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Structural vs cosmetic cracks: how to tell the difference

Grounded in CSIRO BTF-18 and AS 2870. General information, not advice for your specific property.

A practical way to read a crack in your wall, based on width, direction and behaviour, so you know whether it is harmless or worth an engineer's opinion.

Before you panic about a crack, it helps to know that most cracks in Australian homes are cosmetic. Plaster and render shrink as they cure, paint splits over old joints, and clay soils swell and shrink with the seasons, nudging walls a millimetre or two back and forth. None of that threatens the structure. The job is to separate that ordinary movement from the smaller number of cracks that signal the footing or slab has actually moved.

Cosmetic vs structural: the quick comparison

Four signals separate a harmless crack from one worth an engineer's opinion. No single row is a verdict on its own; it is when several point the same way, and the crack keeps growing, that it becomes structural.

Read the width first

Crack width is the single most useful clue, and it is worth measuring rather than guessing. A common engineering damage classification used in Australia and the UK groups cracks by width and by the repair they need. Hairline cracks under about 1 mm and fine cracks up to roughly 5 mm are generally classed as negligible to slight, the sort of thing filler and paint fix. Cracks from about 5 mm to 15 mm move into moderate damage, where doors and windows may start to stick. Anything wider than about 15 mm, or several cracks of that size together, is classed as severe to very severe and warrants a structural engineer.

Keep it in proportion. A 0.5 mm hairline over a doorway is not the same problem as a 10 mm diagonal crack that lets daylight through, even though both are 'cracks'.

Then read the direction and pattern

Direction matters as much as width. Fine vertical cracks in render, or cracks that follow a mortar joint in brickwork, are usually shrinkage or minor settlement. Diagonal cracks that run out from the top corners of windows and doors, or stepped cracks that climb the mortar joints of a brick wall in a staircase pattern, are the classic signature of footing movement, because the wall is being dragged apart as one part of the house drops or lifts relative to another.

Cracks that are wider at the top than the bottom, or that appear on both the inside and outside of the same wall in the same place, also point to structural movement rather than surface shrinkage.

Finally, watch how it behaves

A crack that appeared, stopped and has stayed the same width for years is far less concerning than one that is actively growing. Mark each end of the crack with a pencil line and a date, or bridge it with a small blob of plaster, and check it over a few weeks and across a wet and dry season. Reactive clay sites move seasonally, so some opening and closing is normal; steady, one-way growth is not. Jamming doors, windows that no longer close square, and sloping or bouncing floors are the companion signs that tip a crack from cosmetic into worth-investigating.

If width, direction and behaviour all point the wrong way, the sensible next step is a structural engineer's inspection, not a builder's quote. An engineer diagnoses the cause; a contractor prices the fix. Getting those two in the right order is how you avoid paying for underpinning you may not need.

Common questions

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