In short
Most cracks in walls are cosmetic: hairline cracks under 1 mm that only need filling. Worry when a crack is 5 mm or wider, keeps growing, steps through brickwork or comes with sticking doors and sloping floors. In Hobart, Mineral Resources Tasmania has documented expansive clays under houses in Lindisfarne, Lenah Valley and Taroona, so seasonal footing movement is one cause to rule out.
Key takeaways
- Under 1 mm is usually cosmetic. From 5 mm, AS 2870 rates the damage as moderate.
- Growth beats width: a crack that is still opening matters more than a wide one that stopped years ago.
- On reactive clay, cracks can open over a dry summer and close over a wet winter. Readings across the seasons show which kind you have.
- Diagnose first, then price a repair. The cause is often a leak, a downpipe or a tree, not a failed footing.
A crack is often the first thing that sends a homeowner to a structural engineer. It is a symptom, not a diagnosis. Before anyone fills it, pins it or underpins the wall, you want three answers: how wide it is, whether it is still moving and what is moving the house. This guide covers cracks in walls, brickwork and sandstone, and what Hobart's ground has to do with them.
Cracks in walls: quick facts
- Usually cosmetic
- Hairline cracks under 1 mm that have stopped moving
- Get it checked
- 5 mm or wider, growing, stepped or tapered
- Hobart ground
- Expansive clays documented in MRT house reports
- How to measure
- A crack-width card or fine steel rule at the widest point
- How to watch
- Repeat readings through the dry and wet months
- Our crack report
- From $590 + GST, fixed quote first
How do you tell if a crack in a wall is serious?
Judge a crack on four things together: its width, whether it is growing, its shape, and what the rest of the house is doing. Any one of them alone can mislead.
How wide
Read the widest point. Below 1 mm is normally a finishing issue; at 5 mm and above AS 2870 calls the damage moderate.
Still moving?
A crack that passes your pencil marks over a few months is telling you the cause is still at work.
What shape
Steps, diagonals from corners, tapers and horizontal runs each suggest a different kind of movement.
The rest of the house
Doors that bind, floors that slope and skirting boards pulling away mean more than one wall has moved.
Engineers grade wall cracks with the damage categories in AS 2870-2011, the Australian Standard for residential slabs and footings. It is the nearest thing Australia has to an official crack chart, and our reports use it.
- Cat 0Negligible< 0.1 mm
Hairline cracks.
- Cat 1Very slight< 1 mm
Fine cracks which do not need repair.
- Cat 2Slight< 5 mm
Cracks noticeable but easily filled. Doors and windows stick slightly.
- Cat 3Moderate5–15 mm
Cracks can be repaired; some wall may need replacing. Doors and windows stick, service pipes can fracture, weathertightness often impaired.
- Cat 4Severe15–25 mm
Extensive repair: sections of wall broken out and replaced. Frames distort, walls lean or bulge, some loss of bearing in beams.
- Cat 5Very severe> 25 mm
Major repair, partial or complete rebuilding. Beams lose bearing, walls lean badly and need shoring. Danger of instability.
The scale is a starting point
A wall with many fine cracks can be in worse shape than one with a single wide crack, and an active crack outranks a stable one. Engineers read the category alongside the pattern and a series of readings.
What do different crack patterns mean?
Crack shape shows the direction of movement. These four patterns are the ones an engineer reads first on a house.
Four crack patterns an engineer reads first
- A · Stepped: follows the mortar joints up and across. Often a sign one part of the wall has moved relative to another.
- B · From a window or door corner: openings concentrate stress, so movement tends to show at their corners first.
- C · Vertical: note whether it's wider at the top or the bottom. The taper shows which way the wall has moved.
- D · Horizontal: can point to sideways pressure, roof thrust or corroding wall ties. Worth an engineer's look.
What do cracks in brick and sandstone walls mean?
In brick and stone walls a crack takes the easiest route, which is usually through the mortar, so a crack stepping diagonally through the joints is the classic sign that part of the wall has dropped or lifted relative to the rest. A crack that cuts straight through bricks or stones points to larger or quicker movement.
- A staircase through the joints often traces back to a footing moving, for example where clay has dried out under one corner.
- A vertical crack that narrows at one end: measure the top and the bottom, because the wide end shows the direction of movement.
- A horizontal crack along one mortar course can come from soil or water pushing sideways, a roof spreading the wall, or rusting wall ties.
- The same crack inside and out means the wall itself has moved, not just the plaster or render on it.
Hobart's older sandstone and brick homes, such as the cottages of Battery Point, often carry cracks that were repointed decades ago and haven't moved since. A crack with a clean, fresh face, or one splitting newer repointing, is the one to watch. If the home is heritage listed, repairs may need approval, so start with heritage and older homes.
Why do houses in Hobart crack?
Most cracking in Hobart houses comes from water and clay: reactive clay drying or wetting under the footings, often helped along by trees, leaking pipes or poor drainage. A Mineral Resources Tasmania report on house damage in Taroona put the cause down mainly to expansive clay, moisture change and inadequate foundations.
