For most pre-1950s brickwork in New Zealand, hydraulic lime mortar is not just a valid choice — it is often the correct one. Using ordinary Portland cement mortar on old, soft bricks can cause serious long-term damage. Understanding why will help you make the right call for your home.
Why Mortar Compatibility Matters
Old bricks — particularly those made before the mid-twentieth century — are softer and more porous than modern machine-made bricks. The mortar in a brick wall is designed to act as the sacrificial element: it should be slightly weaker than the bricks themselves, allowing moisture and movement to be absorbed by the joint rather than the masonry unit.
When a hard cement mortar (typically a 3:1 or 4:1 sand-to-cement mix) is used on soft old bricks, the mortar becomes stronger than the brick. Moisture that would normally escape through the joint is instead forced through the brick face, leading to:
- Spalling (the brick face flaking or popping off)
- Salt crystallisation damage
- Cracking as the wall cannot flex with thermal movement
- Trapped moisture causing ongoing damp problems
This is a well-documented problem in heritage conservation and is recognised by building authorities and conservation bodies internationally.
What Is Hydraulic Lime Mortar?
Hydraulic lime (often labelled NHL — Natural Hydraulic Lime) sets through a combination of carbonation (absorbing CO₂ from the air) and a hydraulic reaction with water. It comes in different strengths, graded as NHL 2, NHL 3.5, and NHL 5 — the number indicating compressive strength.
- NHL 2 – feebly hydraulic, very soft set, suited to the most delicate old bricks
- NHL 3.5 – moderately hydraulic, the most commonly used grade for general repointing and bricklaying
- NHL 5 – eminently hydraulic, stronger set, used in exposed or below-ground situations
Hydraulic lime mortar is breathable (vapour-permeable), flexible, and self-healing to a degree — small cracks can re-carbonate and close over time. These properties make it ideal for older masonry.
When Should You Use Hydraulic Lime Instead of Cement?
As a general guide:
| Situation | Recommended Mortar |
|---|---|
| Pre-1950s soft or handmade bricks | NHL 2 or NHL 3.5 lime mortar |
| Repointing heritage or villa-era brickwork | NHL 3.5 lime mortar |
| Below-DPC or exposed retaining walls (old brick) | NHL 5 lime mortar |
| Modern hard-fired bricks (post-1960s) | Cement or cement-lime blend |
| New construction with modern bricks | Standard cement mortar |
If you are unsure what type of bricks you have, a qualified bricklayer can assess them. You can find experienced local bricklayers on tradedirectory.co.nz.
Mixing and Using Hydraulic Lime Mortar
Hydraulic lime mortar is mixed with well-graded sharp sand — the sand type and grading significantly affect the final result. A typical mix ratio is 1 part NHL 3.5 to 2.5–3 parts sand by volume, though always follow the manufacturer's data sheet.
Key practical tips:
- Pre-wet the bricks lightly before applying mortar to prevent the brick from drawing moisture out too quickly and weakening the bond
- Do not add cement to a lime mortar mix — this defeats the purpose and reduces breathability
- Avoid working in frost — lime mortar is vulnerable to freezing before it has cured; protect fresh work with hessian or polythene in cold weather
- Curing is slower than cement — NHL mortar can take weeks to months to reach full strength, so avoid heavy loading or vibration early on
- Repointing depth should be at least 15–20 mm to ensure a durable joint
In New Zealand's varied climate — from the humid north to the frost-prone South Island interior — choosing the right NHL grade and protecting fresh work from extremes is especially important.
NZ Building Code and Heritage Considerations
Repair and repointing work on existing brickwork is generally exempt from building consent under Schedule 1 of the Building Act 2004, as it is considered like-for-like maintenance. However, if you are making structural changes to a brick wall, or if the property is a heritage-listed building, different rules may apply. Always confirm with your local council before starting work.
For heritage properties, your local council or Heritage New Zealand may have specific requirements around mortar specification. BRANZ publishes useful guidance on heritage masonry repair that is worth consulting.
The MBIE Building Performance website is the authoritative source for understanding consent requirements and the NZ Building Code.
Sourcing Hydraulic Lime in New Zealand
NHL products are available from specialist masonry and building suppliers in New Zealand. Check with Placemakers or Mitre 10 for availability in your region, though specialist lime suppliers may stock a wider range of grades. Sika also produces lime-based mortar products suitable for masonry repair.
Always read the product data sheet and safety data sheet before use — lime is caustic and requires gloves, eye protection, and appropriate skin cover when mixing and applying. Refer to WorkSafe NZ guidelines for safe handling of construction materials.
When to Call a Professional
While repointing small areas of brickwork is within the capability of a careful DIYer, there are situations where you should engage a qualified bricklayer:
- Large areas of failing mortar or structural cracking
- Walls showing signs of movement, bulging, or instability
- Heritage or character-listed properties where incorrect work could cause irreversible damage or regulatory issues
- Any work involving structural brick elements (piers, chimneys, load-bearing walls)
- If you are unsure of the brick type, original mortar specification, or appropriate NHL grade
A skilled bricklayer will assess the existing mortar, identify the brick type, and specify the correct mix. Find qualified local bricklayers through tradedirectory.co.nz.
Licensed Building Practitioner (LBP)
