A 1 mm crack and a 15 mm crack look about the same in a phone photo taken at night. One is, by CSIRO’s scale, a very slight crack that doesn’t need repair. The other sits in the “moderate” to “severe” range, where doors jam, pipes can fracture and walls may need rebuilding.
If you’ve just found cracks in walls around your Perth home, that’s the gap you need to close: what size it really is, where it sits, and whether it’s moving. This guide walks through how to read a crack, what the national damage categories say, why Perth houses crack in the first place, and when it’s worth paying an independent structural engineer to look at it.
One thing up front. Nobody can tell you from a blog post that your crack is harmless, and we won’t try. What we can do is help you decide what to watch, what to measure, and what to ask for.
What cracks in walls are actually telling you
A crack is a symptom. The wall has moved, shrunk or been pushed, and the brick, render or plaster was the weakest part. The crack itself doesn’t tell you the cause, and CSIRO’s homeowner guide on footings makes the point that symptoms “do not simply reflect the original cause”. That’s why two cracks that look alike can need completely different responses.
Still, some features are worth noting before anyone else looks at it:
- Direction. Hairline vertical cracks in render or plaster are common. Stepped cracks that follow the mortar joints diagonally through brickwork point to movement below or around the wall.
- Location. Cracks that start at the corners of windows and doors tend to show where movement is concentrated. CSIRO notes vertical cracks at openings and step cracking in mortar as typical signs of erosion under masonry.
- Shape. A crack that’s wider at one end than the other suggests one part of the wall has moved more than another. A crack of even width is harder to read without measuring it over time.
- Company. Doors or windows that started sticking, cornices pulling away, gaps opening under skirting boards. One crack alone tells you less than a crack plus a door that won’t latch.
- Change. A crack that’s the same width in March and September is a different conversation from one that’s grown.
Write these down and take photos with a ruler or tape held across the crack. An engineer who can see a dated series of photos starts from a much better place than one who sees the crack once.
If you already have photos and a rough idea of the size, tell us what you need looked at and we’ll put you in touch with an independent structural engineer who can quote for a crack assessment. You can also call (08) 5550 6237.
How wide is too wide? The CSIRO crack categories
The most useful yardstick for residential masonry is the damage classification in AS 2870, reproduced in CSIRO’s 2024 guide Foundation maintenance and footing performance. It grades damage by crack width and by what the damage does to the house.
| Category | Approx. crack width | Typical damage (paraphrased from CSIRO) |
|---|---|---|
| 0 Negligible | under 0.1 mm | Hairline cracks |
| 1 Very slight | under 1 mm | Fine cracks that don’t need repair |
| 2 Slight | under 5 mm | Noticeable, easily filled; doors and windows stick slightly |
| 3 Moderate | 5 to 15 mm, or several cracks of 3 mm or more in one group | Some wall replacement possible; doors and windows stick; pipes can fracture |
| 4 Severe | 15 to 25 mm, also depends on the number of cracks | Extensive repair; frames distort; walls lean or bulge |
The scale stops at category 4. You’ll see “category 5” quoted around the internet, but the current CSIRO table doesn’t have one.
Two things the table doesn’t do. It doesn’t tell you the cause, and it doesn’t tell you whether the crack is still moving. CSIRO’s own view is that “most cracking found in masonry walls is a cosmetic nuisance only”, and in the same guide it says monitoring cracks “should always be treated seriously”. Both are true at once. A 2 mm crack that’s been 2 mm for five years is a different animal from a 2 mm crack that was hairline last summer.
A cheap way to track it: pencil a short line across each end of the crack, write the date beside it, and photograph it with a ruler every month or two. If the crack runs past your pencil marks or opens up, you’ve got useful evidence.
Which cracks in walls to worry about in a Perth house
Here’s where the table meets real houses. These are illustrations of the patterns Perth owners describe, not records of particular jobs, and the costs quoted are the published figures from our research, not quotes.
