Lime Mortar vs Cement Mortar: Which Is Right for Old Stone Walls?

There is a quick way to tell whether an old stone wall has been pointed in cement. Look at the joints. If they sit proud of the stone, hard and grey, with a hairline crack running along them, the trouble has usually already begun. The stones nearby may be flaking at the edges, or hollow behind the surface. None of this happens because cement is a poor material. It is a very good one, in the right place. On a traditional stone wall, built with soft, porous mortar, it is the wrong place, and the wall pays for it over the following twenty or thirty years.

Breathability: the wall that needs to dry

Old stone walls are thick, and they get wet. Rain drives in, ground moisture rises, and water moves through the fabric as vapour. In a traditionally built wall, the mortar is the pathway. Lime mortar is porous, so vapour passes through the joints and out into the air, then the wall dries out again.

Cement mortar is far less permeable. It is dense to begin with, and pointing tends to be tooled hard and smooth on top of that, which seals the face further. Water still gets in, often through cracks or around window reveals, but it cannot get out the way it came. It travels sideways into the stones and sits there. That trapped moisture sits behind most of the damage described below: damp patches on internal plaster, salts blooming on the wall face, and timber joists that never quite dry out.

Flexibility: soft mortar protects hard stone

Lime mortar is weaker than the stone it holds. That sounds like a fault. It is the whole point.

Old buildings move. A little settlement, thermal expansion, a shift in the ground beneath a corner. When that happens, you want the sacrificial material to give first. Lime flexes and crumbles at the joint, and in a few decades you repoint. Cement is rigid, and often harder than the sandstone or limestone it is touching. Movement is transferred into the stone instead. Joint arrises spall, cracks open through the stones themselves, and what should have been a repointing job becomes stone replacement.

A hard cement joint in a soft wall behaves a little like a girder set into masonry. It does not move with the wall, so it concentrates stress at its edges. That is why the failures always start at the mortar line.

Frost and the freeze–thaw cycle

Water expands as it freezes. A saturated stone frozen repeatedly through a British winter will eventually break up, and no mortar can stop that entirely. What lime does is reduce the amount of water hanging around in the wall in the first place, and give freezing water somewhere to expand into. The joints are porous and slightly yielding; they absorb some of the pressure.

Cement pointing seals the face. Moisture accumulates behind it, saturates the stone, and the frost does the rest. This is why spalling so often appears on walls that have been cement pointed for years without any obvious other cause.

Lime has its own vulnerability. Fresh lime mortar does not like frost while it cures, and it does not like being dried out too fast by sun or wind either. Plan pointing for late spring through early autumn, cover the work with hessian, and keep it damp for the first few days.

What cement mortar does to traditional stonework

The pattern of failure is fairly consistent. Worth checking for:

• Joints standing proud of the stone face, often with a crack along the mortar line
• Flaking, hollow-sounding or crumbling stone faces, especially near the joints
• White salt deposits blooming on the wall surface
• Damp patches inside the building that reappear after rain
• Timber lintels, joist ends and skirting that stay damp in one particular spot
• Mortar that rings hard when tapped rather than sounding dull

Removing cement is not simply a matter of raking it out. It grips the stone, and the stone edges around it are often already weakened. The work needs a careful hand, a narrow chisel and patience. If the stone is badly spalled, get an opinion before anyone starts cutting, because aggressive removal can take the face off with the mortar.

When cement mortar is the right choice

Cement has its place, and this is not an argument for lime everywhere. Modern extensions, blockwork, engineering brick, retaining walls and below-ground work are all reasonable candidates for a cement or cement-lime mortar, and sometimes an engineer will specify one for structural reasons. A wall built in the twentieth century with cement mortar is generally best repointed in something similar.

The useful rule is that mortar should be weaker and more permeable than the material it holds. A gauged mix of cement, lime and sand can be a compromise on less sensitive work, but on historic fabric it is still a compromise. If the building is listed, repointing may need consent, so speak to your local authority conservation officer before you start. For anything older than about 1919, a conservation-accredited surveyor or an experienced lime mason is worth the phone call.

Mixing and pointing: the details that matter

1. Only repoint what needs it. Sound lime mortar stays. Rake out failing joints to roughly two to two-and-a-half times their width, with square edges, and brush out the dust.
2. Pick the lime. Lime putty suits sheltered and internal work; a natural hydraulic lime around 3.5 is a good general choice for external walls, with a stronger hydraulic lime where exposure is severe.
3. Source the sand first. Sharp, well graded, and matched to the colour and texture of the existing mortar. Local sand is usually the closest match.
4. Mix by volume, not by eye. One part lime to two-and-a-half or three parts sand is a common starting point. Add water gradually; a sloppy mix is a weak mix.
5. Dampen the wall, don't soak it. A dry wall pulls the water out of the mortar before it can set.
6. Work in small areas. Press the mortar in firmly in layers rather than smearing it across the stone face.
7. Finish when thumb-dry. Rub back with a churn brush to a slightly recessed profile that matches the original joints.
8. Cure it slowly. Shade and damp hessian for several days, and keep frost and driving rain off the work.

Where to start

Before doing anything, photograph the wall and note where the damp and the spalling appear. Then point a single square metre as a test panel, using the sand and lime you intend to use throughout. Leave it through a winter. You will learn more from that one panel about how the wall behaves than from any amount of reading.

Do not pressure-wash old masonry, and do not grind out joints with an angle grinder. Both cause damage that is difficult and expensive to undo. Work in manageable sections, one elevation at a time, and keep a record of the mix you used so the next repair matches it. Old walls have coped with our weather for centuries. Give them a mortar that moves and breathes with them, and they will carry on doing so.

Photo: congerdesign / Pixabay