Cracked walls are among the most common problems in Australian homes, and most are not serious. Fine cracks in plaster often come from timber and plaster shrinking as they dry. The cracks that matter are wide, growing, diagonal, stepped or horizontal, or they appear with sticking doors and sloping floors. Those usually mean something under the house is moving.
This guide explains why walls crack, how to read the main crack patterns, which crack widths engineers treat as serious, and which repair suits cause. Our approach is simple: find what is moving the house, fix the cause, then fix the damage.
Are Cracked Walls a Sign of Structural Problems?
Not always. CSIRO says most cracking in masonry walls is only a cosmetic nuisance. Cracked walls become a structural concern when cracks are 5 mm or wider, keep growing, return after patching, or appear with sticking doors, sloping floors or gaps at skirting boards. Those signs indicate movement in the footings or in the ground beneath them.
The repairs are completely different. A cosmetic crack needs to be filled and painted. A structural crack needs the movement to be stopped first, or it will reopen. CSIRO’s homeowner guide, Foundation Maintenance and Footing Performance, puts it plainly. Where the ground is being eroded, or a footing is failing locally, cracking “will usually continue to develop until the original cause has been remedied.”
What Causes Cracked Walls in Australian Homes?
Most structural wall cracks start in the ground, not the wall. Reactive clay swelling and shrinking, leaking pipes, poor drainage, tree roots, settling fill, and failing stumps can all cause a footing to move unevenly. Cosmetic cracks usually come from materials shrinking, temperature changes and normal settling in a newer house.
Reactive clay soil
Many Australian homes are built on clay. CSIRO notes that “all clays react to the presence of water by slowly absorbing it, making the soil increase in volume”, and they shrink again as they dry. When one side of a house gets wetter or drier than the rest, the footing moves unevenly, and the walls crack. Our guide to reactive clay and moisture explains how this works and how to reduce the risk.
Leaks and poor drainage
Water is one of the most common triggers. A leaking stormwater, sewer or water pipe can soften the ground under one part of a footing, and CSIRO warns that “even a minuscule leak can be enough to saturate a clay foundation.” Blocked gutters, disconnected downpipes, and garden beds that hold water against the house do the same thing more slowly. Read more on how water leaks cause subsidence.
Trees and garden watering
Tree roots draw moisture from clay, which can cause the ground beneath nearby footings to shrink. CSIRO says roots close to footings “will absorb much of the moisture in the foundation soil, causing shrinkage or subsidence.” Overwatering causes the opposite problem. CSIRO lists misused automatic watering systems as a common cause of saturated soil.
Settlement, poor fill and erosion
Footings can drop where the ground below was poorly compacted, where fill was placed on a sloping block, or where water has washed soil away. This often shows up as cracking at one end or corner of the house.
Timber stumps in older homes
In older homes on stumps, rotting or sinking stumps can cause floors and walls to drop, cracking plaster and cornices. Restumping is the usual fix for failed stumps. We do not do restumping, so if that is your problem, we will tell you and suggest you speak to a restumping specialist.
Shrinkage and normal settling
Not every crack means movement. New plaster, timber and concrete shrink as they dry, and walls move slightly with heat and cold. These cracks are usually fine and stay the same size once filled.
Types of Wall Cracks and What They Mean
The direction, shape and location of a crack tell you more than its length. Hairline and short vertical cracks are usually cosmetic. Diagonal cracks at door and window corners, stepped cracks through brick mortar joints, horizontal cracks, and bulging walls are more likely to signal movement and are worth having checked.
Figure 1. Common wall crack patterns. Simplified and not to scale.
