Lime mortar, repointing and why cement is refused

Cement pointing on an old solid wall traps water in the masonry and pushes the damage into the brick or stone instead of the joint. That is why conservation officers refuse it, and why the refusal is about building physics rather than taste. A lime mortar is softer and more permeable than the units around it, so it takes the wear and lets the wall dry. Cement is harder and less permeable, so the wall wears instead.

If you own a listed building or a solid walled house in a conservation area, this is the single repair decision most likely to cause long term damage if you get it wrong.

What the mortar is actually doing

A solid wall built before roughly 1919 has no cavity and no damp proof course. It handles water by absorbing it and then releasing it, and the mortar joints are the main route out. Joints make up a surprising proportion of the wall surface, and they are deliberately the weakest part of the assembly.

The mortar also accommodates movement. Old buildings move seasonally, and a soft joint takes up that movement in fine cracks distributed along the bed rather than concentrating it in one place. Repointing replaces the outer part of that joint when it has weathered back, typically after many decades.

Two consequences follow. The mortar should always be weaker than the masonry unit. And repointing is a repair to a working system, not a decorative refresh, so matching the original matters.

Why cement causes damage

Portland cement mortar is stronger, denser and far less permeable than the lime mortar it usually replaces. Four things then happen.

Moisture that used to leave through the joint has to leave through the face of the brick or stone. In winter that moisture freezes, and the face spalls. On soft handmade brick and on sandstone the damage can appear within a few seasons.

Salts carried in the moisture crystallise behind the face rather than at the joint, and the resulting pressure blows the surface off.

The mortar is too rigid to accommodate movement, so instead of fine cracks in the joint you get a clean crack at the interface between mortar and unit, which is an open route for water straight into the wall.

Internally, a wall that cannot dry outwards holds moisture, and damp appears on the inside face. Owners then treat the symptom with tanking or injected damp proofing, which traps more moisture again.

Removing cement pointing later is difficult and usually damages the arrises of the units, which is why the second repointing costs more than the first.

Specifying a lime mortar

Lime mortars run from non-hydraulic lime putty, which sets slowly by absorbing carbon dioxide, through natural hydraulic limes graded by strength, usually written as NHL 2, NHL 3.5 and NHL 5. Weaker is generally better on soft masonry. Hot mixed mortars, made by slaking quicklime with the aggregate, have returned to use on many conservation projects.

The aggregate matters as much as the binder. It should be sharp, well graded and matched to the original in colour and particle size, because the aggregate is what you see once the mortar has weathered slightly.

A typical specification gives the binder type, the binder to aggregate ratio by volume, the aggregate source and grading, the joint profile and the protection regime. Where the building is important or the original mortar is unusual, mortar analysis of a sample tells you what was there rather than what somebody assumes was there.

Ask one question of any proposed mix: is it weaker than the brick or stone it sits between. If the answer is uncertain, it is too strong.

Joint profiles and finishes

The profile should sit slightly back from the face of the units, or flush with them, and should follow the shape the wall already has. Three things to avoid:

• Strap or ribbon pointing, where a raised band of mortar sits proudly across the face of the units. It looks wrong and it sheds water onto the face.
• Weather struck profiles applied indiscriminately, particularly where the original was flush.
• Feathered edges spread over the arris of a worn brick to make the joint look neat. They break off, and they hold water.

The finish is worth specifying too. Brushing or scudding the face once the mortar has firmed exposes the aggregate and gives a texture that matches an old wall. A trowel polished surface forms a dense skin that behaves more like cement.

Preparation, season and curing

Rake out by hand with a quirk or a plugging chisel, to roughly twice the joint width in depth, and never with an angle grinder. Grinders widen joints, cut into the units and leave straight edges that are visible forever.

Lime needs frost free conditions to set. In practice that means working between spring and early autumn in most of England, and stopping well before the first frosts. Pointing in November because the scaffold is up is the most common cause of a failed lime job.

After pointing, protect the work. Damp hessian over polythene, misting in dry weather, and shelter from strong sun and wind all keep the mortar from drying too fast. Carbonation is slow, and a mortar that dries before it sets stays friable.

When repointing needs consent

For an unlisted building in a conservation area, repointing is normally maintenance and does not need planning permission. Check whether an Article 4 direction applies, because a few cover works to unlisted buildings in sensitive areas.

For a listed building the position is different. Section 7 of the Planning (Listed Buildings and Conservation Areas) Act 1990 requires listed building consent for works affecting the character of the building as one of special interest. Changing the mortar type, colour, texture or joint profile is capable of doing exactly that, and repointing a whole elevation is rarely treated as a like for like repair. Our note on like for like repairs and listed building consent sets out where that line falls.

Carrying out unauthorised works to a listed building is a criminal offence, and there is no equivalent of the time limits that apply to planning enforcement. Ask the conservation officer in writing before work starts. Most will answer a short email describing the proposed mix and profile.

Where consent is needed, a Heritage Statement describing the significance of the fabric, the condition of the existing mortar and the reasons for the proposed specification is what supports the application. It does not need to be long for a repointing job.

Sample panels and choosing someone who has done it

Conservation officers almost always ask for sample panels, and a condition requiring them is normal. Build at least two, roughly a square metre each, on different elevations, using the actual mix and the actual operative. Leave them for several weeks so the colour settles, then agree one on site.

Keep the approved panel visible for the duration of the works. It is the reference when the second gang arrives.

When choosing a contractor, ask to see lime work they did at least five years ago rather than last month, ask which lime they propose and why, and ask how they will protect the work in hot or cold weather. Anyone who answers all three specifically has done it before. Anyone who offers to add a little cement to help it go off has not.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope drafts heritage documents from your property details and flags what it cannot verify, and on a listed building the assessment of significance should be checked by a conservation specialist. Enter your postcode to see which reports a listed building project is likely to need.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.