Wall cracks: which ones matter, and how to tell
A hairline crack that stays still is often a monitoring job. Cracks around 5 mm or wider, diagonal or stepped patterns, visible growth, sticking openings, sloping floors or water entry deserve professional investigation. AS 2870 gives useful width categories, and an engineer can read those measurements in the context of the whole building and site.
- 01GeometryHow wide, which way it runs, and where it sits
- 02ContextWhat is around it: openings, floors, drainage, the soil
- 03ChangeWhether it is growing. Date it, measure it, come back.
Read growth, width, direction, distortion and location together. Escalate concerning movement.
How serious is it? The damage categories
AS 2870-2011, Residential slabs and footings, classifies wall damage using crack width and typical effects. Use the categories with direction, location, associated distortion and change.
| Category | Crack width | What it means |
|---|---|---|
| 0 | Less than 0.1 mm (hairline) | Negligible, no action |
| 1 | Up to about 1 mm (fine) | Very slight; monitor context and change |
| 2 | About 1–5 mm | Slight, noticeable, usually filled and monitored |
| 3 | About 5–15 mm | Moderate, doors and windows stick, pipes may fracture; investigate and repair |
| 4 | About 15–25 mm | Severe, frames distort, walls lean or bulge; major repair |
| 5 | More than about 25 mm | Very severe, with major structural effects; urgent engineering assessment |
Categories 0 and 1 are negligible to very slight, Category 2 is slight, and from Category 3 up the Standard’s own language runs moderate to severe, which is where professional investigation starts. The width is the entry point, not the verdict.
How to read a crack
Write down the width, the direction, where it sits, anything distorting nearby, and how it changes. Together those signals tell you which professional to call, and what to ask them.
| Signal | Lower-concern pattern | Escalation pattern |
|---|---|---|
| Direction | Vertical, fine, along joints | Diagonal from window/door corners; stepped through brick |
| Associated signs | Localised crack only | Sticking doors and windows, sloping floors, gaps at cornices |
| Where | Plaster or render surface | Through the structure, external brickwork, lintels |
| Over time | Stable, seasonal | Progressive, getting longer or wider |
Why cracks appear
Reactive clay and moisture
Cracks can arise from material shrinkage, seasonal movement, alterations or footing movement. Reactive clay soil swells and shrinks as moisture changes. AS 2870 uses site classifications including A, S, M, H1, H2, E and P. Use the classification with drainage, trees, services, footings and the observed building pattern.
Drainage, trees and original footings
Moisture changes can drive footing movement. Check drainage at the foundations, leaking service pipes, downpipe discharge, large trees, root effects and the actual footing system.
The single most useful test: time
Monitor cracks with dated photographs and a stable reference point. Record width, length, season, drainage conditions and associated changes to doors or floors. Visible growth raises concern, and width, direction, distortion and location still matter. Progression or associated movement is the point to seek professional investigation.
When to bring in a structural engineer
Category 3 width, diagonal direction, sticking doors, sloping floors and visible progression justify professional investigation. A building inspection can flag the concern; a structural engineer can diagnose cause and scope. Use the era guide for related construction questions.
Frequently asked questions
When should I investigate a wall crack?
Seek competent assessment where a crack is changing, relatively wide, stepped or diagonal, appears with sticking openings or floor movement, admits water, affects a structural element, or follows an event. Width categories are only one input; material, location, cause and change over time also matter.
Are cracks in walls a dealbreaker when buying?
Cracks need context. Escalation signals include increasing width, diagonal cracking from openings, steps through masonry, sticking doors, sloping floors and related distortion. AS 2870 Category 3 starts at about 5 mm and describes moderate damage that warrants professional investigation.
How wide does a crack need to be to worry?
AS 2870 classifies wall cracks by width: hairline under 0.1 mm and fine cracks up to about 1 mm are negligible; 1–5 mm is slight; 5–15 mm is moderate and worth professional investigation; 15–25 mm is severe. Also look at direction, location, associated distortion and progression.
What causes structural cracks?
Most commonly movement in the footings, reactive clay soils swelling and shrinking with moisture, poor drainage at the foundations, leaking pipes, large trees, or shallow original footings. Much of southern Australia sits on reactive clay, which makes footing movement a recurring issue in older homes.
Should I get an engineer or is a building inspection enough?
A building inspection can flag concerning cracking. A structural engineer can then diagnose cause, significance and repair scope where indicated.
Is a hairline crack anything to worry about?
Hairline cracks fall within the negligible AS 2870 width category. Monitor progression and associated signs, especially around openings and floors.
How this guide was researched
A triage approach: what the crack looks like, what surrounds it, and whether it is moving.
Limits: A structural engineer can diagnose the significance and cause for the building in front of them. Visible progression or movement affecting doors, floors and openings deserves professional investigation.
Sources
- AS 2870-2011 Residential slabs and footingsStandards Australia · accessed 2026-07-19
- Protect against subsidenceQueensland Building and Construction Commission · accessed 2026-07-20
- Due diligence checklist for home buyersConsumer Affairs Victoria · accessed 2026-07-19
- Brickwork and blockworkYourHome, Australian Government · accessed 2026-07-19
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