The cracks that matter, and the cracks that don’t

I get asked about cracks more than anything else. A buyer walks through an open home in Ripley or Redbank Plains, spots a hairline running out of a window corner, and the whole purchase suddenly feels like a gamble. Or the opposite happens — there’s a 6mm gap you can post a business card into above the front door, and nobody in the room mentions it.

After 20+ years across carpentry, construction, pest control and building inspections, here’s the honest position: most of the cracking I see in Ipswich homes is normal, expected, and specifically permitted by the Australian Standard. A small amount of it is a genuine structural problem that will cost tens of thousands to fix. The difference between the two isn’t obvious to the naked eye, and it isn’t about how ugly the crack looks.

This article is about how to tell them apart on a house you’re thinking of buying — in Ipswich specifically, because the ground we build on here behaves in a particular way.

Fine diagonal crack running from the top corner of a window frame in an Ipswich home — A fine diagonal crack from the head corner of a window. Cracks start at openings because that’s where the wall is weakest — this one is well under 1mm and, on its own, tells you very little.

Now compare it with this one, which looks far worse and matters far less.

Crack running through a concrete footpath beside a brick wall — This is a footpath. It is not the house, it is not the footings, and it is not structural. Path and driveway slabs are thin, unreinforced or lightly reinforced, poured on whatever was under them, and they crack — it’s normal. This one gets pointed at during open homes constantly.

That’s the whole problem in two photographs. The one that might matter is the one you can barely see. The one everybody photographs and sends to their conveyancer is a path.

Paths, driveways, garden edging and freestanding walls are separate elements. They have their own footings, or no footings worth speaking of, and they move independently of the house. A cracked path tells you almost nothing about the slab your bedroom is sitting on. It’s worth noting in a report — sometimes it’s a trip hazard, sometimes it’s a sign of ground movement in that spot — but it is not evidence that the house is failing, and it should never be priced as if it were.

Why Ipswich ground moves

Ipswich City Council publishes its own soils reference, the Soils of Ipswich Field Guide. It lists six major soil types across the local government area, and the first one on the list is Vertosols — cracking clays.

That’s not a marketing line. The Queensland Government describes Vertosols as brown, grey or black soils “which crack open when dry,” with a large water-holding capacity. They’re the most common soil in Queensland. And we have a locality in this city named, with no ambiguity whatsoever, Blacksoil — a name in use since the 1860s.

Reactive clay soil cracked open in the subfloor beneath an Ipswich house — This is the ground under a house. Not a paddock, not a garden bed — the subfloor. That’s reactive clay that has dried out and shrunk, and every one of those gaps closes up again when the wet comes. This is the engine that drives everything else in this article.

Most people never see this, because it lives under the floor they’re standing on. When I’m in a subfloor in February and the ground looks like that, I already know what I’m going to find upstairs.

Reactive clay does one thing: it takes on water and swells, then dries out and shrinks. Not a little. The relevant measure in the Australian Standard is characteristic surface movement — how far the ground surface would rise and fall through a full wet-dry cycle if there were no house sitting on it. On a highly reactive site that figure runs into tens of millimetres.

A house doesn’t sit on an inert platform. It sits on something that breathes. (On older stumped and timber-floored homes the same ground movement shows up differently again — we’ve covered that in our guide to bearers, floor joists and subfloor defects.)

The site classification your house was designed to

Before a slab is poured, a soil test is done and the block is given a classification under AS 2870 (Residential slabs and footings). The footing design follows from that classification. The classes, in the Standard’s own words:

Class A — “Most sand and rock sites with little or no ground movement from moisture changes”
Class S — “Slightly reactive clay sites, which may experience only slight ground movement”
Class M — “Moderately reactive clay or silt sites, which may experience moderate ground movement”
Class H1 — “Highly reactive clay sites, which may experience high ground movement”
Class H2 — “Highly reactive clay sites, which may experience very high ground movement”
Class E — “Extremely reactive sites, which may experience extreme ground movement”
Class P — Problem sites: inadequate bearing, uncontrolled fill, landslip, mine subsidence, abnormal moisture, or significant tree influence

A “-D” suffix (M-D, H1-D and so on) means deep-seated moisture change — the wetting and drying reaches further down the profile, and the footing has to be designed specifically for it rather than picked off a standard table.

