Why Is My Concrete Cracking? What Floor, Driveway and Wall Cracks Mean, and When to Worry
- Jul 27
- 5 min read
Almost every house in New Zealand has a crack somewhere, and almost every owner has stood looking at one wondering whether it matters.
Most cracks are minor. Concrete and buildings crack for many reasons, and many cracks are cosmetic. But some are the visible symptom of movement in the ground or structure, and the difference is not always obvious by eye.
This guide explains why concrete and buildings crack, what common crack patterns can mean, when to take them seriously, and what an assessment involves, so you can tell the ordinary from the significant without panicking or ignoring something important.
Why concrete cracks at all
The first thing to know is that concrete cracks by nature.
As concrete cures and hardens, it shrinks slightly. That can create fine shrinkage cracks in slabs, driveways and paths. This is why slabs are built with control joints: deliberate lines that encourage the concrete to crack neatly where it is expected to.
Temperature movement adds more. Concrete expands and contracts with heat and cold, and long runs of driveway, patio or path without enough joints may crack as they relieve stress.
Cracks like these are usually fine, relatively straight or map-like, stable over time, and level on both sides. They are generally about the material itself, not the ground beneath it.
When the ground is involved
The cracks that matter more often come from below. When the ground under a slab or foundation moves, the concrete has little choice but to move with it.
One common ground-related cause in New Zealand is expansive or reactive clay. These soils swell when wet and shrink as they dry. Through the seasons, that cycle can move the ground beneath slabs and foundations. It is why some cracks open in late summer and close again in winter, and why dry years are often followed by new cracking.
Large trees and their root systems near a house can make this worse by drawing moisture from clay soils and causing localised shrinkage and differential behaviour. Seasonal cracking in known clay areas is common and often manageable, but it depends on whether the foundations were designed to accommodate the ground movement.
Settlement is another major cause. Settlement is ground compressing under the load it carries. A small amount of uniform settlement is usually not a major issue. The problem is differential settlement, where one part of a building settles more than another.
Classic signs include stair-step cracking in brick or blockwork, cracks radiating from the corners of doors and windows, doors and windows starting to jam, and floors becoming noticeably out of level.
Differential settlement can be caused by soft or variable ground, poorly compacted fill, moisture changes, drainage problems, leaking pipes or erosion beneath part of the foundation.
Fill also deserves attention. If part of a house, driveway or retaining wall sits on placed fill, especially fill of unknown quality, it can keep compressing over time. The boundary between fill and natural ground is a common place for cracks to appear.
Water is another quiet culprit. Blocked drains, leaking pipes, stormwater discharging beside a foundation, or poorly managed surface water can soften or erode the ground locally. Cracking near downpipes, wet corners or ones that formed following plumbing issues often points back to water.
Earthquakes can crack buildings directly and can also affect the ground beneath them through settlement, lateral movement or liquefaction-related damage. In Canterbury, distinguishing earthquake-related ground damage from ordinary cracking has been a major issue, and it can matter for insurance.
Reading a crack: rough rules of thumb
No engineer can diagnose a crack remotely without understanding the proper context, and anyone who says they can is guessing. But some patterns can help you decide whether a crack is likely to be minor or worth checking.
Width and change matter more than the mere existence of a crack. Hairline and stable is usually less concerning. Wider cracks, cracks that are growing, or new cracks that are spreading deserve attention.
Pattern matters too. Fine map-like cracking on a slab surface often points to shrinkage. Stair-step cracking through brickwork, cracks radiating from window or door corners, or cracks running through the full depth of a slab are more likely to indicate movement.
Level matters. If one side of a crack sits higher than the other, or the crack is wider at one end than the other, something has moved. That is different from simple surface shrinkage.
Context matters as well. A crack accompanied by jamming doors, sloping floors, gaps at skirtings or cornices, cracked pipes, leaning retaining walls or ground movement outside is part of a wider pattern, and patterns are what deserve assessment.
Timing is also useful. Cracks that open and close with the seasons may point to reactive clay. Cracks that appeared after a drought, storm, earthquake, nearby excavation, new extension or plumbing leak may be telling you something about the cause.
Photographs with a coin or tape measure for scale, taken from the same position every month or two, are simple but genuinely useful. They show whether the crack is changing or staying stable.
When to take it seriously
Get an assessment rather than waiting if:
cracks are wide, growing or multiplying
doors or windows are jamming
floors have become noticeably out of level
cracking follows an earthquake, storm, flood, plumbing leak or nearby excavation
a crack runs through the full depth of a slab or through foundations, ring beams or structural walls
concrete is breaking away (spalling), exposing reinforcing steel or showing signs of rust staining
cracking appears with dropped ground, slumping, leaning retaining walls or other signs of ground movement
the property is on a slope, on known fill, or in an area of reactive clay and the cracking is new or changing
If anything about the building feels unsafe, such as visible movement, sagging, leaning or sudden change, keep people away from that area and get advice promptly.
What an assessment actually does
The point of a professional assessment is to find the cause, not just describe the crack. A repair only lasts if it deals with why the crack happened.
Depending on the situation, the assessment may involve a structural engineer reviewing the damage pattern against the building’s construction, a geotechnical engineer assessing the ground conditions, a levels survey to measure whether the building is out of level, or targeted investigation to check fill, soil type, moisture conditions or drainage.
Sometimes monitoring is the right first step. That can help establish whether movement is live or historic. This distinction matters. Some cracks are the record of movement that finished years ago and only need repair. Live movement needs the cause addressed first.
Patching a crack without knowing which situation you have is how the same crack comes back.
The outcome is usually practical: a cause, a judgement about significance, and a proportionate response. That might be cosmetic repair, drainage fixes, moisture management around reactive clay, targeted foundation work, or, where needed, designed remediation.
Serious cases do exist, but they are not the majority. Even then, understanding the cause early usually gives you more options.
Where we come in
Diagnosing cracking is core work for Cook Costello’s structural and geotechnical engineers.
It is often a two-discipline problem. The crack is visible in the structure, but the cause may be in the ground. Having structural and geotechnical engineers working together is exactly what this kind of assessment needs.
We assess cracking in homes and buildings across New Zealand, from one worrying crack to post-earthquake and storm damage. We have assessed thousands of damaged buildings after New Zealand’s major natural-hazard events, and where insurance is involved, we provide clear engineering evidence to support the claim process.
The aim is always the same: find the cause, establish whether movement is ongoing, and recommend what the situation actually calls for.
If cracking in your home or on your property is worrying you, tell us what you are seeing and when it started. Photos with something for scale help, and we can advise whether it warrants an assessment and what that would involve.
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