How to Tell If Your Foundation Crack Is Serious (And What to Do Next)

The Short Version

Foundation cracks are not all created equal. The width, direction, pattern, and location of a crack each tell you something different about what caused it and how urgent the problem is. Here's how to read them.

Crack Direction Is the First Thing to Note

Vertical cracks run straight up and down and are the most common crack type in poured concrete foundations. In most cases, they're caused by concrete shrinkage during the original cure — a normal, expected behaviour. Hairline vertical cracks (less than 1/8 inch wide) are typically cosmetic and only require sealing to prevent water infiltration. Wide vertical cracks (over 1/4 inch) or vertical cracks that are wider at one end than the other suggest differential settlement — one section of the wall is moving differently from the adjacent section.

Horizontal cracks are almost never cosmetic. A horizontal crack running across a basement wall indicates that the wall is under lateral pressure from soil on the outside. This is a structural failure in progress. The pressure that opened the crack is ongoing — if you have a horizontal crack, you need a professional assessment, not just a sealant.

Diagonal cracks — running at 45 degrees — are typically caused by differential settlement. When one corner of the foundation settles more than the adjacent wall, tension develops diagonally from the high point to the low point. These cracks are particularly common at window and door corners, where stress concentrations are highest. Diagonal cracks that are wide (over 1/4 inch) or still actively growing require stabilization.

Stair-step cracks in brick or block foundations follow the mortar joints in a stair-step pattern. Like diagonal cracks, they indicate differential settlement. In block walls, stair-step cracking is particularly significant because block mortar joints are the weakest plane in the system.

Crack Width Matters — Here's a Rough Scale

Hairline (less than 1/16 inch): Shrinkage-related in poured concrete. Seal to prevent water entry. Monitor for growth.

Fine (1/16 to 1/8 inch): Still likely shrinkage in poured concrete, but worth a professional look. Seal and monitor.

Medium (1/8 to 1/4 inch): This width in a vertical crack can indicate ongoing movement. In a horizontal or diagonal crack at this width, it's urgent.

Wide (over 1/4 inch): Any crack this wide, in any direction, warrants professional assessment.

Is the Crack Active or Dormant?

A dormant crack has stopped moving. An active crack is still widening, shifting, or seeping. Active cracks can't be permanently sealed until the movement is stopped — sealants applied to active cracks will break again, often within a single freeze-thaw cycle.

A simple way to check for activity: mark the ends of the crack with a pencil mark and date. Photograph it with a ruler for scale. Check again in 30, 60, and 90 days. If the crack has extended past your marks or widened, it's active. A professional will use more precise tell-tale monitors, but the pencil-mark method gives you useful preliminary information.

What Water Around a Crack Tells You

Water emerging from or around a crack means the crack goes through the full wall thickness and there's hydrostatic pressure on the other side. This is the single most important indicator of urgency. Even a hairline crack that seeps water under pressure is a sealing priority, because water infiltration accelerates deterioration significantly.

White deposits around a crack — called efflorescence — are mineral salts left behind by water that has evaporated. It tells you water has been moving through that location, even if the crack looks dry now.

When to Call a Professional

Call a foundation specialist (not a general contractor) for: any horizontal crack, any crack wider than 1/4 inch, any crack accompanied by water infiltration, any diagonal crack at window corners in a home showing other signs of settlement (sloping floors, sticking doors), or any crack you're not sure how to classify.

A written assessment from a foundation specialist will tell you exactly what you have, what caused it, and what options exist — from simple crack injection to full structural stabilization. That information is worth having before you decide to spend (or not spend) money on repairs.

One Last Thing

Photos taken over time are genuinely useful data. If you've noticed a crack, start photographing it with a date reference now, before you call anyone. A sequence of dated photos showing whether a crack has changed tells a foundation specialist a great deal about the problem — and can save money on the diagnostic process.