Acceptable Cracks in a Concrete Slab

In Victoria the published threshold for a slab-on-ground system is 2 mm. The Building and Plumbing Commission's Guide to Standards and Tolerances 2026 states that cracks in slab-on-ground systems "may be considered outside tolerance if they are equal or greater than 2 mm in width", and that cracks below 2 mm should be monitored across 12 months to confirm they have stabilised.

That is the number, and it is not the same number as the one for walls. This guide covers what the thresholds are, how to read a crack properly, and where the assessment stops being yours.

The published width thresholds

Two separate tolerances apply, and conflating them is the most common error in this area.

The guide applies from 1 August 2026 and sets the benchmarks finished domestic building work is assessed against in a dispute.

Two practical points follow. A 3 mm crack in a slab is outside tolerance while a 3 mm crack in a brick wall above it is not, so the element has to be identified before the measurement means anything. And where movement continues below the threshold, or a crack progresses up to it, further monitoring applies rather than an immediate remedy.

There is a related set of figures worth knowing, because slab complaints are often really level complaints. The same guide sets slab level tolerances within the first 24 months: not within tolerance if the slab differs in level by more than 10 mm in any room or area, 4 mm in any 2 m length, or 20 mm between any two points across the building footprint.

Step one: measure, do not estimate

Use a crack width gauge and record the measurement in millimetres. "Hairline", "fine" and "significant" are not assessments, and they are the reason most crack disputes go nowhere.

Measure at the widest point of the crack, not at an average point, and measure more than one location along its length.

A crack that is 1 mm at one end and 3 mm at the other is telling you something a single reading hides. Photograph every measurement with the gauge or a rule in frame, and date it. Note recent rainfall, because on reactive clay the weather in the preceding weeks is part of the evidence.

Step two: classify what you are looking at

Width tells you whether the crack is inside a published tolerance. It does not tell you what caused it, and the cause decides what happens next.

Step three: read the pattern, not just the width

1. Plastic shrinkage cracking

Short, random, often parallel cracks appearing within hours of the pour, while the concrete is still plastic. Caused by the surface losing water faster than the concrete can bleed it, which hot or windy conditions accelerate. Usually shallow and usually cosmetic, though it is a curing failure rather than bad luck.

2. Drying shrinkage cracking

Concrete shrinks as it dries and cures. Where that shrinkage is restrained, it cracks. These appear over weeks to months, tend to be fairly uniform in width along their length, and often run between restraint points or from re-entrant corners.

3. Thermal cracking

Cracks that open and close with daily and seasonal temperature swings, more common in long unjointed elements. The giveaway is that the width changes with the temperature rather than with the season's rainfall.

4. Settlement and differential movement cracking

Cracks that widen progressively and are often wider at one end than the other. On a slab these can run diagonally across the footprint, and they usually appear alongside doors binding, floors falling to one corner, or matching cracking in the walls above.

5. Structural cracking

Cracking associated with load rather than volume change, which can present as a crack through the full slab depth, displacement across the crack face, or cracking at a location where the design did not anticipate it. This is the category where measurement stops and an engineer starts.

The single most useful distinction is direction of travel. A crack that reverses, opening in dry months and closing after rain, is a moisture problem. A crack that only ever opens and keeps opening is a support problem.

Step four: monitor across a full seasonal cycle

The guide is explicit that cracking below the threshold is monitored over 12 months, and there is a reason the period is that long. On reactive clay the ground swells and shrinks with soil moisture across the year.

A single reading, or two readings a fortnight apart, tells you nothing about whether movement is cyclic or progressive. Twelve months of dated readings tells you which one you are dealing with.

What to record: the width at fixed marked points, the date, recent rainfall, and a photograph with a scale in frame.

Mark the measuring points on the slab so successive readings are taken at the same place. Where movement continues at the end of the period, or a crack reaches the threshold during it, the guide contemplates a further period of observation rather than an automatic remedy.

Step five: rule out water before you rule in movement

On Melbourne's reactive clay, most movement that looks structural is moisture driven, and a good share of that is a fixable water source rather than the seasonal cycle.

Work through the water before concluding anything about the footing:

• Check the meter with all fixtures closed, to find a leaking water service
• Walk the stormwater and check every downpipe discharge point
• Check the perimeter falls, and whether ground falls toward the building
• Look for damp brickwork, or localised cracking near a wet area
• Note any mature tree within roughly its own height of the affected corner
• Check irrigation lines and garden bed watering against the perimeter

Because the driver is water, minimising foundation movement is largely about controlling where water goes around the finished building, a point the Building and Plumbing Commission makes directly.

Fixing a leak and re-monitoring costs a fraction of a remedial scope, and it is the step most often skipped.

Step six: know where your assessment stops

Measuring, photographing and monitoring is evidence gathering. Assessing structural adequacy is an engineer's call, and recommending a remedy without that assessment carries real liability.

