A concrete step cracking away from the house is one of the most common exterior maintenance issues for NZ homeowners. The good news is that many of these repairs are well within DIY reach, provided the damage is limited to the joint between the step and the foundation or wall — rather than structural failure of the step itself. Here's how to assess the problem and fix it properly.
Why Does This Happen?
Concrete steps often separate from the house due to a combination of factors common in New Zealand:
- Soil movement and settlement — especially in clay-heavy soils that shrink and swell with moisture changes
- Frost heave — in colder regions like Canterbury, Otago, and Central Plateau, ground freezing can push steps outward
- Poor original construction — steps not tied into the foundation or built on uncompacted fill
- Water ingress — water getting into the joint, freezing, and expanding the crack over time
- Age and weathering — concrete naturally shrinks slightly as it cures and ages
Understanding the cause helps you choose the right fix and prevent it recurring.
Assessing the Damage
Before you reach for the filler, take a close look at what you're dealing with:
| Damage Type | Likely Fix | DIY Suitable? |
|---|---|---|
| Narrow crack at joint (under 10 mm) | Sealant or mortar repair | Yes |
| Wide gap (10–25 mm) | Backer rod + flexible sealant | Yes |
| Step tilting or sinking | Re-bed or rebuild | Possibly — assess carefully |
| Crumbling concrete on step surface | Resurfacing compound | Yes |
| Structural cracking through the step | Rebuild required | Call a builder |
If the step has shifted significantly, is rocking, or the concrete itself is crumbling through its full depth, it may need to be rebuilt rather than patched.
Tools and Materials You'll Need
- Cold chisel and hammer (or angle grinder with diamond blade)
- Wire brush and stiff broom
- Vacuum or compressed air
- Concrete bonding agent (e.g. Sika or Ardex products)
- Flexible polyurethane or silicone sealant rated for exterior/concrete use
- Backer rod foam (for gaps wider than about 10 mm)
- Rapid-setting concrete repair mortar or resurfacing compound
- Safety glasses, gloves, and dust mask
Products from Sika and Ardex are well-regarded in the NZ trade for concrete repair. You'll find suitable products at Bunnings, Mitre 10, or PlaceMakers.
Step-by-Step Repair Guide
1. Prepare the Joint
Use a cold chisel and hammer to remove any loose or crumbling concrete from the crack edges. The goal is to create a clean, stable surface for the repair material to bond to. Undercut the edges slightly if possible — this gives the repair a mechanical key. Brush out all dust and debris, then vacuum thoroughly. Allow the area to dry completely before proceeding.
2. Apply a Bonding Agent
For mortar-based repairs, brush a concrete bonding agent onto both surfaces of the crack. Follow the manufacturer's instructions — most require you to apply the repair mortar while the bonding agent is still tacky.
3. Fill the Gap
For gaps under 10 mm: Pack with a stiff concrete repair mortar, pressing it firmly into the joint. Smooth flush with a trowel and allow to cure as directed.
For gaps 10–25 mm: Insert a foam backer rod to the correct depth first (the rod should sit about 6–10 mm below the surface). This gives the sealant the right shape to flex without failing. Apply a flexible polyurethane sealant over the top and tool it smooth.
For larger voids: Build up in layers using rapid-setting repair mortar rather than trying to fill in one go. Large single pours can crack as they cure.
4. Seal and Protect
Once the repair has cured, consider applying a concrete sealer over the step surface. This helps repel water and reduces future cracking. Check the product is suitable for exterior use and slip-resistant when wet — safety on steps is important.
5. Address the Root Cause
If water is pooling near the step, improve drainage by regrading the surrounding ground away from the house. Check that downpipes and spouting are directing water well clear of the area. Ongoing water ingress will undo even the best repair.
NZ Building Code Considerations
Most like-for-like concrete step repairs are exempt from building consent under Schedule 1 of the Building Act — but always confirm with your local council if you're unsure, particularly if you're altering the size or structure of the step. Steps must also comply with MBIE Building Performance requirements for safe access, including appropriate riser heights and a stable, slip-resistant surface. If your step serves as the primary entrance, make sure the repair restores it to a safe, compliant condition.
When to Call a Professional
Call a qualified builder if:
- The step has sunk, tilted, or shifted significantly from its original position
- The concrete is cracked through its full depth or crumbling structurally
- The step is attached to a retaining wall or other structural element
- You're unsure whether the movement is ongoing (active soil settlement)
- The step is part of a larger concrete structure like a porch or landing
A Licensed Building Practitioner (LBP) can assess whether the movement is ongoing and advise on the best long-term solution. You can find qualified local builders on tradedirectory.co.nz.
Safety Note
Always wear safety glasses and gloves when cutting or chiselling concrete. If using an angle grinder, add a dust mask rated for silica dust — concrete dust contains crystalline silica, which is a serious respiratory hazard. See WorkSafe NZ for guidance on working safely with concrete.
Registered Master Builder
Licensed Building Practitioner (LBP)
