Mortar colour and joint profile on a stone cottage extension

On a stone cottage, the pointing changes the look of a wall more than the stone does. Mortar typically covers between a fifth and a third of the visible face of a rubble stone elevation, so a grey cement joint on a warm limestone wall reads as a grey wall. That is why conservation officers spend more time on mortar specification than on stone selection, and why a refusal over pointing is more common than most householders expect.

Get three things right and the extension will sit comfortably against the old building: the binder, the sand, and the joint profile.

Why pointing changes the appearance more than the stone

Look at a random rubble wall from 10 m. You see stone faces of varying size separated by a network of joints, and your eye reads the whole surface as one tone. Widen the joints, raise them proud of the stone, or lighten the mortar and the network dominates. The stone has not changed at all.

Two numbers explain it. Joint width on an older rubble wall varies from a few millimetres to 30 mm or more, because the stones are irregular. And mortar reflects light differently from stone, usually more evenly, so a continuous mortar network reads as a flat plane against a textured one.

The practical conclusion is that a new extension pointed differently from the old building will look like a different wall even if the stone is from the same quarry.

Lime against cement mortar on an older building

Lime is softer and more permeable than the stone around it. Moisture that enters the wall leaves through the joints, and salt damage and movement are taken by the mortar rather than by the stone. That is the whole argument, and it holds on any solid walled building without a cavity.

Cement is stronger and much less permeable. Moisture arriving in the wall cannot leave through the joint, so it leaves through the face of the stone, carrying salts with it. On a soft limestone or sandstone the result is a spalled, powdery face within a few decades, and on a hard gritstone the damage shows instead as failed internal finishes and damp at skirting level.

For a new extension there is a complication worth being honest about. A cavity wall extension in new stone does not need a lime mortar for building physics reasons, because the cavity deals with the moisture. It usually needs one anyway for appearance, since the colour and texture of a lime mortar is what matches the old wall, and because the junction between the two walls is where any mismatch shows.

Hot mixed lime mortars, made by slaking quicklime with the aggregate, are increasingly used on conservation work and produce a softer, more workable mortar. If your mason proposes one, that is a good sign rather than a problem.

Flush, recessed and weathered joint profiles

The profile decides how much shadow the joint casts, and shadow is what the eye reads.

Flush pointing finishes level with the stone face and is the correct profile for most rubble stone buildings. Brushed back with a churn brush when the mortar is green, it exposes the aggregate slightly and matches a weathered wall well.

Recessed pointing sits a few millimetres behind the stone face. It emphasises each stone and casts a shadow line, and on some regional traditions it is correct. On others it looks like a modern conservation fashion applied to the wrong building.

Weatherstruck pointing is cut to a slope to shed water and is more usual on brickwork than on rubble stone. On stone it tends to look mechanical.

Ribbon and strap pointing is proud of the stone face, standing out as a raised band, often with a scribed line. This is the one officers object to, for two reasons. It widens the apparent joint and reduces the stone to a series of islands. And it traps water against the stone face, which accelerates decay rather than preventing it. Removing existing ribbon pointing is often part of a conservation area scheme, and it has to be done by hand.

Ribbon and strap pointing, and why officers object

Worth separating out, because this is where most disagreements happen.

Ribbon pointing became common in the twentieth century because it is quicker. A mason can apply a band of strong mortar over the joint without raking out properly, and the result looks tidy when new.

Three consequences follow. The apparent joint width doubles or more, so the wall looks coarser and the stone looks smaller. The hard mortar band holds water against the arris of each stone. And removing it later is slow, hand work that risks damaging the stone edges it sits on.

An officer asked to approve new ribbon pointing on an extension to a stone cottage will refuse, and an officer asked to approve its removal will usually want a method statement specifying hand tools only, no disc cutters, with a trial area agreed on site first.

Colour matching with local sand

Mortar colour comes overwhelmingly from the sand, not from the binder and not from a pigment. Lime putty is white, natural hydraulic lime is slightly buff, and both take the colour of whatever aggregate they are mixed with.

So the specification should name the sand. A local pit, a named supplier and a stated grading. Many regions have a characteristic sand colour, and using it is the shortest route to a match: yellow sands in parts of the south, red in the midlands, grey in upland areas.

Three practical points.

Pigments are a last resort. They fade unevenly and a pigmented mortar often reads as a slightly wrong colour rather than as a close match.

Mortar dries lighter than it looks when wet, sometimes considerably. Judge a sample panel after at least two weeks, not on the day it is laid.

Specify by volume. A mix of one part NHL 3.5 to two and a half parts of a named sand, measured by bucket on site, is a specification. "Lime mortar to match existing" is not, and it will be refused.

Stone sourcing and coursing

The bond and coursing of the new stonework matters as much as the mortar, and the two are specified together.

Random rubble, squared rubble laid in courses, snecked rubble and ashlar are different walling types, and an extension should follow whichever the original building uses. Copying the coursing height is as important as copying the stone. On a snecked wall, the proportion of small snecks to larger stones is part of the character.

Sourcing takes time. Where the original quarry is closed, a geologically similar stone from a working quarry is the usual answer, and a geological match matters more than a visual match on the sample, because the two will weather towards each other. Reclaimed stone works well where available, with an allowance for sorting and breakage.

Where the scheme is in a conservation area or the cottage is listed, set out the sourcing and coursing in a Heritage Statement with photographs of the existing wall at close range and from a distance. For an unlisted cottage outside a conservation area, the same material belongs in the Design and Access Statement, and the scale and position of the extension will matter more to the officer than the pointing, measured as set out in measuring rear extension depth from the original rear wall.

Evidence to submit for a conservation area scheme

A submission that gets approved without a further round usually contains:

1. Photographs of the existing wall, close up with a scale rule and from 10 m, on an overcast day.
2. A mortar specification stating binder and class, sand source and grading, mix by volume, finish and joint profile by name.
3. The stone: quarry, bed, finish, sizes and the walling type, with the coursing shown on an elevation drawing.
4. A method statement for any repointing of existing fabric, specifying hand raking out to twice the joint width, hand tools only, and protection from frost and rapid drying.
5. An offer of a sample panel of at least one square metre, built on site next to the existing wall and left in place.
6. The junction detail between the existing wall and the new work.

Allow eight weeks for determination of any discharge of condition application, and allow for the sample panel to cure before it is judged. Where an Article 4 direction applies in the conservation area, check what else it catches, since window replacement is frequently caught, as we set out in replacing windows under an Article 4 direction.

Enter your postcode on PlanScope to see which documents your extension application is likely to need.

This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.