Tuesday, January 08, 2008

Stonework care and repair

Above: Dave Bridgewater's house in Bath an example of a Limestone Building

Most commonly used building stones are sandstone and gritstone, limestone, granite and slate. Granite and slate are usually fairly tough and resistant to decay, whereas limestone and sandstone tend to be more affected by the elements, particularly in polluted environments. Once they are removed from the quarry and exposed to the atmosphere they will start to weather and decay - their ultimate aim is to turn into soil. It is important that you know the cause of the problem and the possible range of solutions. There are four aspects to successful care and repair of stonework; maintenance, cleaning, repair and protection.

Maintenance - Stone decay is inevitable and no remedial treatment can halt it, but regular maintenance can prevent small defects developing into something more serious and greatly prolong the life of stonework. General maintenance tips include checking gutters and downpipes regularly for leaks as these concentrate water in the stone work increasing the risk of frost damage and cause unsightly staining. Remove all plants from the stone work as this can cause damage and channel water into the wall. Carry out patch re-pointing using a matching lime mortar. Never rake out sound original pointing.

Cleaning - Should never be undertaken without a detailed survey by an expert to establish the type of stone, the type of soiling and weather the dirt is harming the stone or is just unsightly. If the dirt is harmful then the cleaning will probably be justified. If not, you will need to weigh up the benefits of a cleaner building against the damage caused by cleaning. The main cleaning methods are water washing, mechanical cleaning and chemical cleaning.

Repair and restoration - Stone which is split or fractured can be strengthened by inserting tiny stainless steel pins. Cracks should be filled with fine-grained lime mortar to consolidate the surface and shed water. Repairs must be weaker than the surrounding stone so that future decay takes place in the mortar and not in the stone; its is much easier to renew a repair than to deal with continued stone decay. Damaged areas can be restored by piecing-in new stone or; as a last resort, by replacing an entire block of stone.

Protection - Historic stone can be protected using various lime-based methods, which can help to slow down the rate of decay. Sheltercoat consists of lime putty and fine stone-dust, worked into the surface of the stone. It is good for protecting mortar repairs and blending new work with old but is only suitable for limestone. Limewash, made from lime putty and pigments diluted in water, provides traditional surface protection to masonry walls. Lime render can protect severely decayed masonry whilst still allowing the building to breathe. Many buildings were originally rendered in this way but have been stripped in modern times, exposing the bare stonework and making them vulnerable to water penetration.


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