Why the Stone You Choose Matters

A garden wall is usually the largest bit of traditional craftsmanship on a property, and once it is up, it stays up for decades. So the stone you pick is not just a colour choice. It decides how the wall weathers, how it handles frost, how it looks when wet, how easy it is to build well, and how much it will cost to repair. In the UK we are lucky in that three broad families of stone do almost all the heavy lifting: sandstone, limestone and granite. Each has a character, and each suits certain places better than others.

Sandstone: Warmth, Variety and Workability

Sandstone is sedimentary, laid down as sand that has been compressed and cemented over millions of years. It splits along bedding planes with a clear grain, which makes it a pleasure to dress and shape on site. That workability is why so much of our traditional walling, from Yorkshire coursed stone to the honeyed rubble of the Welsh borders, is sandstone.

Colour is its great gift. Buff, honey, oatmeal, pale grey, deeper reds and rust tones, sometimes all in a single quarry. It takes tooled finishes beautifully, so a garden with formal lines and crisp quoins can be built from the same stone as one with tumbled, weathered faces.

Its weakness is porosity. Soft sandstones can soak up water and, in a hard frost, spall at the surface. This is manageable, and the rules are simple:

• Lay stone on its natural bed, never face-bedded, so water can shed off the surface rather than run along the bedding planes.
• Use a lime mortar soft enough to accommodate movement and allow moisture to evaporate.
• Give the wall a proper coping with a slight overhang to throw water clear of the face.
• Avoid soft sandstone for a wall that never dries out, such as the base of a north-facing retaining wall.

Limestone: Pale, Formal and Quietly Hardy

Limestone is also sedimentary, formed largely from calcium carbonate in shallow seas. It ranges enormously. The oolitic limestones of the Cotswolds and Lincolnshire are creamy, fine-textured and almost luminous in afternoon sun. The harder Carboniferous limestones of the Peak District and the Mendips are grey, dense and much more resistant to weather.

For garden walls this range is useful. Softer pale limestone is ideal for a formal wall, a wall with plinths and rounded copings, a run that frames a border without shouting. It tools cleanly, holds a rubbed or chisel-drafted finish, and takes age gracefully, developing a patina rather than crumbling. Harder limestone is the better choice for exposed sites, long boundary walls and any wall that needs to hold back soil.

Two practical points. First, limestone dissolves very slowly in acidic rain, so avoid pointing it with hard cement mortar that traps water against the stone, and keep aggressive cleaning chemicals away. Second, a cream limestone wall can look harsh on a grey day until the stone has weathered, so a sample panel, even a metre of it, is worth the effort before you commit to the whole run.

Granite: The Long Game

Granite is igneous, cooled from molten rock, and it is properly tough. It is dense, near-impermeable, essentially frost-proof, and available in silver-greys, deeper charcoals and soft pinks. In the UK it belongs to the uplands: Cornish and Dartmoor granite, the granite of Aberdeenshire, Cumbrian and Scottish border country.

Its toughness is also its drawback. Granite resists the chisel, so it is worked with diamond tooling and split with plugs and feathers rather than dressed by hand. It costs more per square metre, and it is heavy. Where it earns its keep is in exposed, wind-driven sites, in retaining walls, in rustic rubble walls that need no mortar, and as coping on top of a softer sandstone or limestone wall, where its impermeability protects everything beneath.

Granite is also the most striking stone for dry stone walling, because large, irregular lumps of it can be laid and locked without any mortar at all. A well-built granite dry stone wall can stand for a century with nothing more than occasional tidying of the cope.

Matching Stone to Mortar, Footing and Exposure

Whichever stone you choose, the wall's long-term health depends on what happens underneath and around it. A few rules of thumb:

• Footings should be at least twice the wall thickness wide, and taken down below the frost line, typically 450mm on a low garden wall.
• Use lime mortar on old or soft stone, and always weaker than the stone itself, so any movement cracks the mortar rather than the wall.
• Keep joints recessed slightly, never flush or proud, and finish with a brushed, not tooled, surface so water drains away.
• Cap the wall with coping laid on a bedding of mortar, or dry, with a slight fall to the outside.
• In a coastal or upland garden, favour granite or a hard limestone and expect more frequent repointing.

Buying, Sorting and Building Well

Buy more stone than you think you need, usually ten per cent extra, and sort it into sizes before you start. Big stones go at the base, medium in the body, small as hearting and packing. A wall built from graded stone, with roughly one-third of its volume buried as hearting, is far stronger than one made only from face stones.

Reclaimed stone is worth chasing. It arrives pre-weathered, matches existing boundary walls, and carries the soft edges that only time produces. Visit the yard, look at wet stone as well as dry, and check a handful for soundness with a tap; a dull thud means a fault inside. Choose patiently, lay carefully, and the wall will look settled from the day it is finished.