The point buyers miss: a house on an H2 site isn’t a worse house than one on a Class S site. It’s a house that was designed for more movement. What matters is whether the footing matches the ground — and whether anything has changed since.

What Ipswich does and doesn’t regulate

There is no reactive-soil overlay in the Ipswich planning scheme. The Ipswich Plan 2024 overlays cover biodiversity, bushfire, character, difficult topography and landslide, and flood risk — soil reactivity isn’t among them. It’s dealt with at building approval instead, through the AS 2870 site classification and the engineer’s design certification. That’s why you’ll never find it flagged in a planning search, and why it never comes up until a buyer is standing in front of a cracked wall.

The table that settles most arguments

AS 2870 classifies damage to walls in five categories, and the QBCC reproduces the same table in its Standards and Tolerances Guide. Crack width is the primary measure:

Category	Severity	Typical crack width	What it looks like
0	Negligible	Under 0.1mm	Hairline cracks
1	Very slight	Under 1mm	Fine cracks which do not need repair
2	Slight	Under 5mm	Cracks noticeable but easily filled. Doors and windows stick slightly
3	Moderate	5–15mm	A small amount of wall may need replacing. Doors and windows stick. Service pipes can fracture. Weathertightness often impaired
4	Severe	15–25mm	Extensive repairs, breaking out and replacing sections of wall. Frames distort. Walls lean or bulge. Some loss of bearing in beams

Where the cracking is in plasterboard on a framed wall rather than masonry, the Standard allows the width limits to be increased by 50% for each category, because it’s far easier to repair.

Now the part that changes how you read a report. The QBCC’s own guidance says, in plain words:

“Category 0, 1 and 2 cracks to walls are not defects. Category 2 damage is a defect if identified within 12 months from date of completion.”

And on footings generally:

“AS 2870 … accepts that although usually no damage occurs … some slight or minor damage to walls and floors due to footing movement is possible. This slight or minor damage is not a defect.“

So a 3mm crack in a masonry wall on a ten-year-old Ripley house is, by the standard the house was built to, not a defect. That is worth knowing before you walk away from a good home — or before you accept a builder telling you a 12mm crack is “just settlement.”

Now the other end of the table.

Twenty millimetre wide crack in a plastered wall measured with a tape measure — Twenty millimetres. Category 4 on that table is 15–25mm — “severe”, the top of the scale. Note the tape: width is the measure, so measure it. An inspector guessing “she’s a fair size” is no use to you when you’re deciding whether to proceed.

That is not a filling job and it is not a conversation about paint. Category 4 in the Standard’s own words means “extensive repair work involving breaking out and replacing sections of walls”, frames distorting, walls leaning or bulging, and some loss of bearing in beams. A crack like that needs a structural engineer before you go anywhere near a contract.

And this is the same story from outside.

Stepped crack running the full height of a brick wall at the corner beside a window — Stepped cracking following the mortar joints, running the full height of the wall at the corner, with the return visibly pushed out of line. Stepped cracks are the signature of footing movement — the wall is failing along its weakest path, which is the joints, and it’s telling you one part of the building has moved relative to another.

Compare that with the hairline at the top of this article. Same house type, same city, same standard — and a completely different conversation.

One more thing worth understanding about how these houses are designed. The QBCC’s subsidence policy states that foundation movement is assessed at “the level which has less than a 5% chance of being exceeded in the life of the structure which may be taken as 50 years.” The design target was never no movement. It was movement inside an envelope.

Diagonal crack running from a door head corner up toward the ceiling — Same story at a door head. What I’m measuring here isn’t just the width — it’s whether the crack is uniform, whether it’s opened at one end, and whether the frame it runs from is still square.

Heave, settlement, and why the direction matters

Three different things get called “cracking” and they have completely different consequences.

Settlement is downward movement under the weight of the building — soil compacting or consolidating. CSIRO’s homeowner guide makes the key diagnostic point: “In the event of construction settlement, cracking will usually remain unchanged after the process of settlement has ceased.” Settlement cracks stabilise. They stop.

