A sandstone wall rarely fails all at once. More often, the warning signs appear in the joints first – crumbling pointing, hairline gaps, damp patches inside, or stone edges starting to break away. If you are trying to choose the best mortar for old sandstone, the answer is usually far simpler than the bagged products stacked in a builders’ merchant suggest: old sandstone nearly always performs best with a carefully matched lime mortar, not a hard modern cement mix.
That matters because mortar is not just there to fill gaps. In traditional stonework, it plays a protective role. It should accommodate slight movement, allow moisture to evaporate, and weather sacrificially before the stone itself begins to suffer. When the wrong mortar is used, particularly on older Edinburgh sandstone buildings, the result is often trapped moisture, accelerated stone decay, and repairs that look tidy for a short while but create deeper problems underneath.
Why the best mortar for old sandstone is usually lime
Most period sandstone buildings were built to work as breathable structures. The stone, mortar and internal finishes all allowed a degree of moisture movement. Rain driven by wind, rising damp, and everyday humidity were managed through evaporation rather than sealed out entirely.
Lime mortar suits that kind of construction. It is more porous and more flexible than cement-rich mortar, which means moisture can escape through the joints instead of being forced into the stone. It also copes better with the slight movement seen in older masonry. Sandstone buildings expand, contract and settle over time. A mortar that is too hard resists that movement, and the stress often transfers into the stone arrises, causing cracking, spalling and localised failure.
This is the key principle many general repairs miss. The best mortar is not the strongest mortar. It is the most compatible mortar.
What goes wrong with cement mortar on old sandstone
Cement mortar became common in later repairs because it sets quickly and appears durable. On modern masonry that may be appropriate in some settings. On historic sandstone, it is often the wrong material.
A hard cement pointing mix can seal the face of the joints while the surrounding stone continues to absorb moisture. In exposed areas, especially on tenements and villas in places such as New Town, Marchmont or Morningside, that trapped moisture can freeze during cold weather and break down the surface of the stone. You then start to see spalling faces, blown edges, and repairs that pull away in awkward patches.
There is also a visual issue. Cement mortar tends to age poorly on heritage stonework. It can look flat, dense and out of keeping with the original character of the building. Traditional lime pointing has a softer, more natural finish that sits properly within old masonry rather than drawing attention to itself.
Choosing the right lime mortar for old sandstone
Even when lime is the correct route, there is no single mix that suits every building. The best mortar for old sandstone depends on the condition of the stone, the exposure of the elevation, the width and depth of the joints, and the original mortar type where evidence remains.
Non-hydraulic lime or hydraulic lime?
Non-hydraulic lime putty mortars are highly breathable and traditionally appropriate in many heritage settings. They cure more slowly and are often used where conditions allow careful workmanship and protection during the curing period.
Natural hydraulic lime, often referred to as NHL, sets more readily and can be suitable for more exposed locations. However, the grade matters. A mortar that is too strong for soft or weathered sandstone can still cause harm, even if it contains lime. Stronger is not automatically better.
For much of Edinburgh’s traditional sandstone building stock, the mortar should be selected with restraint. A modestly hydraulic or carefully gauged lime mortar is often more appropriate than anything overly hard. The mix also needs the right aggregate. Sand colour, grading and texture all affect not just appearance but performance.
Aggregate matters as much as the binder
The sand used in mortar has a major effect on strength, permeability and finish. Well-graded sharp sands usually provide a better structure than very fine uniform sands. Colour matching is equally important on visible elevations. Poor aggregate choice can leave fresh pointing looking bright, patchy or obviously modern against historic masonry.
In heritage restoration, mortar should be matched to the building rather than forcing the building to accept a generic mix. That usually means assessing existing samples, exposure conditions and the state of the surrounding stone before any repointing begins.
Signs your current mortar is unsuitable
You do not always need laboratory analysis to spot a compatibility problem. Certain defects strongly suggest that the mortar is too hard, too dense or simply failing.
If the joints stand proud while the stone around them is wearing back, that is a warning sign. If pieces of sandstone are breaking from the edges while the pointing remains stubbornly intact, the mortar may be stronger than the masonry. Damp staining that lingers after rain, salt deposits, and repeated localised patch failures can also point to moisture being trapped rather than released.
A common issue on older properties is mixed repair history. One section may have original lime joints, another cement patching from decades ago, and another a cosmetic filler repair. That unevenness changes how moisture moves through the wall, so isolated defects are not always isolated causes.
Repointing old sandstone is about more than the mix
Using the best mortar for old sandstone will not deliver a lasting repair if the workmanship is poor. Repointing historic masonry requires careful raking out by hand or with very controlled methods, avoiding unnecessary damage to the stone edges. Joints should be cleaned properly, prepared to the right depth, and dampened where needed so the new mortar cures evenly.
Application technique matters just as much. Mortar should be compacted into the joint in layers where appropriate, then finished to suit the building and exposure. Overly neat, ribbon-like pointing can look wrong on traditional stonework and may not shed water as intended. The aim is a repair that works with the masonry and looks as though it belongs there.
Curing is another part of the process that often gets overlooked. Lime mortar needs time and the right conditions. If it dries too quickly, is exposed too soon, or is applied in poor weather, it can fail before it has had a fair chance to carbonate or set properly.
When patch repairs are not enough
Sometimes the mortar problem is actually a stone problem in disguise. If joints have been left open for too long, water ingress may have already weakened adjacent sandstone. In those cases, repointing alone will not resolve spalling, delamination or cracking. Localised indent repairs or stone replacement may be needed before new mortar is introduced.
This is where specialist assessment matters. A surface-level repair can make a wall look improved while leaving the underlying causes untouched. A preservation-led approach considers the condition of the whole elevation, not just the obvious gaps in the joints.
At A & M Sandstone Restoration Edinburgh, that is central to how we approach historic masonry. The objective is not to make old stonework look newly finished. It is to keep the building sound, breathable and visually consistent with its original character.
How to know what mortar your sandstone building needs
If you own or manage an older sandstone property, the sensible starting point is not choosing a product from a shelf. It is understanding the building. Age, exposure, previous repairs, stone condition and listing status all influence what is appropriate.
A tenement gable taking the full force of weather will not always need exactly the same treatment as a sheltered rear wall. Likewise, a listed townhouse with surviving traditional detailing deserves a more careful specification than a generic repointing job. The mortar should be chosen as part of the restoration strategy, not as an afterthought.
That is why experienced stonemasons often begin with small test areas, mortar sampling and close visual inspection. The best result is usually a balance – enough strength to endure the environment, enough permeability to protect the stone, and the right finish to respect the building’s age and appearance.
If you are seeing failing joints, loose pointing, damp ingress or stone faces beginning to break away, it is worth acting before a straightforward lime repair turns into more extensive restoration. The right mortar will never be the most conspicuous part of a sandstone building, and that is exactly the point. When chosen properly, it quietly protects the masonry for years while the character of the stone remains the part people notice.
