The Building Conservation Directory 2023

128 T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 2 3 | C E L E B R AT I N G 3 0 Y E A R S C AT H E D R A L C O M M U N I C AT I O N S REPAIRS TO LIME RENDERS DAVID SLEIGHT L IME-BASED COATINGS to external surfaces of structures have been around for thousands of years. The Romans used them to create a smooth surface layer for their frescos and wall paintings. In medieval Europe a mixture of sand, clay, straw and even animal dung was daubed onto a wattle, a panel of woven sticks or strips, to form weather-resistant walls. In the UK in areas of clay-rich subsoil, earth mortars were applied as daub (as in ‘wattle and daub’), and clays have been used as simple plasters and renders. Elsewhere, lime coatings have been used on the external elevations of buildings and applied either by hand, by throwing the material at the wall (termed harling or roughcast), or by trowel or float (render, plaster or stucco). Roughcast (also called ‘wet-dash’ in Cumbria) is a lime-based mortar with a coarse finishing coat containing gravel thrown in a premixed state onto a wall. It is distinct from pebbledash (or ‘dry dash’) in which the aggregate is applied separately onto wet mortar so the aggregate is visible, not coated with mortar. Historically, roughcast was popular for rural buildings across much of the UK, except the south- eastern half of England. Application techniques for versions of roughcast typically involve two base coats of material applied by trowel and just the final top coat of material thrown at the surface Harling is predominately a Scottish version of roughcast, but in this case all lime coats are thrown or ‘harled’ against the wall. The appearance and character of the harling or roughcast would depend upon the type and size of local aggregate available at the time of construction. This could result in the surface finish of buildings varying in appearance due to the size and type of aggregates used in the render, and it is even possible that individual elevations of a building may vary if aggregate sources changed during construction. All cast renders are fairly quick to apply and less inclined to shrinkage than those applied by trowel as their layers are relatively thin. Casting or throwing the lime mortar against a wall surface, especially an uneven undulating one, has the advantage that, when thrown with some force, the mortar is compacted on impact, expelling air and ensuring a good bond between the mortar and the wall surface. Trowel applied renders cannot provide the same pressure to any undulations and depressions on the wall surface as cast renders as it is difficult to maintain a constant pressure. If care is not taken depressions within the surface of the wall may not receive the same compaction and the render may adhere less in those areas. Deep undulations may require dubbing out with porous tiles or stones bedded in render to bring the wall to a level surface. Dubbing out with just lime mortar may lead to uneven drying and poor carbonation as well as possible shrinkage. Until the 18th century the majority of lime renders were applied in two coats, but eventually three coat work became the norm. At the end of the 18th and in the early 19th century new types of renders were developed to imitate the appearance of fine ashlar better. These were based on various hydraulic limes and natural cements, such as Parker’s ‘Roman cement’ (patented in 1796), Joseph Aspdin’s ‘Portland’ cement (patented 1824) and many others. Non hydraulic lime was gradually abandoned in the 19th century in preference for these new binders, and in some cases these would be mixed with various sands and limestones to resemble natural stones such as Portland stone. During the latter part of the 19th century natural cement renders faced increasing competition from Portland cement. The use of this new fast-setting cement in renders resulted in variations of roughcast and pebbledash that were based on a combination of lime, sand and cement. At the beginning of the 20th century typical specifications may have been 1:1:6 cement:lime:sand. By today’s standards that would be a very dense impermeable mix, although the cement of the early 20th century was not as strong as the Portland cements of today. Stucco is a type of render applied to rough exterior surface of walls to imitate flat ashlar or fine plaster finish. Historically, binders have included gypsum, limes, sometimes with pozzolanic additives such as brick powder; cements; and linseed oil (mastic). Aggregates have included marble dust, crushed stone and sand. All have been used in varying mixes and strengths with a wide range of colorants and other additives. Common forms include common stucco , an exterior render prepared from hydraulic lime, sand and hair; rough stucco , a fine plaster of sand and lime made from chalk or a very pure limestone which was used internally to imitate stonework; bastard stucco , a superior render prepared from non-hydraulic lime and fine washed sand which was applied to a good backing coat, scoured and either polished or left floated; and trowelled stucco – a non-hydraulic lime render applied as a normal finishing coat, scoured, polished and then painted. Pebbledash is a modern term for a type of roughcast render generally containing sand and cement and historically some lime. A simple roughcast render in Sherston, Wiltshire which has been limewashed Fine Regency stucco in Cheltenham