Slating in South-West England
Total Page：16
File Type：pdf, Size：1020Kb
Slating in South-West England SPAB Regional Technical Advice Note Terry Hughes BSc (Hons), FIoR Dp Man Society for the Protection of Ancient Buildings 37 Spital Square, London, E1 6DY 020 7377 1644 [email protected] www.spab.org.uk 1 Contents This advice note describes the production and use of slates in the counties of Cornwall, Devon and Somerset, and deals with the maintenance and repair of slate roofs and wall cladding. It considers regional details specific to the traditions of roofing and cladding in south-west England and explains, where appropriate, adaptations to incorporate modern requirements such as improved thermal performance. 1 Introduction ...................... 3 2 History ...................... 6 3 Characteristics ...................... 6 4 Winning and production of slates ...................... 9 5 Work in general ...................... 13 6 Recording and assessment ...................... 16 7 Repairs and reslating ...................... 18 8 Specification and detailing ...................... 26 9 Scantle slating ...................... 33 10 Rag slating ...................... 36 11 Patent slating ...................... 38 12 Wall cladding ...................... 40 13 Maintenance ...................... 40 14 Notes and references ...................... 41 15 Other advice ...................... 43 16 Glossary ...................... 45 2 1 Introduction Vernacular slate roofs are important elements in It is important that the details of roofs are the character of historic buildings and settlements recorded before the slating is stripped and that the in south-west England, and help give individual information obtained is used to derive appropriate buildings, towns and villages their sense of place. specifications for re-slating. Each surviving This is in part due to the materials themselves, with example is likely to contain valuable information their different colours, textures and sizes, but also about traditional craft techniques and local to the way in which they are laid and detailed. variations. The massive rag slates of north Cornwall are seen in In general, the existing form and details of each impressively large numbers on roofs in Launceston, historic building should be respected and retained for instance, where the hilly topography makes during any campaign of repair. This extends not them more prominent. The small scantle slates by only to the overall appearance of the building but contrast give a completely different character to also to the construction methods employed. the mining settlements and historic landscapes of The broad principle of retaining existing details west Cornwall. Slate-hanging on walls is common does not mean that they should simply be in both Devon and Cornwall, but less so in other replicated without thought. There are situations south-west counties. The use of large rag slates at where details may need to be sensitively the roof verges and eaves and, in the South Hams adapted to improve performance but if so, this district of Devon, coloured slates for decorative should follow careful analysis in such a way cladding are important local features. The loss that the traditional local form and appearance of these distinctive local slating traditions would is maintained and with appropriate statutory result in blandness and uniformity. consents. This advice note explains the basics of south- West Country slate roofs use a variety of slating west slating practices, including many of the local systems: common; rag; scantle and sized (Table 1). variations. Their geographical extent is generally unrecorded Table 1: West Country slating techniques 3 and their boundaries may be blurred. Some of Terminology the techniques are not unknown from other parts of the United Kingdom – most notably along the Technical terms used in this advice note and not west coast – but they are less prominent in those explained in the text are defined in section 15 regions. Today the most frequently used type of and a fuller glossary of slate and stone roofing is slate roofing for new building is sized slates laid referenced in 14.2. The more common terms are to a constant head lap to the detriment of the used with local variations bracketed. The meanings regional roofscape. This type is not included in this of some words are variable and need to be advice1. understood before reading on: Common slating - large (mounter) slates at the Slate length: The length of slates sold by quarries eaves with small slates laid in thirds over the rest of is the overall dimension from the tail to the head the slope - was a medieval style and such roofs are (bottom edge to top edge), But for top-fixed slates now uncommon2. Its decline began when, in the the effective length is the dimension from the tail late 17th century, quarries began supplying slates to the bottom of the fixing hole and this is typically as finished products and selling them in specific 25 mm or one inch less than the overall length. This size ranges, such as rags and scantles. Previously is important because it is the length to the fixing they had been supplied as rough blocks or rivings hole which is used to establish the gauging of the to be finished by the roofer3 The visually similar roof and hence the area which can be covered with and distinctive roofs seen in the South Hams a given quantity of slates. Failure to appreciate district of Devon may be derived from this system. this can result in expensive under-estimates of the cost of a roof. When carrying out roof surveys or Rag roofs and scantle roofs remain very common recording, it is the length to the fixing hole which within their regions but are in decline due to should be measured. difficulties of supply in the case of rags, lack of knowledge or skill or ill-judged attempts to Pin slating: Triple-lap slating is known as either improve. Rag roofs are most common in east Three-and-a-half-pin or four-and-a-half-pin (Figure Cornwall and into west Devon. Scantle slating is 2). In the former the first count is taken on the slate either dry-laid or wet-laid - tail bedded in mortar. and in the latter it is on the scantle stick but they Wet laying is employed in the more exposed areas both mean the same thing and in this advice note of west Cornwall and dry in the more sheltered the term three-and-a-half pin is used. Devon and at sheltered locations within the Scantle. In slating the term scantle has several predominately wet-laid areas. meanings and this leads to confusion in specifications. It can mean small slates typically Slating in thirds appears to be rare (or perhaps 12 to 6 inches long4 (305 to 152 mm). But it is also under-recognised) in Cornwall and the extent of its used to mean their use when triple-lapped (Figure use in other counties still needs to be established. 2). In this advice note any reference to scantle Historically, all the systems used slates from the slates means small slates laid in this way. A scantle nearest sources within their own counties (Figure is also the name of the gauging stick used to sort 1). As transport systems improved and the quarries slates into length sets and to set out the slating. closed, slates were used from more distant sources. This meaning is derived from the more general term for any small timber. In recent years there have been misunderstandings of the traditional systems and confusion about Dimensions: Although metric dimensions were their details leading to roof failures. In part this adopted long ago many slaters still work in inches is because of attempts to apply British Standard and the slate manufacturers still make imperial size recommendations and detailing. These are not slates and quote approximate metric equivalents. always appropriate for conservation because In this guide where past practices are described they do not cover vernacular techniques and where direct quotations are provided from comprehensively. The issue of differences between historic documents the inch dimensions are used. modern and historic slating should be dealt with at Otherwise and for descriptions of modern practice the design stage of a project. approximate metric equivalents are given with the imperial dimensions in brackets. 4 Figure 1 Figure 2 5 2 History Slate has been exploited to provide effective styles and those quarries which were too small and long-lasting roof coverings for buildings in to compete closed down. In spite of attempts by south–west England for many centuries. There the Arts and Crafts movement to encourage the is archaeological evidence throughout England use of Cornish slates and vernacular techniques, of roofing slates from excavations of Romano- the industry continued to decline throughout the British sites but the earliest post-Roman records 19th and 20th centuries. The last slate quarries in of slate quarrying date from the beginning of Devon ceased trading in the early 20th century the 12th century. At this time roofing slate use and Treborough quarry in Somerset closed in 1938. was restricted to important buildings, including Today, only the Cornish quarries Delabole and some as far away as Kent and Essex, but it was Trevillet (Mill Hill) still produce roofing slates. not until the 15th century that slate began to generally replace thatch on ordinary buildings. Alongside the introduction of tally slates, sawn In more inaccessible areas, such as the Scilly Isles, battens became available and these allowed slates the change did not take place until the early 18th to be centre-nailed rather than top-hung. Slates century. fixed in this way do not need to be bedded in mortar because they are inherently resistant to Whilst the slate itself is durable and may last wind uplift. Bedding therefore ceased for all but hundreds of years, slate roofs have a finite life the smaller slates which continued to be top-hung.