Concrete cracking is one of the most common concerns NZ homeowners face — whether it's a new driveway, a patio slab, or a garage floor. The good news is that not all cracks are serious. The bad news is that some are a sign of a deeper structural problem that needs professional attention. Understanding why your concrete is cracking is the first step to knowing what to do about it.
Common Causes of Concrete Cracking
Concrete cracks for a surprisingly wide range of reasons. Here are the most frequent culprits:
- Shrinkage during curing — As fresh concrete dries and loses moisture, it shrinks slightly. If control joints weren't cut or formed at the right intervals, the slab will crack where it wants to rather than where you planned.
- Poor mix design or too much water — Adding extra water to make concrete easier to work with weakens the final product and increases shrinkage. This is one of the most common causes of premature cracking in DIY or budget pours.
- Inadequate sub-base preparation — If the ground beneath the slab wasn't properly compacted, or if fill material was used without adequate settlement time, the slab can crack as the ground moves beneath it.
- Tree roots — Roots from nearby trees or shrubs can exert significant upward pressure over time, heaving and cracking slabs from below.
- Thermal expansion and contraction — NZ's climate varies considerably by region. Concrete expands in heat and contracts in cold. Without adequate expansion joints, this movement causes cracking.
- Overloading — Slabs designed for foot traffic can crack if heavy vehicles (like a skip bin truck or concrete mixer) are driven or parked on them.
- Corrosion of reinforcing steel — If moisture reaches the steel mesh or rebar inside the slab, it rusts and expands, causing the concrete above to crack and spall. This is common in coastal areas.
- Settlement or subsidence — Soil movement, poor drainage, or erosion beneath the slab can cause sections to sink and crack.
How to Diagnose Your Crack
Before reaching for the crack filler, take a few minutes to properly assess what you're dealing with.
Step 1: Measure the crack width
Use a ruler or crack gauge. As a general guide:
- Hairline cracks (under 0.3 mm) — Usually cosmetic, very common in surface concrete.
- Narrow cracks (0.3–1 mm) — Monitor these; they may be stable or slowly growing.
- Wide cracks (over 1 mm) — More likely to indicate movement or structural issues.
Step 2: Check for displacement
Run your finger across the crack. If one side is higher than the other (step cracking), the slab has moved — this points to sub-base failure or tree root heave rather than simple shrinkage.
Step 3: Monitor for growth
Mark the ends of the crack with a pencil line and date it. Check again after 4–6 weeks. A crack that is growing or widening needs professional assessment.
Step 4: Look for associated problems
Check for:
- Water pooling near or in the crack
- Crumbling or spalling concrete edges
- Multiple cracks forming a pattern (map cracking or crazing)
- Rust staining, which suggests corroding reinforcing steel inside
| Crack Type | Likely Cause | Action |
|---|---|---|
| Hairline surface crazing | Rapid drying during cure | Monitor; seal if desired |
| Straight shrinkage crack | No control joints, excess water | Fill and seal |
| Step crack (displaced) | Sub-base failure, tree roots | Call a concreter |
| Map/pattern cracking | Alkali-silica reaction or overwatering | Professional assessment |
| Rust-stained crack | Corroding reinforcing steel | Call a concreter |
| Crack with settlement | Soil movement, poor drainage | Call a concreter |
What You Can Safely Fix Yourself
For stable, non-structural cracks — ones that aren't growing, aren't displaced, and aren't associated with drainage or structural problems — a homeowner can carry out basic repairs.
Filling hairline and narrow cracks
- Clean the crack thoroughly with a wire brush and blow out dust and debris. Let it dry completely.
- Choose the right filler. For narrow cracks, a polyurethane or epoxy-based concrete crack filler works well. Products from Sika (such as their concrete repair range) are widely available at Bunnings and Mitre 10.
- Apply the filler per the manufacturer's instructions, slightly overfilling the crack.
- Smooth and finish with a putty knife or trowel before the product skins over.
- Seal the surface once the repair has cured. A penetrating concrete sealer helps prevent water ingress and future cracking.
Preventing further cracking
- Keep the area around the slab well drained — water sitting against or under concrete is a major cause of long-term damage.
- Avoid parking heavy vehicles on domestic slabs not designed for that load.
- Keep tree roots in check; consider root barriers if planting near concrete.
How to Prevent Concrete Cracking in Future Work
If you're planning new concrete work, these steps make a real difference:
- Insist on the correct mix — don't let concreters add excess water on site to make the pour easier.
- Ensure proper sub-base preparation — at least 100 mm of well-compacted AP20 or similar granular material under most residential slabs.
- Specify control joints — saw-cut or formed joints should be placed at regular intervals (generally no more than 3–4 metres apart for a standard 100 mm slab) to control where shrinkage cracking occurs.
- Curing matters — fresh concrete should be kept moist and protected from direct sun and wind for at least three days. In hot NZ summers, this is critical.
- Use appropriate reinforcing — mesh or rebar adds tensile strength and helps hold cracks together if they do form.
BRANZ publishes useful guidance on concrete construction best practice for NZ conditions.
When to Call a Professional
Call a qualified concreter if you notice any of the following:
- Displaced or step cracks — the slab has moved and the cause needs to be identified and fixed before any repair will last.
- Cracks that are growing — ongoing movement means a surface repair will just re-crack.
- Rust staining — corroding reinforcing steel requires concrete removal and proper repair, not just surface filling.
- Structural slabs — any cracking in a suspended slab, foundation, or retaining wall must be assessed by a professional. This may involve a Licensed Building Practitioner (LBP) or structural engineer. See LBP for how to verify a practitioner's licence.
- Drainage issues — if water is the underlying cause, you may also need a plumber or drainlayer. Drainlaying is restricted work under the Plumbers, Gasfitters and Drainlayers Act and must be carried out by a licensed tradesperson registered with the PGDB.
A good local concreter can assess the situation, identify the root cause, and recommend the right repair — whether that's grinding and patching, full slab replacement, or improving drainage. You can find qualified concreters in your area on tradedirectory.co.nz.
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