Alt text: Diagram of a wall elevation showing five crack patterns: a fine hairline crack and a vertical crack in a plastered wall, a diagonal crack running up from the top corner of a window, a stepped crack following brick mortar joints, and a horizontal crack along a mortar bed in a brick wall.
| Crack type | What it looks like | Common causes | Level of concern |
| Hairline | Very fine lines in plaster or paint, often at joins and corners | Shrinkage, temperature changes, and normal settling | Low. Fill and paint, then watch for change. |
| Vertical | Straight, up-and-down crack, often at wall junctions | Shrinkage, minor footing movement | Low to moderate. Higher if wider at one end or growing. |
| Diagonal | Runs at an angle, often from a door or window corner | Uneven footing movement | Moderate to high, especially if it tapers or grows. |
| Stepped | Zig-zags along brick mortar joints like a staircase | Uneven footing movement | Moderate to high if wide or growing. |
| Horizontal | Runs along a mortar bed or across a wall | Soil or water pressure behind the wall, corroded wall ties, and roof movement | High. Have it checked. |
| Bulging, leaning or separating | Wall out of plumb, gaps at corners, ceilings or cornices | Advanced movement or wall instability | Severe. Get advice promptly. |
Hairline cracks
Hairline cracks in walls are the most common type and often follow plaster joins and cornices. If one stays the same after filling and painting, it is almost always cosmetic.
Vertical cracks
Vertical cracks in walls often appear where two walls meet, or an extension joins the house. Even width usually means shrinkage. A crack wider at the top or bottom suggests one side is moving more than the other.
Diagonal cracks
Diagonal cracks in walls usually start at a door or window corner, because openings are weak points. CSIRO notes it is “usual to see cracking at weak points, such as openings for windows or doors.” A diagonal crack that is wider at one end, or that comes with a sticking door, is a sign of uneven footing movement.
Stepped cracks in brickwork
Brick wall cracks often follow the mortar joints in a staircase pattern. CSIRO describes this as “step cracking in the mortar beds in the body of the wall or above/below openings such as doors or windows.” Fine, stable steps may only need repointing. Wide or growing steps usually mean the footing below is moving.
Horizontal cracks
Horizontal cracks in walls are less common and more concerning. They can mean that soil or water is pushing on a retaining or subfloor wall, that wall ties have corroded, or that the roof is spreading. Have one checked rather than filling it.
Bulging, leaning or separating walls
A wall that leans, bulges, or pulls away from a corner or ceiling indicates that movement is well advanced and requires prompt professional advice.
How Does Footing Movement Crack a Wall?
When part of a footing drops or lifts more than the rest, the wall above has to bend. Brick and plaster cannot bend much, so they crack at the weakest point, often a door or window corner. If one end settles, the crack is often widest at the top and narrows towards the footing.
Figure 2. How uneven footing settlement can crack a wall. Simplified, with movement exaggerated.
Alt text: Diagram of a wall on a footing where the right-hand end of the footing has dropped into softer ground. The wall has cracked diagonally, widest at the top and narrowing to almost nothing near the footing.
The taper shows which part of the house is moving, helping identify the cause. It is also why filling the crack does not fix it.
How Wide Does a Wall Crack Need to Be Before You Worry?
As a guide, cracks under 1 mm are usually cosmetic, cracks up to 5 mm are classed as slight damage, and cracks of 5 mm or more are moderate damage that needs a closer look. Growth matters as much as width. A crack that keeps widening needs attention, whatever its size.
Australian engineers grade crack damage in walls using Appendix C of AS 2870, the residential slabs and footings standard. CSIRO reproduces the table in its homeowner’s guide.
| AS 2870 category | Approx. crack width | Typical damage |
| 0: Negligible | Less than 0.1 mm | Hairline cracks |
| 1: Very slight | Less than 1 mm | Fine cracks that do not need repair |
| 2: Slight | Less than 5 mm | Noticeable cracks, easily filled; doors and windows stick slightly |
| 3: Moderate | 5 to 15 mm, or several cracks 3 mm or wider in one group | Doors and windows stick; pipes can fracture |
| 4: Severe | 15 to 25 mm, depending on the number of cracks | Window and door frames distort; walls lean or bulge |
State building regulators use similar figures to judge defects in new building work:
- Victoria: the Building and Plumbing Commission’s Guide to Standards and Tolerances 2026, which applies to work from 1 August 2026, treats masonry wall cracks of 5 mm or wider as outside tolerance, and plasterboard cracks wider than 1 mm within 24 months of completion.
