Mortar that shrinks and cracks after drying is one of the most common issues homeowners notice on brick, block, and stone work in New Zealand. The good news is that in most cases it's a straightforward repair — known as repointing — that a confident DIYer can tackle on a small scale. For larger areas or structural concerns, a qualified bricklayer is your best bet.
Why Does Mortar Crack and Shrink?
Understanding the cause helps you avoid the same problem after your repair. Common reasons include:
- Too much water in the mix — a wet mix is easier to work with but shrinks more as it dries.
- Incorrect mix ratio — too much sand or cement out of balance weakens the mortar.
- Rapid drying — hot sun, wind, or low humidity pulls moisture out before the mortar cures properly.
- Using the wrong mortar type — hard Portland cement mortars used on older, softer bricks can crack as the wall moves slightly.
- Movement in the substrate — minor settlement, thermal expansion, or moisture movement in the wall can open joints over time.
- Poor surface preparation — repointing over dusty or loose joints without proper cleaning leads to poor adhesion and cracking.
In New Zealand's variable climate — from the humid north to the freeze-thaw conditions in the South Island — mortar joints take a real beating. Freeze-thaw cycles in particular can accelerate cracking in exposed joints.
Assessing the Damage
Before you start mixing, take a good look at what you're dealing with:
| Situation | Likely Action |
|---|---|
| Hairline cracks in a few joints | DIY repointing |
| Crumbling or hollow-sounding mortar | Rake out and repoint |
| Cracks running through bricks, not just joints | Possible structural issue — get a professional assessment |
| Large sections of mortar falling out | Consider a bricklayer for efficiency and quality |
| Cracks with water ingress or damp patches | Investigate moisture source first |
If cracks are running through the bricks themselves, or if you're seeing stair-step cracking across multiple courses, this can indicate foundation movement or structural issues. In that case, stop and consult a Licensed Building Practitioner or structural engineer before doing any repair work.
What You'll Need
- Cold chisel and club hammer (or an angle grinder with a mortar raking disc for larger jobs)
- Wire brush and vacuum
- Pre-mixed repointing mortar or the ingredients to mix your own (cement, lime, and sand)
- Pointing trowel or hawk and trowel
- Spray bottle with water
- Safety glasses and dust mask (P2 rated — mortar dust contains silica)
You'll find repointing mortar and tools at Bunnings, Mitre 10, or PlaceMakers.
Step-by-Step: How to Repoint Cracked Mortar
1. Rake Out the Old Mortar
Using a cold chisel and hammer, carefully remove the cracked mortar to a depth of at least 15–20 mm. Don't go so deep that you risk loosening the bricks. For long runs, a mortar raking disc on an angle grinder speeds things up — but always wear eye protection and a P2 dust mask. Silica dust is a serious health hazard; check WorkSafe NZ guidance on working safely with masonry dust.
2. Clean the Joint
Brush out all loose material with a wire brush and vacuum or blow out the dust. Dampen the joint with a spray bottle — this stops the dry brick from sucking moisture out of your new mortar too quickly, which is a leading cause of shrinkage cracking.
3. Mix Your Mortar
For most modern brick and block work, a 1:1:6 mix (1 part cement : 1 part lime : 6 parts sand) is a good general-purpose repointing mortar. For older, softer bricks (pre-1950s), use a softer, lime-rich mix — something like 1:2:9 — to avoid trapping moisture and damaging the brick face. If in doubt, ask at your local building merchant or check BRANZ guidance.
Add water gradually. The mix should hold its shape when squeezed but not be sloppy or wet.
4. Apply the New Mortar
Pack mortar firmly into the joint in layers if the joint is deep, allowing each layer to firm up slightly before adding the next. Finish the joint to match the existing profile — weathered, flush, or raked — using your pointing trowel.
5. Cure Slowly
This is where most DIY repairs go wrong. Keep the new mortar damp for at least 3–4 days by misting with water, especially in hot or windy weather. Avoid working in direct sun or on days above 30°C if possible. Shade the area with hessian or shade cloth if needed.
Preventing Cracks in Future Repairs
- Never add extra water to a stiffening mix — discard it and mix fresh.
- Work in the early morning or evening in summer.
- Match the mortar strength to the brick type.
- Ensure good drainage and weatherproofing around the wall to reduce ongoing moisture movement.
When to Call a Professional
Some situations are best left to a qualified bricklayer:
- Large areas of failed mortar across an entire wall or chimney
- Any sign of structural cracking through bricks or blocks
- Heritage or character brick buildings where matching mortar colour and profile matters
- Repointing at height (scaffolding and working at heights carry their own safety requirements under WorkSafe NZ guidelines)
- Retaining walls, which may have specific requirements under the NZ Building Code
A skilled bricklayer will also be able to identify the root cause of the cracking and advise whether any consent is needed for remedial work. You can find qualified local bricklayers on tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
