Brick Mortar Mix Ratio Guide: Which Mix for Which Job?
Mortar holds brickwork together, but the right mix depends on where the wall is and what it needs to do. Using a mix that's too strong can cause cracking; too weak and it won't cope with moisture or load. Here's how to match the ratio to the job.
The basic mix: cement and sand
Standard bricklaying mortar is a mix of Portland cement and sharp sand, combined by volume. The ratio describes how many parts sand per part cement. A higher number means more sand — a weaker, more flexible mix. A lower number means more cement — stronger but less forgiving of movement.
| Mix ratio (cement:sand) | Typical application |
|---|---|
| 1:3 | Below ground, high moisture exposure, engineering bricks |
| 1:4 | Below DPC level, retaining walls, hard-wearing surfaces |
| 1:5 | External walls above ground, most general brickwork |
| 1:6 | Internal non-structural walls, light partitions |
Adding lime
Hydrated lime is commonly added to mortar to improve workability and reduce the risk of shrinkage cracking. It makes the mix easier to spread and gives it a slight flexibility that pure cement mortar lacks — useful in walls that may experience minor movement.
A typical lime-modified mix for external brickwork is cement:lime:sand in a 1:1:6 ratio. The lime partially replaces some of the cement's stiffness without significantly reducing final strength for standard applications.
| Mix (cement:lime:sand) | Use |
|---|---|
| 1:0:3 to 1:0:4 | Below ground, high exposure (no lime) |
| 1:½:4 to 1:½:4.5 | External walls, moderate exposure |
| 1:1:5 to 1:1:6 | External walls, sheltered; most general use |
| 1:2:8 to 1:2:9 | Internal walls, low load |
Plasticisers as a lime alternative
Liquid plasticisers can be added instead of lime to improve workability. They introduce tiny air bubbles into the mix, making it easier to spread without adding water. Dosage varies by product — follow the manufacturer's instructions rather than a fixed ratio, as over-dosing weakens the finished mortar.
How much water to add
Water is added until the mortar reaches a workable consistency — it should hold its shape on a trowel without slumping, and spread smoothly without being stiff or dry. Adding too much water reduces strength and causes the mortar to run down the face of the bricks.
Mix the dry materials thoroughly before adding water, then add water gradually and mix again. It is much easier to add a little more water than to correct a mix that is too wet.
How much mortar per square metre
As a rough guide for standard brickwork with 10mm mortar joints, allow approximately 30 to 40 litres of mixed mortar per square metre of single-leaf wall. This accounts for bed joints (horizontal) and perpendicular joints (vertical) but not collar joints in cavity walls. Add 10% for waste and overfilling.
Common mistakes
- Using the same mix everywhere — a 1:3 mix on an internal partition is overly rigid and prone to cracking with normal building movement
- Adding too much water — weakens the mortar and causes staining on brick faces
- Inconsistent batching — measuring by eye rather than a fixed container leads to variable strength across a wall
- Working in cold conditions without precautions — mortar should not be mixed or applied when temperatures are at or below 2°C, as freezing before curing destroys the bond
Putting it together
Before mixing mortar, it helps to know exactly how many bricks you're laying and how much material you'll need. Our brick calculator gives brick count and pallet estimates for any wall size, and the bricks per square metre guide covers how coverage varies by brick type.
FAQ
What is the correct mortar mix ratio for bricklaying?
For most external brickwork, a 1:5 ratio (1 part cement to 5 parts sand) by volume is standard. For internal non-structural walls, a weaker 1:6 mix is common. For below-ground or high-moisture areas, a stronger 1:3 or 1:4 mix is used. Adding a small amount of hydrated lime (typically 1 part lime to the cement) improves workability without significantly reducing strength.
Can I use the same mortar mix for all brickwork?
No. The right mix depends on where the wall is and what load it carries. A 1:3 mix used for an internal partition is unnecessarily strong and more likely to crack due to rigidity. A 1:6 mix used below ground level will not have adequate water resistance. Match the mix to the application.
What does a 1:5 mortar mix mean?
A 1:5 mortar mix means 1 part cement to 5 parts sand, measured by volume. So for every bucket of cement, you add 5 buckets of sand, plus water until the mix reaches a workable consistency.
How much mortar do I need per square metre of brickwork?
As a rough guide, standard brickwork with 10mm joints uses approximately 0.5 to 0.7 litres of mortar per brick, depending on whether joints are on all faces or only the bed and perpendicular joints. For a single-leaf wall using approximately 60 bricks per square metre, that works out to around 30 to 40 litres of mixed mortar per square metre.