- Queensland: the QBCC’s guidance on protecting your home against subsidence says hairline cracks and wall cracks under 5 mm that are easily filled are acceptable, while cracks over 5 mm that mean part of a wall must be replaced are not.
- New South Wales: the NSW Guide to Standards and Tolerances sets the minimum standard for building work.
These are quality benchmarks for new work, not a diagnosis for an older home. A 3 mm crack that has doubled in six months is a bigger worry than a stable 6 mm crack that has not changed in years.
Structural Warning Signs to Look For
A single crack rarely tells the whole story. Structural movement usually shows up as a group of signs: cracks that widen or return, doors and windows that start sticking, floors that slope, and gaps opening at skirting boards, cornices or wall junctions. The more of these you see together, the more likely the ground is moving.
- A crack 5 mm or wider, or several cracks 3 mm or wider in one area
- Cracks that are wider at one end, or that come back after patching
- Doors and windows that have started to stick or will not latch
- Sloping or uneven floors
- Gaps between walls and ceilings, cornices or skirting boards
- Cracks in several rooms, or inside and outside the same wall
- A wall that leans or bulges
- Damp patches, soggy ground or a jump in your water bill near the cracks, which can point to a leak
If you are still unsure, our guide on whether it is just a crack or a structural issue walks through a step-by-step decision checklist.
Why Do Wall Cracks Vary Across Australia?
Cracking varies because soils vary, often within the same suburb. Sandy and rocky sites move little in response to moisture changes, while highly reactive clay sites can move substantially between wet and dry seasons. Your site classification, found in your engineering or soil report, tells you which kind of ground your house sits on.
| Site class | What CSIRO says it means |
| A | Most sand and rock sites, with little or no ground movement from moisture changes |
| S | Slightly reactive clay sites, which may experience only slight ground movement |
| M | Moderately reactive clay or silt sites, which may experience moderate ground movement |
| H1 | Highly reactive clay sites, which may experience high ground movement |
| H2 | Highly reactive clay sites, which may experience very high ground movement |
| E | Extremely reactive sites, which may experience extreme ground movement |
The QBCC notes that most Queensland homes are built on clay soils that dry out and get wet quickly. Reactive clays are also found in parts of Sydney, Melbourne and other cities, but conditions change from block to block, so a site report tells you more than a suburb rule of thumb.
How to Monitor a Wall Crack
Mark each end of the crack with a fine pencil line and the date, measure the widest point with a ruler or crack gauge, and photograph it from the same spot. Check every few weeks, and after heavy rain or a long dry spell. A crack that lengthens or widens needs professional advice.
Figure 3. A simple way to monitor a wall crack.
Alt text: Illustration of a diagonal wall crack with a short pencil line and date at each end, a ruler held across the crack, and a second pencil mark and date beyond the top end showing that the crack has grown.
- Do not fill the crack while you are monitoring it. Filling hides the evidence that an engineer would use.
- Keep a simple log: date, width, length, weather and any doors that have started to stick.
- Note whether the crack opens in dry months and closes in wet months, which points to reactive clay.
Repair Options for Cracked Walls
The right repair depends on the cause. Cosmetic cracks need filling and painting. Cracks from past movement may need repointing, brick stitching or epoxy crack injection. Cracks from ongoing movement need the cause fixed first, such as a leak or drainage fault, and sometimes footing stabilisation with resin injection or underpinning.