A stepped crack above a window, two streets from the beach. Say you own a brick house in Rockingham and a crack has appeared that steps up through the mortar from the top corner of a window frame. The brickwork just above the window looks slightly lifted and there’s a rust stain. Near the coast, one cause worth ruling in or out is a corroding steel lintel: a WA building-inspection firm describes coastal WA as “the perfect rust storm” for lintels, with rust expansion cracking the brick and mortar above windows and doors. That’s a secondary source, not a standard, and a crack in that spot can have other causes too. It’s also the kind of thing an engineer wants to see in person. Our guide to rusted lintels in Perth goes into what the report should cover.
A Hills home whose back doors started sticking after a wet winter. Now picture a house in the Perth Hills. The eastern side of Perth has large areas of Guildford Formation clay, and clay sites turn up “particularly in the Perth Hills”, according to a Perth geotechnical firm. Clay swells when it gets wet and shrinks when it dries, which CSIRO says causes doors to bind and cornices to crack. The owners find three cracks of about 3 mm in a group near the laundry. On the CSIRO scale, several cracks of 3 mm or more in one group already reads as category 3, “moderate”. CSIRO also flags overwatering from badly set automatic reticulation as a common cause of saturated foundations, so the engineer will likely ask about the watering schedule, not just the walls. If the report ends up recommending underpinning or footing replacement, that work isn’t exempt from a building permit in WA. A certified building permit for a house costs 0.19% of the estimated value of the work, with a minimum of $121.00, plus the Building Services Levy ($61.65 where the work is $45,000 or less).
A 1990s brick-and-tile in the northern suburbs with a crack by the corner. Finally, a single vertical crack beside a bedroom window, near a corner where a downpipe empties onto the ground instead of into a soakwell. Most of the Perth metro area sits on sand, and CSIRO warns that sandy soil “is particularly susceptible to being washed away”, with roof water from unconnected downpipes concentrating next to the footings. A leaking sewer or stormwater pipe does the same thing out of sight. The remedies CSIRO lists here are plumbing and drainage ones: fix leaks, add grated drains, and lay a paved apron at least 900 mm wide falling at 1:60 away from the house. Whether that’s all this house needs is the engineer’s call, made after seeing the crack and the site.
None of these three is diagnosed by the description. Each one is a reason to get somebody qualified to look, with your photos in hand.
Want an engineer’s view on a crack like one of these? Send us the details through the form or call (08) 5550 6237. We’re a referral service, not an engineering firm: we pass your enquiry to an independent structural engineer, who quotes you and contracts with you directly.
Why Perth houses crack in the first place
Perth’s ground is less forgiving than it looks. Geoscience Australia’s Perth hazard report describes Bassendean Sand “over much of the central Perth region”, Tamala Limestone along the coastal strip with “numerous solution channels and cavities”, clayey Guildford Formation over much of the eastern suburbs, and clayey alluvium on the Pinjarra Plain in the Peel region. Sand, limestone and clay each move for different reasons. We don’t assign a soil class to any particular suburb, because the published mapping doesn’t do that reliably and your block can differ from the one next door.
The causes CSIRO lists that come up most in Perth:
- Water where it shouldn’t be: leaking pipes, downpipes that aren’t connected, garden reticulation soaking the ground next to the slab. CSIRO says “even a minuscule leak can be enough to saturate a clay foundation”.
- Erosion of sand under footings, often from the same water sources.
- Trees, whose roots can lift footings or dry out clay and cause shrinkage.
- Seasonal wetting and drying of clay, which makes slabs dish or dome.
- Soakwells placed too close to the house. The City of Joondalup, for example, requires a soakwell to sit at least the width of the soakwell away from buildings and boundaries so the digging doesn’t undermine footings.
Age matters too. CSIRO notes that “older masonry has little resistance” to this kind of movement, and WA has a lot of masonry. An HIA-commissioned report cited by the ABC found 68 per cent of detached houses in WA are brick, the only state above half. There’s more on this in our piece on why Perth houses crack.
Engineer’s report or building inspection?
