FixMeasure
Basics
Mortar and plaster mixes: ratios explained
Mortar is not just cement and sand: the right mix depends on the job, and a mix that is too strong can be as bad as one that is too weak. This guide explains common ratios and how to mix them.
1. What each ingredient does
Cement gives strength. Sand is the bulk of the mix and should be clean and well graded; fine, soft sand for brickwork and sharper sand for render and screeds. Lime makes the mortar more workable and flexible and helps it hold water. Water is added only to a workable consistency; too much weakens the mix.
2. Common mix ratios
| Use | Typical mix (by volume) |
|---|---|
| General bricklaying | 1 cement : 1 lime : 5–6 sand, or 1 cement : 4–5 sand with plasticizer |
| Below ground, exposed walls | 1 cement : 3–4 sand |
| Render, first coat | 1 cement : 1 lime : 6 sand |
| Floor screed | 1 cement : 3–5 sharp sand |
These are typical examples; product instructions and local standards take priority. Ready-mixed bagged mortars only need water.
3. Stronger is not always better
Mortar should be slightly weaker than the bricks or blocks. Then any cracking happens in the joints, which are easy to repair, rather than in the units. Old soft bricks and stone need lime-rich mortar; hard cement mortar can damage them.
4. How to mix
Measure with a bucket or gauge box, not shovelfuls, so every batch is the same. Mix the dry ingredients until the color is even, then add water gradually. Good mortar holds its shape on the trowel and spreads easily. Mix only what you can use within about 1–2 hours, and do not re-temper mortar that has started to set. The Mortar calculator gives cement and sand quantities for a wall.
5. Weather and curing
Do not lay mortar in frost or on frozen surfaces, and protect fresh work from rain and hot sun. Keeping new joints and render slightly damp for the first days helps them cure properly. See How to pour a concrete slab: step-by-step guide for curing concrete.
Common mistakes
- Measuring by shovelfuls, so batches vary.
- Adding too much water to make mixing easier.
- Using strong cement mortar with soft old bricks.
- Re-mixing mortar that has started to set.
- Working in frost or letting fresh render dry too fast.
Frequently asked questions
What is the best mortar mix for bricks?
For general work, about 1 cement : 1 lime : 5–6 sand, or 1 cement : 4–5 sand with a plasticizer.
Why add lime to mortar?
It improves workability and flexibility and helps the mortar retain water.
How long can mortar be used after mixing?
Usually about 1–2 hours, less in hot weather.
Can I use the same sand for mortar and concrete?
Not ideally: mortar uses fine building sand; concrete needs coarser sharp sand and gravel.
How much water should I add?
Only enough for a workable mix that holds its shape on the trowel.