| Repair | What it involves | Best suited to | Fixes the cause? |
| Fill and patch | Rake out, tape, fill, sand and paint | Hairline and slight cracks in plaster and render | No |
| Repointing | Remove cracked mortar and replace it | Mortar cracks in stable brickwork | No |
| Brick stitching | Stainless steel helical bars set into mortar joints across the crack | Stepped or vertical brick cracks once movement has stopped | No. Reinstates wall strength |
| Epoxy crack injection | Resin is pumped into the crack through ports to bond and seal it | Structural cracks in concrete and masonry | No. Bonds the cracked section |
| Leak and drainage repairs | Fix pipes, reconnect downpipes, regrade paving | Cracking triggered by water | Yes, for water-related movement |
| Resin injection | Polyurethane resin is injected into the ground below the footing, where it expands to fill voids and firm up loose soil. | Footings settling on soft or washed-out ground | Yes |
| Underpinning | New support was built under the existing footing to reach firmer ground | Footings that need deeper support | Yes |
| Restumping | Replace failed timber or concrete stumps | Older homes on stumps | Yes (we do not offer this) |
Order matters. Fix the cause, confirm the movement has stopped, then repair the wall. Repairing a wall over a moving footing usually means paying for the same repair twice.
We carry out brick stitching, epoxy crack injection, resin injection and underpinning as part of our wall crack repair service. If you want to know what each option is likely to cost, see our guide to what it costs to fix cracks in a wall.
How to Prevent Wall Cracks
You cannot change the soil, but you can control the water around it. Keep moisture under the house as evenly as possible: fix leaks quickly, keep downpipes connected to stormwater, slope paving away from the walls, and avoid heavy watering or thirsty trees close to the footings.
- Extend paving at least 900 mm from the house with a fall of at least 1:60, as CSIRO recommends, and more on highly reactive sites.
- Keep gutters clear and downpipes connected to the stormwater system.
- Fix plumbing leaks straight away, including slow drips and taps leaking near walls.
- Plant lawn or plants that need only light watering next to the house, with thirstier plants further out.
- Choose trees with their mature size in mind, and get advice before removing a large tree, because removal can change soil moisture, too.
- Check your walls once or twice a year and after extreme weather, and note any changes.
Frequently Asked Questions
Are cracked walls always serious?
No. Most cracked walls are cosmetic, caused by plaster and timber shrinking or small seasonal movement. They become serious when cracks are 5 mm or wider, keep growing, are wider at one end, return after patching, or come with sticking doors and sloping floors. Those signs suggest the footings are moving.
Which wall cracks should I worry about?
Worry about diagonal cracks from door and window corners, stepped cracks through brick mortar, horizontal cracks and any crack that is growing. Cracks wider than 5 mm, or several cracks of 3 mm or more in one area, fall into the moderate damage category in AS 2870 and should be assessed.
Do cracks come back after patching?
Cosmetic cracks usually remain closed if they are properly taped and filled. Structural cracks usually recur because the filling does not stop the movement behind them. If a patched crack reopens within months, treat that as a sign the cause has not been fixed, and have the footings checked before patching again.
Can cracked walls be fixed without underpinning?
Often, yes. Many cracked walls only need a cosmetic repair, repointing or brick stitching. Where a leak or drainage fault caused the movement, fixing it may be enough. Where the ground under a footing has softened, resin injection can re-support it. Underpinning is for footings that need deeper support.
Who should I call about a cracked wall?
For an independent diagnosis, call a structural engineer. Engineers Australia’s National Engineering Register lists registered engineers. Call a plumber if you suspect a leak. For repairs, call a licensed foundation repair contractor who will explain the cause, not just quote to fill the crack.
The Bottom Line
Cracked walls are common in Australian homes, and most are cosmetic. The ones to act on are wide, growing, tapering, stepped or horizontal cracks, especially with sticking doors or sloping floors. Monitor before you patch, find the cause, and choose a repair that fixes the movement, not just the crack.
Get Your Cracked Walls Assessed
Not sure what your crack means? Start with a cracked wall assessment. We find what is moving the house, explain your options in plain language, and tell you honestly if the fix is something we do not offer. Our teams cover Sydney, Melbourne and Brisbane. Contact us or call 1300 053 835.
