Standard lime mortar mix ratios

Lime mortar mix ratios are expressed as lime:sand by volume, not by weight. The ratio depends on the hydraulic lime strength (NHL 2, 3.5, or 5) and the application (repointing, building, or rendering).

NHL 3.5 (most common)

1:2.5 or 1:3 (lime:sand by volume). Use 1:2.5 for sheltered walls or when you need slightly stronger mortar. Use 1:3 for general heritage repointing.

Example: 1 x 25kg bag NHL 3.5 + 2.5 buckets of sharp sand (if measuring by buckets). Or weigh it: 1 x 25kg NHL 3.5 + 60-75kg sharp sand.

NHL 5 (exposed locations)

1:3 (lime:sand). NHL 5 is harder and less workable than NHL 3.5, so it needs more sand to remain workable. Use for coastal locations, hilltop sites, or chimneys.

NHL 2 (soft masonry)

1:2 or 1:2.5 (lime:sand). NHL 2 is very weak, so it doesn't need as much sand. Use for very soft bricks, sandstone, or internal work where strength isn't critical.

Non-hydraulic lime putty

1:2.5 or 1:3 (lime putty:sand). Lime putty is supplied pre-slaked in tubs. Mix by volume with sharp sand. Some mixes add a small amount of hydraulic lime (10-20% NHL 3.5) to speed up the set, but this is optional.

Choosing the right sand

Sand type matters as much as the lime:sand ratio. Use the wrong sand and the mortar will be too weak, too strong, or won't carbonate properly.

Sharp sand (correct)

Sharp sand (also called grit sand or concreting sand) has angular grains and a mix of particle sizes from fine to coarse. This is what you want. The angular shape helps the grains lock together, and the varied particle sizes fill gaps, producing strong mortar.

Look for washed sharp sand with particle sizes ranging from 0.1mm to 5mm. Most builders' merchants stock it in 25kg bags or bulk bags (850kg).

Building sand (wrong)

Building sand (also called soft sand or bricklaying sand) has rounded grains and is too fine. Mortar made with building sand is weaker and more prone to shrinkage cracks. Don't use it for lime mortar.

Beach sand or unwashed sand (wrong)

Beach sand contains salt, which prevents proper carbonation and causes efflorescence (white salt stains). Unwashed sand contains clay and organic matter, which weaken the mortar. Always use washed sharp sand.

How to mix lime mortar

Lime mortar should be mixed the day before use and left to 'fatten' overnight. This improves workability and reduces shrinkage.

By hand (small batches)

Use a mixing board (a large flat board or piece of ply) and a shovel or hoe.

  1. Measure 1 part NHL 3.5 and 2.5-3 parts sharp sand by volume (use buckets for accuracy).
  2. Mix the lime and sand dry first. Spread them out on the board and turn the mix 5-6 times until the colour is even.
  3. Form a crater in the centre and add water gradually. Mix in the water from the edges, adding more until the mortar is stiff but workable (it should stand on a trowel without slumping).
  4. Cover with damp hessian or plastic sheet and leave overnight (12-24 hours).
  5. Next day, remix briefly. Add a splash more water if the mortar has stiffened. Use within 2 days.

By mixer (large batches)

Use a drum mixer (not a paddle mixer). Paddle mixers over-work lime mortar and break down the lime particles.

  1. Add sand and lime to the mixer dry. Run for 2 minutes to mix.
  2. Add water gradually while the mixer runs. Stop when the mortar holds together but is still stiff.
  3. Tip into a wheelbarrow, cover, and leave overnight.
  4. Next day, put back in the mixer, add a splash more water, run for 1-2 minutes, then use.

How much water to add

There's no fixed water:lime ratio because it depends on sand moisture and ambient humidity. Add water gradually until the mortar is stiff but workable. Too wet and the mortar will slump and shrink. Too dry and it won't stick to the wall.

Test: pick up a handful and squeeze. It should hold together without water running out. Drop it on the board. It should stay in a lump, not spread out.

How much lime mortar do you need?

For repointing

Estimate 1.5-2kg of mortar per m² of wall at 15mm joint depth (typical for brick). For a 50m² wall, you'll need 75-100kg of mortar.

One 25kg bag of NHL 3.5 mixed with 60-75kg of sand (1:2.5-3 ratio) makes 85-100kg of mortar, enough for 40-50m² of repointing.

Example: a typical house front (40m² of wall) needs 60-80kg of mortar, so 1 bag of lime + 2-3 bags of sand.

For building

Estimate 25-30kg of mortar per m² of single-skin brickwork (100mm wall). A 25kg bag of NHL 3.5 + sand makes enough mortar for 3-4m² of wall.

Common mistakes

Wrong sand type

Using building sand instead of sharp sand produces weak mortar that erodes quickly. Always use sharp sand with angular grains and varied particle sizes.

Too much lime (less than 1:2 ratio)

Mortar with too much lime shrinks excessively and cracks. It also costs more (lime is more expensive than sand). Stick to 1:2.5 or 1:3.

Too much sand (more than 1:3 ratio)

Mortar with too much sand is weak and crumbly. It won't bond properly to the masonry. Maximum sand ratio is 1:3 for lime mortar.

Adding cement

Some builders add cement to lime mortar thinking it makes it stronger. It does, but it also makes it rigid and waterproof, which defeats the purpose of using lime. If you need stronger mortar, use NHL 5 instead of NHL 3.5. Don't add cement.

Not leaving it to fatten

Lime mortar mixed and used immediately is harsh and difficult to work. Leaving it overnight allows the lime particles to hydrate fully, making the mortar creamy and sticky. Always mix the day before.

Using mortar after it has dried out

If lime mortar dries out (goes hard), don't just add water and re-mix. The lime has started to carbonate, and adding water won't reverse that. Make a fresh batch instead.

Try the mortar calculator

Enter your wall area and joint depth to see how much lime and sand you need.

Summary

The standard lime mortar mix ratio is 1 part lime to 2.5-3 parts sharp sand by volume. NHL 3.5 at 1:2.5 or 1:3 is the most common mix for UK heritage repointing. Mix the day before use and leave to fatten overnight.

Use sharp sand, not building sand. Don't add cement. A 25kg bag of lime mixed with 60-75kg of sand makes enough mortar for 40-50m² of repointing.