crack
The Government of Canada recommends monitoring all foundation cracks for moisture and growth, even small ones, by marking the ends of the crack with pencil to track changes in size or direction (Canada.ca). A simple routine:
- Mark both ends with a short pencil line across the crack tip, and write the date beside it.
- Measure the width at the widest point with a crack gauge, feeler gauges or a ruler with millimetre markings. Write it on the wall.
- Photograph it with a tape measure in the frame, from the same spot each time.
- Check again every few months, and especially after spring thaw and after a dry summer. Note any new length, width or offset.
- Look for company: new cracks nearby, doors or windows that start sticking, gaps opening at trim, or floors that start to slope.
If a crack is stable over a full year of seasons, doesn’t leak, and has no offset, it is very likely a shrinkage crack. If it grows, call a professional.
Sealing a minor crack
For minor, stable cracks, the Government of Canada describes a straightforward DIY repair: make sure the crack is completely dry, brush out old caulking or paint, vacuum the debris, then use caulking or hydraulic cement for very small hairline cracks and epoxy filler injection for larger cracks under 0.5 cm wide, finished with waterproofing sealant (Canada.ca).
Concrete Saskatchewan notes that epoxy injection, dry-packing, or routing and sealing are all used to repair cracks, and that drainage around the foundation should be assessed and corrected before repairing leaking cracks (Concrete Saskatchewan). If the outside is still sending water at the wall, a sealed crack often just moves the leak. See our guide to stopping a leaking basement in spring for the drainage side.
A note on materials: rigid epoxy bonds the two sides of a crack together, while flexible polyurethane foam fills and seals but allows a little movement. Follow the product instructions, and don’t use a repair to hide a crack that is still moving.
When to call a professional, and which one
Call a professional if you see:
- a horizontal crack in a poured or block wall
- a crack where one side is offset from the other
- a wall that bows, leans or bulges
- stair-step cracking in a block wall, especially with bowing
- a crack that is getting longer or wider
- cracks wider than about 5 mm (1/4 in.), or a network of cracks
- a crack that leaks and comes back after sealing
- cracks together with sticking doors and windows, sloping floors, or gaps at trim and drywall
Who to call: For a structural question, a licensed structural or geotechnical engineer gives an independent assessment, not a sales pitch. In Ontario, engineers are licensed by Professional Engineers Ontario (PEO), and you can confirm a licence on PEO’s public directory. For leak repairs on a stable crack, a foundation repair or waterproofing contractor may be the right call. Be cautious of anyone who diagnoses a serious structural problem and quotes an expensive fix in the same visit; a second opinion is reasonable.
The Government of Canada also notes you should check with your local government about permits and inspections required for foundation repairs (Canada.ca).
New home in Ontario? Check Tarion first
If your home is under Ontario’s new home warranty, contact your builder before paying for repairs. Tarion’s two-year warranty covers water penetration through the basement or foundation walls. Only actual water leakage is covered, not dampness or condensation (Tarion: Construction Performance Guidelines). Tarion’s seven-year major structural defect coverage includes significant damage due to soil movement and major cracks in basement walls (Tarion: Warranty coverage). Keep dated photos and measurements: they are exactly what a warranty claim needs.
Cracks and radon
Radon is a radioactive gas from the soil that can enter homes through cracks, gaps around pipes and other openings in the foundation. Health Canada’s guideline is 200 Bq/m³ for the average annual level in normally occupied areas, and it recommends corrective action above that level (Health Canada: Radon guideline). The only way to know your level is to test.
Sealing cracks helps, but it isn’t the main fix. Health Canada says active soil depressurization is the most effective method and can reduce radon levels by 90% or more, and it recommends certified professionals trained through the Canadian National Radon Proficiency Program (C-NRPP) (Health Canada: Reduce radon in your home).
Safety note
- Don’t remove soil from against a bowing or leaning wall, and don’t drive heavy equipment close to the foundation. Both can make lateral pressure worse.
- If a wall is visibly bulging inward or cracks open quickly, stay out of the area and call an engineer right away.
- Wear eye protection and a dust mask when grinding or chiselling concrete, and follow epoxy and foam product safety instructions.
- Before digging outside, request a free locate through Ontario One Call.
- This guide can’t diagnose your foundation. Local soil, groundwater, construction age and wall type all matter. When in doubt, have it assessed.
FAQ
Are vertical foundation cracks normal?
Thin vertical cracks in poured concrete walls are usually caused by shrinkage and are common. Monitor them and seal them if they leak. Get advice if they widen, offset or grow.
Are horizontal cracks always serious?
Horizontal cracks are a common sign of soil or water pressure on the wall, so they should always be assessed by a professional, especially if the wall bows inward.
Can I sell a house with foundation cracks?
Yes, but buyers’ inspectors will notice them. Dated monitoring records, repair invoices and, for anything significant, an engineer’s report make the conversation much easier.
Do foundation cracks get worse in winter?
Freeze-thaw cycles and frost in wet soil add stress, and spring melt saturates backfill. That’s why good drainage away from the foundation is one of the best ways to protect it.
Should I seal a crack that doesn’t leak?
Sealing a dry, stable crack can help reduce moisture and radon entry, but it’s not essential. Monitoring it is more important.
The bottom line
Most foundation cracks are a normal part of how concrete behaves. Measure them, date them, watch them and keep water away from the wall. Pay attention to direction, movement, offset and water: horizontal cracks, bowing walls and cracks that keep growing are the ones that need a professional. And while you’re in the basement, plan a radon test.
This article is general information, not professional or engineering advice. See our Disclaimer.