Using a straight cement mix instead of proper mortar for bricklaying is a mistake that many DIYers make — and it can lead to cracked bricks, failed joints, and costly repairs down the track. Proper brick mortar is a carefully balanced mix of cement, sand, and often lime, designed to bond bricks while still allowing a small amount of movement and moisture management. Here's what you need to know before you mix up a batch.
What's the Difference Between Cement Mix and Mortar?
Cement (typically Portland cement) is the binding agent — it's just one ingredient. Mortar is the finished product made by combining cement with sand and water, and often with hydrated lime added to improve workability and flexibility.
A straight cement-and-water mix (or even cement with coarse aggregate, which is closer to concrete) is far too rigid and strong for most brickwork. When it cures, it becomes harder than the bricks themselves, which means any movement — from temperature changes, ground settlement, or moisture — causes the bricks to crack rather than the joint. In bricklaying, the mortar joint is designed to be the sacrificial element: it's much easier to repoint a joint than to replace a cracked brick.
Why Mortar Needs to Be Weaker Than the Brick
This is the key principle most DIYers miss. Mortar should always be softer and weaker than the masonry unit it's bonding. If the mortar is stronger, stress concentrates in the brick face, leading to spalling and cracking — especially in NZ's variable climate where freeze-thaw cycles (in colder regions like Canterbury, Otago, and the Central Plateau) and coastal salt exposure put real pressure on masonry.
BRANZ research consistently highlights that incorrect mortar specification is one of the leading causes of premature masonry failure in New Zealand homes.
Correct Mortar Mixes for Common Bricklaying Jobs
Mortar mixes are described by the ratio of cement : lime : sand by volume. Here's a general guide for common applications:
| Application | Cement | Lime | Sand | Notes |
|---|---|---|---|---|
| General brickwork (sheltered) | 1 | 1 | 6 | Good flexibility, easy to work |
| General brickwork (exposed) | 1 | 0.5 | 4–4.5 | Slightly stronger for weather exposure |
| Retaining walls / engineering brick | 1 | 0 | 3 | Higher strength, less flex |
| Repointing older soft brick | 1 | 2 | 8–9 | Very soft — critical for heritage brick |
Note: These are general guides only. Always check the brick or block manufacturer's recommendations, as some products specify particular mortar grades. Confirm with a qualified bricklayer or BRANZ if you're unsure.
The Role of Lime
Hydrated lime (not agricultural lime or quicklime) is what gives mortar its workability, water retention, and slight flexibility after curing. It also has mild self-healing properties — tiny hairline cracks can re-carbonate and seal over time. For most residential brickwork, including a lime component is strongly recommended. You'll find bagged hydrated lime at Bunnings, Mitre 10, and PlaceMakers.
If you'd rather skip the batching, pre-mixed mortar products are available from the same suppliers — just make sure the product is specifically labelled for bricklaying or masonry, not for tile adhesive or general patching.
Mixing Tips for DIYers
- Use clean, well-graded bricklaying sand (also called plasterer's sand or brickie's sand) — not beach sand, which contains salt, and not coarse concrete sand.
- Mix dry ingredients thoroughly before adding water.
- Add water gradually — mortar should hold its shape when squeezed but not be crumbly or sloppy.
- Mix only what you can use in about 60–90 minutes; don't add water to re-temper mortar that has started to stiffen.
- In hot or windy weather, dampen bricks slightly before laying to prevent them drawing moisture out of the mortar too quickly.
NZ Building Code Considerations
If your bricklaying project involves structural masonry — such as a load-bearing wall, a retaining wall over 1.5 m, or any work attached to your home — it may require a building consent from your local council. New Zealand's Building Code and standards such as NZS 4210 (Masonry Construction) set out requirements for mortar specification in structural applications. Always check with your council before starting, as Schedule 1 exemptions have limits and conditions that vary by project type.
For non-structural work like garden walls under 1 m, letterbox pillars, or decorative features, consent is generally not required — but confirm with your council first.
When to Call a Professional
While small DIY bricklaying jobs are within reach of a capable homeowner, there are situations where you should bring in a qualified bricklayer:
- Any structural or load-bearing masonry work
- Retaining walls over 1 m (and definitely over 1.5 m)
- Work on heritage or soft old bricks, where the wrong mortar can cause irreversible damage
- Brickwork that forms part of a weathertight envelope (NZ Building Code clause E2 covers external moisture)
- Large projects where consistent joint quality matters for strength and appearance
You can find qualified, experienced local bricklayers on tradedirectory.co.nz — it's always worth getting two or three quotes and checking that your tradesperson has relevant experience with your type of project.
Licensed Building Practitioner (LBP)