Stop and engage a structural engineer when any of these apply:

• A crack in the slab reaches or exceeds 2 mm
• Cracking widens across successive seasons rather than reversing
• Floors are measurably out of level, or falling toward one corner
• There is displacement across the crack face, or the crack runs full depth
• Movement cannot be attributed to any water source you can find
• Cracking in the slab is matched by cracking in the walls above it

Why slabs crack at all, and which causes are normal

Concrete shrinks as it cures and it has low tensile strength, so some cracking is an expected characteristic of the material rather than a defect. Reinforcement does not prevent cracking; it controls where cracks form and how wide they get. That is why the published thresholds exist.

They separate the cracking that comes with the material from the cracking that indicates something has gone wrong, and they do it with a number rather than an opinion. Curing is the biggest controllable factor. A slab cured properly cracks less than an identical slab left to dry out, and curing is also the cheapest stage of the work to get right.

What to record so an engineer can act on it

Hand over evidence, not impressions:

• Width readings in millimetres at fixed, marked points, each dated
• Photographs with a gauge or rule in frame, dated
• A sketch or mark-up showing where each crack runs across the slab
• Floor level readings across the floor plate
• Rainfall notes for the period covered
• The building's age, construction type and footing system where known
• Any previous remedial work, and any excavation on the site or next door
• The site classification and the geotechnical report if you have them

Twelve months of that costs a few minutes a month and turns an argument into an assessment.

When a crack means the footing, not the slab

A slab crack that widens progressively, runs diagonally across the footprint and appears alongside matching wall cracking is usually reporting on what is underneath it rather than on the slab itself.

At that point the question moves from the slab to the footing system, and the threshold question becomes whether the movement is progressive or seasonal.

Whether a house actually needs underpinning turns on exactly that distinction, because a footing rising and falling with the seasons has not lost its support and underpinning will not stop the cycle.

Where the footing design itself is in question, what the footing was required to do under its site classification is the starting point for the engineer's assessment.

Frequently asked questions

1. What is acceptable cracking in concrete in Australia?

In Victoria, the Building and Plumbing Commission's Guide to Standards and Tolerances 2026 states that cracks in slab-on-ground systems may be considered outside tolerance at 2 mm or greater, and cracks in masonry walls at 5 mm or greater. Below those widths, the crack is monitored over 12 months to confirm it has stabilised rather than repaired immediately.

2. How big of a crack is acceptable in concrete?

For a slab-on-ground system the published Victorian threshold is under 2 mm. A crack at or above 2 mm may be considered outside tolerance. The figure differs by element, so a 3 mm crack is outside tolerance in a slab but not in a masonry wall, where the threshold is 5 mm.

3. Is it normal for a concrete slab to crack?

Some cracking is expected. Concrete shrinks as it cures and has low tensile strength, so it cracks where shrinkage is restrained. Reinforcement controls where cracks form and how wide they get rather than preventing them. What separates normal cracking from a defect is width against the published tolerance, and whether the crack is stable or progressive.

4. What are the acceptable crack tolerances for concrete in Australia?

The tolerances that get applied in practice are published at state level for domestic building work. In Victoria the current benchmarks are in the Building and Plumbing Commission's Guide to Standards and Tolerances 2026, which applies from 1 August 2026 and gives 2 mm for slab-on-ground systems and 5 mm for masonry walls, with 12-month monitoring below those widths.

5. How long should I monitor a crack before acting?

Twelve months, which is what the guide specifies for cracking below the threshold. The period covers a full seasonal cycle, and on reactive clay that is the only way to tell cyclic movement from progressive movement. If movement continues at the end of the period, or the crack reaches the threshold during it, further observation applies.

6. Do cracks in a slab mean the footing has failed?

Not usually. Most slab cracking is shrinkage, and most movement-related cracking on Melbourne clay is seasonal moisture change or a fixable water source. A footing problem is indicated when cracking widens across successive seasons without reversing, floors are out of level, and no water source explains it. That combination is an engineer's assessment, not a builder's.

7. Can a cracked slab be repaired, or does it need replacing?

It depends entirely on the cause and whether movement has stopped. Stable shrinkage cracking is generally a sealing or filling job. Cracking driven by ongoing movement will return through any repair, because the repair does not address the mechanism. That is why the cause and the direction of travel get established before anyone prices a remedy.

Measure it, date it, then get it assessed

Nearly every crack dispute comes down to the same thing: nobody measured anything and nobody wrote the date down. The thresholds are published, the monitoring period is published, and both are easy to work to.

Urban Pour pours reinforced concrete slabs and structural foundations across Melbourne. If an engineer has assessed a slab or footing and there is remedial concrete work to scope, send through the report and we will tell you what it involves.