Natural stone replacement is cutting out a damaged or worn stone and setting a matching new one in its place. In building conservation the partial replacement of decayed stone is called indenting. It is painstaking work, because the new stone has to match not just in size but in type, colour, bedding direction and surface finish.
Repair or replace?
- Repair suits small spalled corners, hairline cracks and surface damage. It is done with a repair mortar matched to the stone in colour, aggregate and strength — deliberately slightly weaker than the stone, so that future damage occurs in the repair rather than in the original
- Replace where there are through cracks, broken-out load-bearing sections, stone powdering through its full depth, or serious salt damage
- A slip or plated indent — a thinner inset piece — is the middle course when only the face has gone
The method, step by step
- Establish the cause. Replacing without addressing the cause gives you the same damage in five years. Look for leaking copings, rusting cramps and dowels, hard cement pointing and rising damp
- Identify the stone — on historic work, involve a stonemason or conservation adviser. The original quarry bed is often no longer available, so a well-matched alternative is needed
- Prop anything load-bearing before removing a single stone
- Cut the stone out by sawing or raking the joints, not by knocking it out. Neighbouring stones must survive intact
- Clean the bed and remove all old, hard cement
- Set the new stone in a lime-based mortar, with stainless dowels and cramps where the originals were iron
- Point up with a mortar softer than the stone — in practice, lime-based
Points to watch
- Respect the bedding. Stone is set the way it grew; face-bedded sedimentary stone delaminates in layers
- Never leave rusting iron in place. Expanding rust is the most common cause of split natural stone
- Colour difference is normal. New stone is lighter and only weathers in over years; ask for a sample and judge it outdoors, wet and dry
- On a listed building consent is almost always required, even for work that looks like maintenance. Talk to the conservation officer first
- Costs. Replacing a single bluestone threshold or a section of window cill, including removal and setting, typically runs from several hundred to over a thousand euros or pounds per piece; bespoke cutting and scaffolding raise that quickly
Related terms
- Natural stone maintenance
- Natural stone finishes
- Stone dressing
- Lime mortar
- Bluestone
- Brick toothing
- Listed building
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