Yes, as a homeowner you can carry out a basic visual inspection of your concrete floor slab to check for cracking — and it's a smart thing to do periodically. However, interpreting what you find, especially whether cracks are structural or cosmetic, often requires the trained eye of a qualified building inspector or structural engineer. Here's how to go about it safely and what to look for.
Why Concrete Slabs Crack
Concrete cracking is extremely common and not always cause for alarm. Slabs crack for a range of reasons:
- Shrinkage cracking — normal as concrete cures and dries, usually fine and shallow
- Settlement or subsidence — the ground beneath the slab shifts or compresses unevenly
- Heave — expansive soils (common in parts of NZ with clay-rich ground) push upward
- Overloading — heavy point loads from machinery, vehicles, or stacked materials
- Poor construction — inadequate reinforcing, incorrect mix design, or missing control joints
- Tree root intrusion — roots from nearby trees lifting or displacing the slab
- Moisture and drainage issues — water eroding the sub-base over time
Understanding the likely cause helps you judge severity. BRANZ has useful guidance on concrete slab performance and common defects in NZ construction.
What You'll Need for a DIY Inspection
You don't need specialist tools for a basic check. Gather:
- A torch or headlamp for low-angle raking light (this makes cracks far more visible)
- A tape measure or steel ruler
- A crack width gauge or a credit card (roughly 0.8 mm thick) as a rough comparator
- A notepad or phone to photograph and record findings
- Chalk or masking tape to mark and date cracks so you can monitor them over time
How to Inspect Your Slab Step by Step
1. Clear the Area
Move furniture, rugs, and coverings so you can see the full slab surface. Check under floor coverings where possible — vinyl, carpet, and tiles can hide significant cracking.
2. Use Raking Light
Shine your torch at a low angle across the slab surface. This technique highlights even fine cracks that are invisible under overhead lighting.
3. Map and Measure Every Crack
Record the location, length, width, and direction of each crack. A simple sketch or photos with measurements is ideal. Pay particular attention to:
- Cracks wider than 0.3 mm (roughly the thickness of a credit card edge)
- Cracks wider than 1 mm — these warrant professional assessment
- Cracks that run diagonally, especially from corners of door openings or columns
- Cracks that appear to follow a pattern across a large area
4. Check for Differential Movement
Run your hand or a straight edge across each crack. If one side of the crack is higher or lower than the other (called a step crack or differential settlement), this is a more serious sign than a flat crack of the same width. Even a 1–2 mm step deserves attention.
5. Check for Active vs Dormant Cracks
Mark the ends of cracks with chalk and note the date. Re-inspect after 4–8 weeks. Active cracks that continue to grow or widen indicate an ongoing problem; dormant cracks that stay the same size are generally lower risk.
6. Look at the Perimeter and Edges
Check where the slab meets walls, piles, or footings. Separation or cracking at these junctions can indicate foundation movement and should be assessed professionally.
Crack Severity at a Glance
| Crack Width | Likely Classification | Recommended Action |
|---|---|---|
| Hairline (< 0.1 mm) | Cosmetic / shrinkage | Monitor; generally no action needed |
| 0.1–0.3 mm | Minor | Monitor; seal if moisture ingress is a concern |
| 0.3–1 mm | Moderate | Investigate cause; consider professional assessment |
| > 1 mm | Significant | Professional inspection recommended |
| Any width with step | Potentially structural | Professional inspection required |
NZ Building Code Considerations
Under the NZ Building Code, concrete floor slabs must meet performance requirements for structural stability and durability. If your home was built under a building consent, the slab design should have complied with the relevant standards at the time. Significant cracking or settlement may indicate the slab is no longer performing as intended, which could have implications for your home's compliance and insurance. If you're buying or selling a property, a pre-purchase building inspection is strongly recommended.
When to Call a Professional
Contact a licensed building inspector or structural engineer if you find:
- Any crack wider than 1 mm
- Step cracks or differential movement across a crack
- Cracks that are actively growing
- Multiple cracks forming a pattern (e.g., a grid or radiating from a point)
- Cracking accompanied by sticking doors, sloping floors, or wall cracks
- Any cracking near load-bearing walls, columns, or piles
- Cracking in a slab that also serves as a foundation (slab-on-ground construction)
A qualified building inspector can assess whether the cracking is cosmetic or structural, identify likely causes, and recommend remediation. For suspected structural issues, a chartered structural engineer (CPEng) should be engaged. You can find qualified local building inspectors and structural engineers on tradedirectory.co.nz.
For guidance on what building inspectors look for and what constitutes restricted building work, visit Licensed Building Practitioners and MBIE Building Performance.
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