Shrinkage or subsidence is downward movement because the soil has lost moisture and reduced in volume — drought, tree roots, a drainage change.

Heave is upward movement because the soil has taken on water and swelled. And heave is the one that cycles. It doesn’t stop. It opens in one season and closes in another.

Which part of the slab moves tells you where the water is:

Edge heave — the perimeter lifts relative to the middle, and the floor reads as dished, low in the centre. The soil under the middle of a house stays at a fairly stable moisture content because the house itself covers it. The edges are what get rained on, watered, and leaked on. CSIRO describes the early signs precisely: “the floor appears slightly dished,” with “some doors binding on the floor or the door head, together with some cracking of cornice mitres,” and externally “visible dishing of the hip or ridge lines.”
Centre heave — the middle rises relative to the perimeter, and the floor reads as domed. This happens as moisture slowly migrates inward under the slab over years, or as the perimeter dries out and drops, classically from tree roots.

Here’s a detail relevant to us specifically. CSIRO notes that “in areas where warm, wet summers and cooler dry winters prevail, water migration tends to be toward the interior and doming will be accentuated.” That is south-east Queensland’s climate exactly. Our default bias is toward centre heave — the domed floor, not the dished one.

On site, the tools are unglamorous: a long straightedge or a laser level across the floor, a crack gauge, and doors and windows tested one by one. Then I go looking for the cause outside, because the cause is almost always outside.

What actually causes it — and it’s usually water management

Reactive clay is the precondition. Water is the trigger. In the field, the same handful of things turn up again and again.

Disconnected downpipe discharging roof water at the base of an external wall — A downpipe that stops short of anything. Every storm, this discharges concentrated roof water straight into the soil at the slab edge. This is the single most common cause I find behind perimeter movement.

CSIRO puts it bluntly: “Downpipes not positively connected to a proper stormwater collection system will direct a concentration of water to soil.” A single roof plane can deliver thousands of litres in one storm to one spot.

Probe rod inserted into a void washed out beneath a concrete slab edge — This is what that water does. That rod is disappearing into a void that’s been washed out under the edge of the slab. From standing height at the open home, this looks like an ordinary path.

Plumbing leaks are the quiet one. CSIRO again, and this is the line I’d underline for any buyer: “Even a minuscule leak can be enough to saturate a clay foundation.” A weeping joint under a slab does not announce itself. It shows up as a floor that isn’t level any more.

Then there’s the garden. Overwatering near the building, garden beds hard against the slab edge, and irrigation systems running on a timer are all common culprits — CSIRO calls misuse of automatic watering systems “a common cause of saturation and water migration under footings.” And it makes the point that the soil affecting your footings extends well beyond the building line.

Trees work the other way — they dry the soil out and pull the edge down. The QBCC requires builders to identify “trees on-site or adjoining properties within mature height distance from the building,” which gives you the working rule of thumb: a single tree within roughly one times its mature height of the house is close enough to matter. Groups of trees need more distance again. And removing a big established tree isn’t automatically the fix — the soil then rebounds as it re-wets, which is its own problem. That’s a decision for an arborist and an engineer, not a chainsaw on a Saturday.

Reading the inside of the house

Cracks start at openings, because that’s where the wall is weakest. Diagonal cracks from window and door corners, cracks along cornice mitres, and cracks at wall-to-ceiling junctions are the classic set.

Crack opening along a cornice mitre at the wall and ceiling junction — Cornice mitres opening up. CSIRO lists this alongside binding doors as one of the first noticeable symptoms of edge movement.

What I’m reading, beyond width:

Is it fresh or old? Paint bridging the crack, or dust and cobwebs inside it, means it’s been there a while. A clean edge with paint flakes on the floor beneath it means recent movement.
Has it been filled before — and has it come back? Filler cracked through the middle is the most useful thing in the room. It means someone repaired this and the house kept moving.
Do the doors and windows agree with the cracks? Cracking with square, freely operating doors is a different story to cracking with three doors that won’t latch.
Is it consistent with a pattern, or is it isolated? A single crack in one room usually isn’t foundation movement. A pattern that runs through the plan is.
Is the floor level? This is the one most buyers never check and it’s the most informative single test in the house.

