🧱 Lime Mortar Mix Ratios
Choosing the correct lime mortar mix for heritage brickwork
Lime mortar mix ratios are one of the most important parts of any heritage repointing or restoration project. The wrong mix can damage brickwork, fail prematurely, or trap moisture inside the wall. The correct mix protects the building, allows it to breathe, and performs exactly as intended.
This guide explains the correct lime mortar mix ratios for different situations, the differences between NHL grades, and how to choose the right mix for your property.
🧱 What Determines the Correct Lime Mortar Mix?
Several factors influence the correct lime mortar ratio:
• Type of lime (NHL 2, NHL 3.5, NHL 5, or lime putty)
• Exposure level (sheltered, moderate, exposed, coastal)
• Brick softness
• Age of the building
• Location on the property (chimney, gable, internal, garden wall)
• Sand type and grading
The golden rule:
🧱 Standard Lime Mortar Mix Ratios
⭐ NHL 2 Mix Ratio
1 part NHL 2 : 2.5–3 parts sand
Best for:
• Internal work
• Sheltered external walls
• Soft bricks
• Older cottages and period homes
⭐ NHL 3.5 Mix Ratio
1 part NHL 3.5 : 2.5–3 parts sand
Best for:
• Most external brickwork
• Victorian and Edwardian homes
• General repointing
• Moderate exposure
This is the most commonly used lime for UK brickwork.
⭐ NHL 5 Mix Ratio
1 part NHL 5 : 2.5–3 parts sand
Best for:
• Chimneys
• Coastal areas
• Highly exposed walls
• Harder brick or stone
⭐ Lime Putty (Fat Lime) Mix Ratio
1 part lime putty : 2.5–3 parts sand
Best for:
• Specialist heritage work
• Very soft bricks
• Internal plastering
• Traditional conservation projects
🧱 Choosing the Right Sand
Sand is just as important as the lime.
✔️ Sharp sand
Adds strength and structure.
✔️ Soft sand
Adds workability but must be used carefully.
✔️ Blended sands
Often the best choice for colour and texture matching.
✔️ Local sands
Help match original mortar on heritage buildings.
The sand must be well‑graded, clean, and free from salts.
🧱 Why Mix Ratios Matter
Using the wrong ratio can cause:
• Cracking
• Weak joints
• Mortar failure
• Trapped moisture
• Damp inside the home
• Spalled brick faces
• Long‑term structural damage
Using the correct ratio ensures:
• Breathability
• Flexibility
• Durability
• Compatibility with historic brickwork
• A long‑lasting finish
🧱 Aftercare: The Critical Step
Even the perfect mix will fail if not cured correctly.
Lime mortar needs:
• Gentle misting in warm weather
• Protection from wind
• Protection from frost
• Protection from direct sun
• Slow, controlled drying
This is why lime mortar must be handled by a specialist.
🧱 Why You Need a Lime Mortar Specialist
Lime mortar requires:
• Correct mix selection
• Correct sand grading
• Correct application
• Correct curing
• Understanding of heritage materials
• Knowledge of brick softness and moisture movement
Most general builders use cement‑based methods that can permanently damage older properties.
A specialist ensures the work is safe, breathable, and conservation‑grade.
🧱 Related Pages