70 THE BUILDING CONSERVATION DIRECTORY 2024 CATHEDRAL COMMUNICATIONS sometimes used in conjunction with these earth and clay mortars to build structures which were well adapted to many different locations and durable in a wide range of climates. Earth mortars were used in rubble-built structures like houses, farm steadings and cottages as bedding mortars for masonry and as a filling material for wall cores, often mixed with aggregate and straw. They were common in areas of clay-rich subsoils. Heavy clay from a silicate-rich soil was needed to bind the matrix of the mortar together and support compression loads. When properly maintained, earth and clay mortars are resilient, durable, flexible, allow for thermal expansion, have good insulating properties and can handle occasional wetting. However, because of their susceptibility to erosion by water, both earth and clay mortars can be difficult to find and evidence of their use is easily overlooked. Vernacular buildings where they were used were often externally pointed or harled with lime mortar as additional weather protection. As a result, identifying them deep within the body of the wall usually requires some type of intervention in the building fabric, like creating a small opening, as often happens accidentally during building works. Where they are not washed out of the core of the wall as a result of poor maintenance and accidental leaks, earth mortars can be identified by their appearance – a soft, crushable material like fine powder, and reddish or yellow in colour. Sometimes a reddish or brown staining appears on external masonry where these mortars are being washed out. THATCH Thatch is one of the oldest roof coverings and was once common, but today only one or two examples survive in areas where there may have been hundreds. Thatch was usually made out of secondary products from agriculture, livestock and land management, eliminating waste. This included straw from harvested cereal crops, heather or bracken from grazing pastures, and rush and reed from riverbeds. Heat is slow to escape from a thatched roof and the material provides natural ventilation. When combined with thick walls of solid masonry, thatch helps the building stay warm in winter and cool in summer. It also sheds water the same way as a traditional slate roof, by passing water down the slope of the roof; stiffer materials at the eaves (like reed or straw) hanging down and over the wall head diverts run-off from the walls and foundations so the walls remain dry. They also resist windlift. In the Northern and Western Isles of Scotland, common vernacular details include: • netted thatched roofs • no sharp angles at ridges and hips • no projections at the eaves • skews anchored with large turves or by roping anchor points on the gables • projecting wallheads to deflect the wind • swept thatch up against the sides on chimneys and wrapped rope around them. Base layers and the roof structure were protected by thatch. These are often the oldest part of a thatched building and should be retained, recorded and analysed. A complete rethatch requires noting the fixing methods and materials from previous phases to carry out a likefor-like rethatch. Materials and fixings could include reed and straw fixed with hazel spars or ‘scobs’ (large staples), heather tied on with twisted rope or sewn into rafters using a stitching needle, or straw and marram (especially in the Hebrides) using coir rope or old fishing nets weighted down with field stones. OTHER ROOF COVERINGS Vernacular roof coverings in Scotland include widely used materials like slates, leadwork and clay pantiles, and more rarely: • Flagstone, used in local areas as they were difficult to transport. Caithness flagstone and Carmylie slate were common as they could be split along a bedding plane to form tiles. • Stone corbelling, constructed using a drywalling technique where each stone course projects slightly beyond the one below, gradually bringing each side of the structure closer together. • Timber shingles, comprising wooden slates applied in courses, similar to stone slates. Only rare, isolated examples of shingles exist. Since the 19th century, vernacular roof coverings like thatch have often been protected by corrugated iron. Patented in 1828, it was often used for roofs, walls, ridging details, ventilators, and windows. It was popular in rural areas and in agricultural buildings. Corrugated iron is light, portable and adaptable. Flat iron sheets are passed through rollers to create corrugations for rigidity and structural strength. From the late 1800s, they were also galvanised with zinc to protect from rust; later, perforated sheets were used to tackle issues of condensation. Sheets can be up to three metres (10 ft) in length and in a variety of thicknesses (0.5–1.5 mm) and pitches (25–153 mm, with 76 mm common in domestic properties). They come in several different profiles: • regular curved peaks and troughs, sometimes known as ‘sinusoidal’ • curved corrugations with an area of flat edging • ‘weatherboard’ or stepped corrugations (rather than curved) • small, wide-spaced ribbed corrugations. They are also sometimes covered in protective coatings, usually oil-based paints (especially linseed), red lead, pitch or bitumen. Specialist fixings were needed for corrugated iron sheets and where these exist they should be cleaned and re-used. TIMBER FRAMES Cruck frame cottages and houses were once common in Scotland, as they were simple to build and used local materials. The cruck frame, or cuppill as it can be known in Scotland, was made with pairs of large, curved timbers (couples), selected to suit the width and height of the house or cottage. These rose from the ground or the base of the walls and curved inwards to support the roof. The two halves were joined together to form the cruck frame by a short and stout capping member close to the apex and by a long heavy collar lower down. Running between the crucks from end to end were long, thin purlins which supported roof poles or rafters. The timber joints were often held together with wooden pegs instead of metal nails. Because the couples held up the roof, the walls carried little weight Stone weights and netting secure the thatch at the eaves of a traditional croft (Photo: Historic Environment Scotland)
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