The ground changes quickly across Greater Hobart. Jurassic dolerite forms the higher hills and kunanyi / Mount Wellington, Permian mudstone and siltstone sit under the foothills and the city, and Triassic sandstone lies under the valley floor and CBD, with pockets of Tertiary clays and basalt. Clays weathered from dolerite are documented as expansive, with MRT reports on houses in Lindisfarne, Lenah Valley and Taroona. Site class still varies from block to block, which is why each new footing gets its own site classification.
What moves a house? Four common causes
- Trees and roots draw moisture from the soil near a footing, so clay under that side shrinks.
- Reactive clay swells when wet and shrinks when dry, moving footings with the seasons.
- Leaking pipes or poor drainage soak and soften the ground under part of the house.
- Poorly compacted fill keeps settling under a footing long after the house is built.
Because water and trees start so much of it, the first fix is often a plumber or an arborist, not a builder: a repaired pipe, a downpipe that no longer dumps water at the wall, a tree managed. CSIRO's homeowner guide, Foundation Maintenance and Footing Performance, explains how to keep moisture even around footings. Our guide to Hobart's reactive clay soils covers site classes in detail.
Is the crack seasonal or still getting worse?
A seasonal crack breathes with the weather, opening as the clay dries and closing as it wets, while a progressive crack ends each year wider than it started; only readings taken across dry and wet months separate the two. Seasonal movement can often be calmed with drainage and steady moisture. Progressive movement needs its cause found and, sometimes, a repair designed.
Cosmetic or structural: how do the signs compare?
A cosmetic crack is fine, stable and sits in the surface finish; a structural crack is wider, still moving or patterned, and goes through the wall.
| Check | Points to cosmetic | Points to structural: get advice |
|---|---|---|
| Width | Hairline, under 1 mm | 5 mm or more, or clearly tapered |
| Movement | Unchanged over years | Lengthening, widening, or cycling with the seasons |
| Shape | Short, random, along sheet joints | Staircase through brickwork, diagonal from openings, horizontal |
| Depth | Paint, plaster or render only | Right through the wall, visible on both faces |
| Company | Nothing else wrong | Binding doors, sloping floors, gaps under skirting |
| Timing | Appeared once, then stopped | Followed a leak, a dry spell, tree removal or digging next door |
A guide only. The cause is confirmed on site.
How do you monitor a crack in a wall yourself?
Keep a dated record for a few months: a short series of readings tells an engineer far more than one photo, and it is the first step in proper crack monitoring.
- First visit
Pencil the tips
Rule a short line across each tip of the crack and write the date next to it.
- First visit
Take a width reading
Read the widest point with a crack-width card or fine steel rule, and mark the spot.
- First visit
Photograph with a scale
Square-on, with a ruler beside the crack, from a spot you can return to.
- Monthly or so
Repeat through the year
Same spot, same method, plus an extra reading after a dry spell or a run of heavy rain.
- On any change
Act on it
Past the marks, wider, or new cracks joining it: the movement is active, so book an engineer.
When should you call a structural engineer about cracks?
Call a structural engineer when any of the signs below shows up, or when you want an independent answer before spending money on a repair.
- The crack reaches 5 mm, or is clearly wider at one end
- It has passed your pencil marks, or new cracks keep joining it
- Doors and windows have started binding, or skirting boards are pulling away
- A floor has developed a slope, or floor tiles are cracking in a line
- You've been quoted for underpinning and want a second opinion before signing
- The house you're buying shows cracking the inspector couldn't explain
Get the diagnosis before the repair quote
Our cracks and foundation movement report measures and grades the cracks against AS 2870, finds the likely cause and lists your options, from $590 + GST. Already holding an underpinning quote? Our second opinion starts from $390 + GST.
Do
- Date, measure and photograph the crack
- Look for leaking taps, pipes, gutters and downpipes
- Write down when you first saw it and the weather at the time
- Get an engineer's view that isn't tied to a repair
Don't
- Patch a wide crack before anyone has measured it
- Treat underpinning as the default answer
- Take advice only from the company selling the fix
- Wait out a crack that keeps opening
Need an engineer?
Worried about a crack?
Send photos with a ruler in the frame. You'll have a fixed quote for a crack report within 1–2 business hours.
Sources
- Standards Australia, AS 2870-2011 Residential slabs and footings. Appendix C, Table C1: classification of damage to walls
- CSIRO, Foundation Maintenance and Footing Performance: a homeowner's guide (formerly Building Technology File 18)
- Mineral Resources Tasmania, report UR1987_71 (house in Lindisfarne). Dolerite-derived clays are expansive and have caused cracking in Hobart and Launceston
- Mineral Resources Tasmania, report UR1988_39 (Lenah Valley). Expansive clays
- Mineral Resources Tasmania, report UR1977_35 (house damage in Taroona). Damage mainly from expansive clay, moisture change and inadequate foundations
- AusIMM Tasmania, The geology beneath the Hobart CBD (T. Webster)