This is where a lot of Perth owners spend money on the wrong thing.
Many products sold as a “structural inspection” in Perth are pre-purchase building inspections to AS 4349.1, done by building inspectors. They’re useful, and the published prices are easy to find: Perth firms advertise building inspection reports starting from about $440 including GST, and one quotes approximately $495 including GST for an average Perth home. But one Perth inspection firm states plainly that its service does not include structural design, engineering drawings or engineering certification. A building inspector can tell you a crack exists. They’re not the person to design a fix or certify one.
An engineer’s crack or structural report is a different product. Perth firms describe it as covering the foundation, walls and roof structure, load-bearing floors and ceilings, the slab and footings, and signs of cracking, sagging or shifting. No Perth engineering firm we checked publishes a fixed price for one. Engineers quote each job; the price depends on the house, the structure and what you need the report for, whether that’s peace of mind, a purchase, a builder dispute or an insurance claim.
Our guide to structural inspections versus building inspections sets out the difference in more detail, and our structural inspections page explains what to expect from the process. If cost is the main worry, see what a structural engineer costs in Perth, which covers what is and isn’t published.
On registration: WA began registering structural building engineers on 1 July 2024. A transition period runs until 1 July 2027, after which structural engineering work must be done by a registered building engineer. You can check an engineer’s status on the Building Engineer Register kept by Building and Energy (DEMIRS). The WA Government’s building engineering registration guide explains the scheme.
What happens after the engineer’s report
The report should give you a view on what’s causing the cracking and what, if anything, should happen next. Sometimes that’s monitoring and fixing a leaking pipe. Sometimes it’s more.
A few practical points once you’ve got it:
- The engineer assesses and designs; a registered builder builds. If the report calls for lintel replacement, crack stitching or underpinning, that’s building work for a builder to price. Underpinning and replacing footings fall outside WA’s permit exemptions, so expect a building permit.
- Insurance is its own question. Many home policies exclude gradual subsidence and ground movement, and some exclude damage from long-term leaks the owner should have known about. Check your own policy’s exclusions before assuming either way.
- Disputes have time limits. If you think a builder’s work caused the cracking, Building and Energy handles building service complaints generally up to six years after the work was completed, and you must serve a Notice of Proposed Complaint at least 14 days before lodging.
- Keep monitoring. Even after repairs, dated photos of the repaired area are cheap insurance.
For the full technical background on how footings and soil interact, CSIRO’s Foundation maintenance and footing performance guide is free and written for homeowners.
If you’ve measured your cracks and they’re growing, grouped or sitting alongside doors that now stick, that’s the point to get an engineer out. Tell us what you need looked at and we’ll pass it to an independent structural engineer for a written quote, or call (08) 5550 6237.
Frequently Asked Questions
Are hairline cracks in walls normal?
Hairline cracks under 0.1 mm are category 0, “negligible”, on the CSIRO scale, and fine cracks under 1 mm sit in category 1. That doesn’t make a particular crack harmless. If a hairline crack is new, is getting wider, or has company such as sticking doors, measure and photograph it and consider getting it assessed.
How can I tell if a crack in my wall is structural?
You can’t be sure from looking, which is the honest answer. Stepped cracks in brickwork, cracks wider at one end, cracks at the corners of openings and cracks that grow over time are signs of movement worth having an engineer assess. A structural engineer’s report looks at the walls, slab, footings and site together rather than at the crack alone.
Is a building inspection enough for a crack?
A pre-purchase building inspection records defects, including cracks, and Perth firms advertise these from about $440 including GST. It’s not an engineer’s report, and at least one Perth inspection firm says its reports don’t include engineering design or certification. If you need a cause, a repair design or a document for a dispute or insurer, you need a structural engineer.
How much does a structural engineer’s crack report cost in Perth?
No Perth engineering firm we checked publishes a price for one. Engineers quote each job based on the house, the extent of the cracking and what the report is for. Sending clear, dated photos with a ruler across the crack helps the engineer give you an accurate quote.