The Ipswich curveball: it isn’t always clay

Not every cracked house in this city is a reactive soil problem. Ipswich has a genuine second cause that almost nobody writes about: historic underground coal workings. Collingwood Park sits above the decommissioned Westfalen No. 3 and New Redbank collieries, and there were documented subsidence events in 1988 and again in April 2008. The Queensland Government legislated a guarantee to repair subsidence damage to affected properties or purchase them at market value, and monitors the ground with seismic stations and extensometers. Current advice is that the ground is stable.

It’s a small area and it shouldn’t frighten anyone off the suburb. But it’s a reminder that “the house is cracked” has more than one possible answer here, and the answer changes what you do about it.

What to actually do about it as a buyer

1. Don’t panic over Category 0–2. Under 5mm in masonry, with square doors and a level floor, is normal for a house that’s been through a few Queensland summers. It is explicitly not a defect.

2. Take Category 3 and above seriously. Over 5mm, or a group of cracks 3mm and up, or cracking with doors and windows that have gone out of square — that needs a structural engineer, not an opinion.

3. Look for the cause before you price the repair. If the cause is a disconnected downpipe and a garden bed against the slab, that’s a few hundred dollars and the movement may well settle down. If the cause is a leaking sewer under the slab or a mature tree three metres from a corner, that’s a different conversation.

4. If the house is under warranty, know your clocks. In Queensland, structural defective work can be pursued with the QBCC up to 6 years and 6 months from completion; non-structural is far shorter, around 12 months. The clock for a QBCC insurance claim, for a direction to rectify, and for the statutory warranties in the QBCC Act are three separate things with three separate timeframes — and they’re all shorter than people assume. If a house you’re buying is a few years old and cracking, check the dates before you do anything else. (If you’re still mid-build, stage inspections are how you catch footing and slab problems while the builder is still on the hook for them.)

5. Monitoring beats guessing. Where movement is genuinely uncertain, tell-tale gauges get fitted across the cracks and read over time. A Brisbane forensic engineering firm puts the minimum honestly: “A minimum of 12 months is essential… Without 12 months of data, cyclical movement can be mistaken for progressive cracking, leading to unnecessary remediation.” Twelve months captures a full wet and dry cycle. Anything less and you’re reading a season, not a house.

6. Understand the cost gap. Fixing drainage is hundreds. Underpinning or re-levelling a house is commonly in the tens of thousands, and severe cases run well beyond that. Those are indicative field figures rather than a quote — but the gap between “manage the water” and “underpin the corner” is the single biggest number riding on getting the diagnosis right.

If you own the house already

Most of the damage I see was preventable with maintenance nobody told the owner about. CSIRO’s guidance is specific and cheap to follow:

Get every downpipe positively connected into the stormwater system, and get the discharge away from the building.
Install paving around the perimeter, extending out at least 900mm — more on highly reactive soil — with a minimum fall away from the building of 1:60.
Keep garden beds, and the watering that goes with them, back from the slab edge. Lawn and light-watering plants closest to the house; thirsty plants and trees furthest away.
Turn off or move automatic irrigation near the footings.
Fix leaking taps, hot water overflows and plumbing promptly. The minuscule leak is the one that gets you.

The goal isn’t dry soil. It’s evenly moist soil — the same all the way around, all year. Differential moisture is what cracks houses.

The bottom line

Reactive clay is not a defect in an Ipswich house. It’s the ground this city is built on, and the building code has a whole system for dealing with it. The houses that get into trouble are the ones where something changed — a downpipe came apart, a pipe started weeping, a garden bed went in against the slab, a tree got big, or the footing was never designed for the ground underneath it.

That’s a diagnosis, and it’s one you want made before you sign, not after. Every inspection I do includes a level assessment of the floors, crack measurement and categorisation against AS 2870, a full external drainage assessment, and thermal imaging where concealed moisture is suspected. You get the report the same day, and you get me on the phone to walk through what it means.

If you’re buying in Ipswich, Ripley, Springfield Lakes, Redbank Plains, Deebing Heights or anywhere across the corridor and there’s cracking you can’t read, book an inspection and let’s find out what’s actually driving it.