Use of materials
Using traditional materials and techniques will ensure that repairs are visually and structurally compatible with the historic fabric. Modern equivalents common in more recent buildings are generally not appropriate for historic buildings, because of the different construction techniques employed. For example, while the modern cavity wall ensures that no water enters the inner face of the wall, most historic buildings rely on a solid wall construction for its watertightness. If the walls become wet from rain or rising damp the thickness of the masonry ensures that moisture evaporates before it reaches the inner face. Traditional materials such as stone, brick, timber and lime are porous and allow moisture to penetrate the fabric and evaporate again quickly without causing harm.
This natural cycle of harmless water absorption and evaporation relies on the compatibility of the building's materials as well as on internal ventilation and good maintenance. Any alterations of parts of the building will inhibit the process. For example, repointing with hard cement mortar will upset the balance as it is much more impervious than soft lime mortar which enhances moisture evaporation. Likewise, the use of impermeable modern emulsion paints will affect the building's behaviour negatively. Such serious alterations will lead to a dramatic increase of moisture in the walls and decay of the adjacent plaster, masonry and timber.
In order to ensure the continued good performance of historic buildings it is important to use only traditional and "like-for-like" materials and techniques in their repair and maintenance. These may be occasionally complemented by modern materials and techniques but only after careful consideration by the inspecting architect and other qualified contractors and conservators.
Lime is a key material in the conservation of historic buildings. It was used in the construction of most historic buildings in Britain for mortars, plasters, renders and washes. Lime mortar allows for the quick evaporation of water from walls and is strong and flexible enough to accommodate structural movement without cracking. Working with lime is slow and requires skill and patience. However, this is outweighed by the beneficial effects of lime in old buildings.
Not every type of stone has the same characteristics, for example, sedimentary stones, such as sandstone or limestone, behave very differently from igneous stones, such as granite, or metamorphic stones, such as marble. These three categories describe stones which have been formed by different natural processes which result in different physical characteristics. However, within these categories there are countless variations again and, for example, not every limestone can be used for repairs of a limestone building. Only those stones should be used which correspond closely to the original stone, in terms of colour, weathering qualities and porosity. If possible, stone for repairs, alterations and extensions should come from the original quarry.
Your inspecting architect can advise you which materials and techniques are appropriate for the maintenance and repair of your church.
November 2007
Further Information
SPAB, Technical Q&A 11: Repointing
SPAB, Technical Q&A 12: Soft Distemper
SPAB, Technical Q&A 14: Lime
SPAB, Technical Q&A 18: Limewash
(available on the SPAB website)
