Concrete Cracks vs Structural Issues: When Should You Worry?

Not every crack in a strata building is a crisis. Some cracks are entirely normal: concrete shrinks as it cures, buildings settle slightly as they age, thermal movement opens and closes micro-cracks every day. But some cracks are serious, and the consequences of misreading them go both ways. Treating a structural crack as cosmetic is clearly dangerous. Calling an emergency engineer review for a normal shrinkage crack wastes the committee's money and creates unnecessary anxiety. This is a plain-English guide to telling them apart.

Start With the Basics: What Kind of Crack Is It?

The first thing to establish is the nature of the crack - not just its width.

Shrinkage Cracks

Fine, shallow, often random-patterned cracks in concrete or render. They typically appear in the first few years of a building's life as the concrete cures and dries. They are usually less than 0.2mm wide, do not run through the full depth of the element, and are not associated with any movement or displacement.

Risk level: Low. Generally cosmetic. Fill and paint as part of normal maintenance.

Thermal Cracks

Long, roughly straight cracks that run horizontally or vertically, typically appearing in long exposed walls. They are caused by temperature-driven expansion and contraction. They may open slightly in summer and close in winter.

Risk level: Low to moderate. If water can enter the crack, seal it. If the crack is widening over time, investigate the movement joint detail.

Settlement Cracks

Diagonal cracks, often starting from corners of openings (windows, doors). Typically caused by differential settlement in the early years of a building's life. Old settlement cracks that are not actively moving are usually not a current concern.

Risk level: Moderate. Assess whether movement is ongoing. Dormant = monitor and seal. Active = engineering input required.

Structural Cracks

Cracks with visible displacement (one side is higher or further forward than the other), wide cracks (1mm+), diagonal cracks at window corners that are widening, or cracks that pass through beams, columns, or load-bearing walls. These indicate that the structural element is under stress beyond its design tolerance.

Risk level: High. Stop, document, and call an engineer before authorising any cosmetic repair.

A Simple Decision Guide

Use this as a first-pass assessment. It is not a substitute for professional advice on anything above low severity.

For a deeper look at the concrete cancer pathway (spalling + rust = rebar corrosion), see What is Concrete Cancer? A Sydney Guide.

When to Call a Structural Engineer

Structural engineers in Sydney typically charge $1,800 to $4,500 for a building crack assessment and written report. The cost is almost always justified when:

• Any crack shows visible displacement (one side higher, deeper, or more forward than the other)
• You see a crack appearing or widening over a short period (weeks to months)
• The crack is in a beam, column, or wall you believe is load-bearing
• Residents or committee members are expressing concern about safety
• An insurance claim, building sale, or legal dispute is involved

The engineer's report gives you something a tradesperson's verbal opinion cannot: a documented professional opinion that the building is (or is not) structurally sound, a recommendation for remediation if needed, and protection for the strata manager and committee who acted in good faith based on qualified advice.

For background on what a full building defect report covers, see Building Defect Reports: A Sydney Guide.

When Monitoring Is Enough

Not every crack needs an engineer. Many cracks can be appropriately managed by installing crack monitors and checking them monthly. A crack that shows zero movement over three to six months is dormant. Once you have that data, a tradesperson can fill and paint it without the cost of an engineering report.

Crack monitors cost almost nothing and take five minutes to install. They are the right first step for any crack that is:

• Under 0.5mm wide
• Not showing visible displacement
• In a rendered or plastered wall (not a structural element)
• More than five years old and not recently changed in appearance

When NOT Acting Is the Bigger Risk

The biggest risk we see is not over-reacting to normal cracks - it is under-reacting to cracks that have been present "forever" and assumed to be fine.

Old cracks that have never been attended to are often stable. But they are also open water pathways. In a Sydney building over 20 years old, a crack that has been open for a decade has been admitting water for a decade. The concrete behind that crack may have been slowly carbonating or the rebar may have been slowly corroding for years before any visible spalling appears.

Sealing old cracks - even dormant ones - is cheap and removes a water ingress risk. It costs $50 to $300 per crack to fill and seal. The alternative is finding out the hard way.

The Bottom Line

Most cracks in strata buildings are not structural emergencies. Some of them are, and the cost of missing one is serious. The right approach is a documented first assessment (is there displacement? is it moving?), crack monitors for anything uncertain, and an engineer's report for anything that shows active movement or displacement. Do not let "it's been there for years" stop you from sealing a crack against water. Contact us if you would like a building assessment or see our remedial works service for crack repair and structural rectification.