Diagonal cracks in concrete

Direction is genuinely informative. A crack takes the path of least resistance through the material, so where it runs tells you something about where the stress was and where the slab or wall was weakest.

It tells you something — not everything, and not enough to conclude a cause. A diagonal crack is a reason to look at what is happening at each of its ends, which is usually where the actual story is.

What it is

A diagonal crack runs at an angle to the edges of the element it is in. In flatwork, diagonals very commonly start at an inside corner — a step in the slab outline, a corner cut around a post, the inside angle of an L-shaped patio.

Those inside corners concentrate stress. The material has less continuous cross-section to resist movement at exactly the point where two directions of shrinkage or settlement meet, so a crack that has to form tends to form there and run away at roughly 45 degrees.

In a wall, a diagonal crack often runs between an opening and a corner, following the same logic: it goes where the wall is least able to resist the movement being imposed on it.

What seeing this cannot establish

• The angle does not identify the cause. A diagonal crack from a re-entrant corner caused by ordinary shrinkage and a diagonal crack caused by one part of a structure settling relative to another can both run at similar angles.
• A diagonal crack in flatwork and a diagonal crack in a load-bearing wall are entirely different levels of concern, and no reading of the angle changes that. In a wall it is a question for an engineer.
• Direction cannot tell you whether the movement that caused the crack has finished. That is a monitoring question, not a geometry question.

What this can be associated with

Candidate explanations, not findings. Nobody can establish which applies to your concrete without seeing it.

• A re-entrant corner in the slab outline, where stress concentrates by geometry rather than by any fault
• Differential settlement, where one area has moved relative to another and the crack runs along the boundary between them
• Shrinkage in a slab whose shape gave it nowhere convenient to relieve
• In walls, movement between two parts of a structure founded differently or at different depths
• A slab poured around an obstruction — a post base, a utility penetration — without an isolation detail

What you can safely check yourself

• Trace the crack to both ends and photograph each. If one end lands exactly at an inside corner, that geometry is likely part of the story.
• Note whether the crack changes width along its length. A diagonal that is wide at one end and closes to nothing at the other suggests rotation rather than straight separation.
• Lay a level across the crack at two or three points along it and photograph. Differential settlement often shows as a consistent height change along the whole crack.
• Look for the same angle repeated elsewhere. Several parallel diagonals in the same area point at a shared cause rather than a local one.
• If the crack is in a wall, note whether doors or windows nearby have become harder to operate, and when you first noticed.

When this is frequently cosmetic

• A single diagonal running from an inside corner of an exterior slab, stable, level, and no wider than a hairline
• A diagonal in a patio that has been there for years without changing and where nothing has dropped

When professional evaluation matters more

• It runs through a foundation wall, a load-bearing wall or a retaining wall — the case where an engineer rather than a repair contractor is the right first call
• There is a consistent height difference along the crack, suggesting one area has moved relative to another
• Doors or windows in the vicinity have started to stick or show gaps that were not there before
• Several parallel diagonals have appeared in the same area
• The crack has extended in length between two dated observations

Evidence worth having before you ask for estimates

Every item here is something you can produce without tools or risk, and every one of them makes an estimate more accurate.

• Photographs of both ends of the crack, not just the middle
• A level laid across the crack at several points, each photographed
• A sketch or photograph showing the shape of the slab and where the crack sits within it
• The date you first noticed it, and the date any nearby door or window changed behaviour
• Notes on any excavation, new landscaping, tree removal or drainage change nearby

Questions to ask whoever comes out

• Where do you think this crack started, and why there?
• Does the geometry of this slab explain the angle, or do you think something has moved?
• Would you want an engineer to look at this before anyone repairs it?
• If the cause is still active, what does repairing the crack now actually buy me?
• How would you detail this differently if you were pouring it again?

Where estimates for this commonly differ in scope

These are the places two prices for apparently the same work stop being comparable.

• A repair that treats the crack and a repair that addresses the corner detail causing it are different scopes. On a re-entrant corner, filling the crack without changing anything often means it returns in the same place.
• Estimates for diagonal cracking in walls sometimes include structural work and sometimes only cosmetic treatment. The difference is very large and is not always spelled out.

Who tends to do this work

• A concrete repair specialist
• A licensed structural or geotechnical engineer
• A residential flatwork concrete contractor

ConcreteOwner does not inspect, certify or approve contractors. This describes which trade usually holds this work, not a recommendation of anybody in particular.