If your home was built before around 1919, as most of Glasgow's sandstone tenements and Victorian villas were, the mortar holding it together is almost certainly lime, not cement. Choosing the wrong material for repairs is one of the most common and most expensive mistakes we see on old stone buildings across the west of Scotland.
Traditional stone walls work as a system. The blonde Giffnock and red Locharbriggs sandstone used across Glasgow is porous, and the walls were built without cavities or damp proof courses. They stay dry by breathing: moisture soaks in during wet weather and evaporates back out through the joints. Lime mortar is softer and more permeable than the stone, so the joints act as the escape route for that moisture.
Cement mortar turns that system on its head. It is harder and far less permeable than sandstone, so water that gets into the wall becomes trapped behind the pointing. With Glasgow seeing well over 1,100mm of rain in a typical year, that trapped moisture has to go somewhere, and it goes through the stone itself.
When moisture can no longer evaporate through the joints, it evaporates through the face of the stone instead, carrying dissolved salts with it. Over the years this causes the surface to blister and crumble, a process masons call spalling. You will see it on tenements all over the city: stones with their faces eaten back 20mm or more while the hard cement joints stand proud around them.
The damage is slow, often taking 10 to 20 years to show, which is why cement repointing can look perfectly tidy for a decade before the real cost appears. By that point the fix is no longer repointing but stone repair or indenting in new stone, which costs many times more. Sound lime pointing, by contrast, sacrifices itself gradually and can simply be renewed every 40 to 60 years without harming the stone.
Original lime mortar is usually off white to buff coloured, relatively soft, and will scratch or crumble slightly with a key or coin. Cement mortar is grey, hard, and often applied as a raised strap or ribbon over the joints, a style common in cheap repointing jobs from the 1960s onwards. Many Glasgow tenements now carry a patchwork of both.
If your building is listed or sits in one of the city's conservation areas, such as Dennistoun, Pollokshields or the West End, like for like lime repair is generally what the council conservation officers expect to see, and using cement can create problems when you come to sell. Historic Environment Scotland publishes free guidance, and any competent stonemason should be happy to tell you exactly what mix they intend to use and why.
Good practice is to rake out the old joints by hand to a depth of at least twice the joint width, avoiding disc cutters on the stone, then repoint with a lime mortar matched to the original, typically a natural hydraulic lime such as NHL 2 or NHL 3.5 with a well graded sharp sand. New lime work needs protecting from rain and frost while it cures, which is why the main repointing season in central Scotland runs roughly from April to October.
Costs depend on access, joint condition and how much cement has to be removed first. As a rough guide, hand repointing in lime tends to run from around £60 to £120 per square metre in the Glasgow area, with scaffolding priced on top for anything above ground floor level. Removing hard cement pointing without damaging the stone arrises is slow work and adds to the labour, which is worth budgeting for on previously botched walls. It is dearer than a quick cement job, but it is the version you only pay for once a generation.
It is best avoided even for small repairs, because each cement patch creates a local point where moisture gets trapped and the surrounding stone starts to decay. Small tubs of ready mixed lime mortar are inexpensive and just as easy to use for minor jobs.
Natural hydraulic lime firms up within days but continues gaining strength for weeks, and it needs protection from rain, frost and drying winds during that period. This is why reputable masons in Scotland avoid lime work in late autumn and winter.
Not necessarily all at once. If the stone behind is still sound, removal can be prioritised on the worst affected elevations, usually those facing the prevailing weather, and phased over time. A survey will show where the damage is actually progressing.