Cracks in walls, gaps under skirting boards, doors that won’t close: most houses develop some of these, and most are not structurally serious. But some are the first sign of movement that needs attention. The trick is telling the difference, and that comes down to the type of crack, its pattern, its width, and whether it is getting worse.
Our structural engineers have inspected a great many cracked houses. Here is how we think about cracks and gaps, and when it is worth getting an engineer to look.
What Causes Cracks in Houses?
These are the causes we see most often:
- Foundation movement from reactive clay: slab heave and shrinkage as the soil under the house gets wetter or drier, often driven by drainage, leaks or trees.
- Settlement: footings sinking into soft ground or poorly compacted fill.
- Concrete shrinkage: concrete shrinks as it dries, leaving fine cracks in slabs and paths. These are usually cosmetic.
- Corrosion of steel: rusting reinforcement, lintels or wall ties expand and crack the masonry or concrete around them.
- Brick growth: clay bricks expand slowly over many years after firing, which can crack brickwork, especially at corners and long walls without control joints.
- Thermal and seasonal movement: materials expanding and contracting with temperature and moisture, often at joints between different materials.
- Construction issues: misplaced reinforcement, missing articulation joints, or alterations that changed how load is carried.
- Impact, overload or storm damage.
Reading the Crack: Pattern, Shape and Location
The pattern of cracking often says more than any single crack:
- Diagonal cracks from the corners of doors and windows usually indicate differential movement of the footings.
- Stepped cracks along mortar joints in brickwork also suggest movement, with the direction of the steps hinting at which part of the wall has moved.
- Tapered cracks, wider at the top or at the bottom, help show whether part of the building has lifted or dropped.
- Horizontal cracks in masonry can point to corroding steel, brick growth, or a wall being pushed, such as by a retaining load.
- Fine random cracks in concrete slabs are usually shrinkage.
- Cracks at cornices and wall-to-ceiling junctions are common and often minor, but can also reflect movement of the frame or footings.
Where the cracks are in the house matters too. Damage concentrated in the middle of a house and damage concentrated around the outside point to different mechanisms. See centre heave vs edge heave.
How Wide Is Too Wide?
Crack width is a useful guide, but it isn’t the whole story. AS 2870, the Australian Standard for residential slabs and footings, includes a classification of wall damage based mainly on crack width and how easy the damage is to repair. It ranges from hairline cracks that are barely visible, through fine cracks that are easily filled, up to wide cracks of many millimetres that need significant repair.
A hairline crack that is stable is rarely a concern. A crack that is a few millimetres wide, or one that keeps growing, deserves attention, even if it looks tidy. Measure cracks with a crack width gauge and record the date, so you can see whether they are changing.
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Gaps Under Walls and Skirting Boards
Gaps opening under internal walls or skirting boards usually mean the floor and the wall are moving relative to each other. That often happens when part of a slab lifts or drops while the wall above is held by the roof or the framing. Gaps that change with the seasons are a classic sign of reactive clay movement.
Is Your Floor Moving?
Sloping floors, rolling marbles and doors that swing open or stick can mean the floor has moved. We measure floor levels across the whole house with a digital water level, which can “see” around corners. A floor level survey shows the shape of the movement, which is often the key to understanding the cause.
If you want to start recording levels yourself, see how to record and plot floor levels.
Found Some Cracks? What to Do First
- Record them. Photograph each crack with a ruler or gauge beside it, note the date, and mark the ends with a pencil. Our crack diary guide and crack monitoring kit explain how.
- Look for water. Check for leaking pipes, overflowing gutters, disconnected downpipes, ponding against the house and gardens being watered next to the walls.
- Don’t rush to patch. Filling cracks before the cause is understood hides the evidence, and the cracks often come back.
- Watch for change. Cracks that grow, new cracks appearing, or doors and windows getting worse are signs to get an engineer in.
Some movement is seasonal and comes and goes. Read more in seasonal movement and cracks.
When to Call a Structural Engineer
Get an engineer to look if:
- cracks are wider than a few millimetres, or are growing;
- cracks come back after being repaired;
- floors are noticeably out of level, or doors and windows are binding;
- there is rust staining, bulging or displacement in masonry;
- the cracking appeared after a storm, flood, excavation or construction nearby; or
- someone has recommended expensive repairs, such as underpinning, and you want an independent view.
How We Can Help
Our inspections are carried out by experienced structural engineers. We work out what is causing the cracking, whether it matters structurally, and what should be done. Often the first fix is something simple, like drainage or a leak, rather than expensive concrete work.
Sometimes crack repairs are covered by insurance, and sometimes there is a dispute about who is responsible. We prepare independent reports for homeowners, insurers and builders. See our structural inspections and reports and forensic engineering pages, or read how to fix slab heave.
Send us the property address, photographs of the cracks (with a ruler for scale), a short description of when you noticed them and how they have changed, and any reports you have. We will review the information and let you know whether the matter is a good fit for Cornell Engineers before preparing a fee proposal.