How do I mix mortar correctly?

Mortar strength is a matter of matching the wall, not of making it as strong as possible — a mix that is too hard damages old brickwork.

Safety

What you'll need

• Building sand and cement, or a bagged pre-mix
• A mixing board or tub, and a shovel
• Waterproof gloves and eye protection

Step by step

1. Choose the ratio for the job.4:1 sand to cement for general above-ground brickwork. 3:1 for chimneys, copings and exposed positions. 5:1 or 6:1 with added lime for soft old brick.
2. Use a weaker mix on old buildings.Mortar must be weaker than the brick, so moisture and movement are taken by the joint rather than the brick face. Cement pointing on soft Victorian brick causes the brick faces to spall off.
3. Consider lime for pre-1919 walls.Lime mortar is flexible and breathable and is the correct material for solid-wall historic construction. It is a different technique and worth reading about specifically.
4. Use the right sand.Soft building sand for bricklaying and pointing. Sharp sand for screeds and concrete. Using sharp sand for pointing gives a harsh unworkable mix.
5. Mix dry ingredients first.Turn sand and cement together until the colour is completely uniform, with no streaks. Uneven mixing gives patchy strength and patchy colour.
6. Add water gradually.A depression in the middle, water in, fold the dry material in from the edges. Adding too much water at once is very hard to correct.
7. Aim for the right consistency.It should hold a ridge when you draw the shovel through it, and stay on a trowel held at an angle without running. Too wet and it slumps out of the joint.
8. Add plasticiser rather than washing-up liquid.Proper mortar plasticiser entrains air and improves workability and frost resistance. Washing-up liquid is a persistent myth and weakens the mix.
9. Mix only what you can use.Mortar starts to go off within about two hours, less in hot weather. Retempering with more water after it has begun to set severely weakens it.
10. Match colour on repairs.Sand colour drives mortar colour. Take a sample of the existing mortar to a builders merchant, and make a test batch — it dries much lighter than it looks wet.
11. Protect it while it cures.Cover new work in hot sun, heavy rain or frost. Mortar that freezes before it sets is permanently weakened.

Tips

• Mortar should always be weaker than the brick. Stronger mixes damage old brickwork.
• It dries much lighter than it looks wet — always make a test batch for repairs.
• Never use washing-up liquid as a plasticiser. Buy the proper additive.

Common mistakes

• Using a strong cement mix on old soft brick — The mortar becomes harder than the brick, so movement and moisture are forced through the brick face instead of the joint. The faces then spall off and the damage is irreversible.
• Adding water to mortar that has started to stiffen — Retempering after the cement has begun hydrating breaks the developing structure and substantially reduces final strength. Mix smaller batches instead.

If it doesn't work

Questions people ask