Using Lime Mortar for Traditional Stone and Brick Walls

Why Old Walls Need to Breathe

If you own a period property, you have probably heard the phrase "the walls need to breathe". It sounds romantic, almost poetic, but it describes a hard physical reality. Traditional stone and brick walls were built to manage moisture, not to seal it out. Rain soaks into the outer face of the wall, travels slowly through the masonry, and evaporates away again when conditions allow. The mortar in the joints plays a central role in that process, because it is softer and more porous than the stone or brick around it.

Modern cement mortar breaks that arrangement. It is hard, dense and largely impermeable, so moisture trapped behind it has nowhere to go. Over the years, that dampness has to escape somewhere, and it usually finds its way through the face of the brickwork, blowing the surface, or drives inward to cause damp patches and rotting timbers. Lime mortar, by contrast, flexes with the wall, absorbs moisture and releases it again as vapour. That is what "breathable" means in practice.

The Difference Between Lime and Cement

Lime mortar is made from limestone burnt at a relatively low temperature, which produces a material that sets by carbonation — absorbing carbon dioxide from the air over weeks and months rather than hardening in hours. The result is a mortar that stays slightly flexible and sacrificial. If movement occurs, the mortar gives way before the masonry does, which is exactly what you want in a wall that has already settled over two or three centuries.

Cement, fired at far higher temperatures, creates a rigid, brittle material. It is stronger than the bricks it holds, so when the wall moves, the bricks crack instead. It also traps salts and moisture, accelerating decay at the edges of the joint. Once a traditionally built wall has been repointed in cement, the damage tends to spread quickly.

Choosing the Right Lime

There is no single product called "lime mortar". The main choices are non-hydraulic lime putty, natural hydraulic lime (NHL) and hydrated lime. Each behaves differently, and matching the mortar to the wall matters as much as the mixing.

• Lime putty — non-hydraulic lime that has been slaked and stored wet. It sets slowly by carbonation, is highly breathable and is the traditional choice for soft, porous stone and pre-1900 brickwork. It needs air and time to cure.
• Natural hydraulic lime (NHL 2, 3.5 and 5) — lime with natural impurities that give it a faster, more predictable set. NHL 2 is the softest and most breathable; NHL 3.5 is a good general-purpose choice for most exposed walls; NHL 5 approaches cement-like strength and should be used sparingly in heritage work.
• Hydrated lime — bagged dry lime, often used to improve workability or in lime washes. It is not a substitute for putty in most repointing jobs.

As a rule, choose the weakest and most breathable lime that will do the job. If in doubt, a sample panel repointed and left over a winter will tell you more than any specification sheet.

Mixing and Applying It Correctly

Lime mortar is forgiving, but it rewards care. The traditional mix is one part lime to three parts well-graded sharp sand, though you may go to one-to-two for exposed or heavily weathered joints. The sand should be washed, free of clay and silt, and matched in colour and grading to the original mortar. Soft building sand alone is rarely suitable — it lacks the range of particle sizes that gives lime mortar its strength.

Mix on a clean board, adding water gradually until the mortar holds its shape but is still workable. Let it stand for a short while before use, and never add cement "just to firm it up". On the wall, dampen the masonry first so it does not pull water out of the mix too quickly.

• Rake out the joint to a depth of roughly twice its width, using hand tools where the masonry is soft.
• Brush out dust and loose material, then wet the joint thoroughly.
• Press the mortar in firmly in layers, filling the joint fully with no voids behind.
• Finish with a wooden or bristle tool to match the original profile — never a metal jointer on soft stone.
• Protect the work from direct sun, wind and frost for several days, and damp it down gently if it dries too fast.

Curing, Weather and the Long View

Lime does not dry; it cures. Carbonation takes weeks in good conditions and can continue for months in thick joints, so the mortar will remain green and vulnerable for a while. In a British climate, that means watching the forecast closely. Frost will ruin fresh lime, and drying winds will cause it to shrink and craze before it has gained strength. Hessian sheeting, kept damp, is a simple and effective safeguard.

Once cured, a lime-repointed wall is a quiet pleasure. Water moves through it as it always did, salts crystallise harmlessly in the sacrificial mortar rather than the brick, and the whole elevation settles back into a steady rhythm. Repointing in lime is slower, more expensive and less tidy than a quick cement job — but it is the repair that respects what the building already knows how to do.