The process of sympathetic restoration is exactly that and the exact same materials and techniques which were used hundreds of years ago must be employed in the 21st century.
The example below is what lime mortar looks like when it is in need of replacement. Masonry bees have severely damaged it, it's weathered and there's evidence of cement mortar.
After. Natural hydraulic lime, although slightly cementitious in colour, makes a more than adequate mortar for period stonework.
Below: A well intentioned repair which unfortunately falls short of the benchmark.
The above photo shows a repair where the lime mortar has been mistakenly trowelled in to try and achieve a weather-struck finish in the same way a bricklayer would finish repointing brickwork with cement. One should never attempt weather-stuck with stone. The problem now is that it has cracked, is not weather-proof and will fall out.
The above photo was taken whilst doing some red brick restoration work in St Clements, Oxford. I've included it to demonstrate just how much mortar actually needs to be removed during repointing.
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Cement mortar compared to lime mortar:
Why bother using lime when comparatively cheaper cement is available? Traditional building construction is based on the use of relatively soft and porous materials such as stone, brick, timber and earth used together with a lime based mortar for bedding and plastering. Buildings made from these materials usually have solid walls with no cavity and are often built on insubstantial foundations. They are therefore liable to settlement and movement associated with seasonal changes in ground conditions. Lime mortar is softer and weaker than the stone or brick which it bonds and is therefore able to accommodate slight movements caused by settlement or temperature changes without significant cracking. It is also permeable and allows evaporation of rising and penetrating damp from within the wall. It is this permeability, or 'breathing', which helps to keep the building dry inside without a damp proof course or chemical treatments.
What is wrong with cement mortar? Modern cement pointing is very different from lime mortar. It is hard and brittle and completely waterproof. Its use on traditional masonry is damaging in several ways...
Cement pointing: as it is harder than soft brick or stone and is too rigid to accommodate settlement or movement in a wall or house it means that when movement occurs the edges of the stone or brick are forced against the hard mortar spaling the masonry.
Below: Here even the wrong type of sand has gone into the cement mortar mix and the stone has deteriorated badly. It is in desperate need of repair.
Further damage is caused by rainwater seeping into the cracks in the pointing and around the edges of the stones. As the mortar is not permeable this moisture cannot evaporate from the mortar joint once rain stops. Instead it is forced to evaporate through the face of the brick or stone and soluble salts present (sodium sulphate for example) in the water crystallise in the surface layers of the masonry leading to crumbling and decay. This is sometimes so severe that the entire face of the stone is lost and the hard cement pointing is left standing proud. Further rainwater is trapped and the decay continues. The concentration of trapped water in the masonry also increases its susceptibility to frost damage in winter.
In contrast, soft lime mortar allows moisture movement and, being more porous than the masonry, encourages evaporation and salt deposition in the mortar joints. Thus it is the mortar which decays and not the stone or brick. It is much easier and cheaper to repoint a wall than to repair or replace damaged stone and there is less loss of important historic fabric.
Above: This style of re-pointing is known as 'double struck'. It may look attractive at first glance but it has been done with cement mortar and trowelled in. Although many bricklayers and builders may create this effect, it indicates insufficient training and experience in traditional methods and materials. The fact is, both weather-struck and double struck were useless inventions born out of the belief that striking the mortar in different ways would somehow be beneficial as far a rain water run off was concerned. A 16th/17th Century craftsman would not have recognised this at all: Lime mortar should have been used and the final effect should be brushed joints with the aggregates in the sand showing.
Below: Before: For some reason the previous contractor had smeared a rich cement mortar very thinly over the joints and stone of this Witney cottage; in effect waterproofing the whole house and turning it a horrible grey colour.
Below: After: I raked out the joints with masonry friendly tools; applied the correct lime mortar mix and used the right brushes to achieve this professional affect. Not only does this Witney property look better, it has increased the saleability of the house.
Sympathetic restoration: On older houses some of the original masonry will deteriorate so badly it needs to be replaced. Here what is needed is experience as only small sections can be restored at a time - only so many pieces of stone can be removed without compromising the structural integrity of a house or wall.
The restoration of a property does not just mean hydraulic lime rendering, re-pointing or stone replacement but can extend to organic paints, sheep's wool insulation, lime wash, lime putty and imperial hand made bricks.
Cement render: This causes slightly different problems. Hairline shrinkage cracks inevitably form in the surface of the render as it sets or afterwards by slight movement in the wall. The rainwater is drawn by capillary action into these cracks and then diffuses into the wall. Once inside the wall this moisture, together with any rising damp, is trapped as it cannot evaporate through the hard, impermeable render. Moisture levels start to build up in the wall and the moisture tends to diffuse towards the inner surface of the wall resulting in internal dampness and damage to plaster and decorations. So, strange as it may seem, applying a waterproof render can actually increase levels of damp inside the house or wall. A porous lime render encourages evaporation of moisture from its surface, helping to minimise the effects of penetrating and rising damp.
For lime mortar repointing in Bicester, Launton, Dadford, Buckingham, Brackley, Pury End, Towcester, Northampton, Stratton Audley, Milton Keynes, Fringford, Stowe, Chackmore, Lillingstone.