The Building Conservation Directory 2020

77 C AT H E D R A L COMMU N I C AT I O N S T H E B U I L D I N G CON S E R VAT I ON D I R E C TO R Y 2 0 2 0 ROOFING 3.1 Where the method is a vernacular style, authentic conservation and repair are precarious at best. Not only are they dependent on supplies of the original large slates or stones being available, but also on people recognising the roofs’ heritage and architectural value. For patent slated listed buildings the future is secure because the slates are readily available from the original quarries and modern joint sealing techniques are well established. Beyond that all that is needed is that once a roof has been repaired it should be inspected periodically without walking directly on the slates. Recommended reading and references Charles Rawlinson, The directory for patent- slating. Calculated to instruct noblemen, gentlemen ... in the use of the new-invented method of covering the roofs of churches, houses … with slates, etc. Londo n, printed for the patentee at Lostwithiel, 1772 (British Library General Reference Collection 1651/474) Terry G Hughes, 2016, Slating in South West England , SPAB Regional Technical Advice www.spab.org.uk/advice/technical-advice- notes David Gwyn, Welsh Slate: Archaeology and History of an Industr y, RCAHMW 2015 p44 (quoting DD Pritchard Aspects of the Slate Industry, 1943 and J Lindsey, A History of the North Wales Slate Industry , 1974) Stone Roofing Association leaflets: Patent Slating – www.stoneroof.org.uk/historic/Historic Roofs/Patent_slating.html St George’s Church Everton – www.stoneroof.org.uk/historic/Historic_ Roofs/Everton_2.html TERRY HUGHES BSc FIoR DpMan is the secretary of the Stone Roofing Association and a specialist consultant. His company, Slate & Stone Consultants, based in Caernarfon, advises on slate and stone roofing construction, materials and conservation, and on sourcing new material to match the existing. Fig 6: An example of patent slate cladding in Tavistock (Photo: Richard Jordan) Figure 7: William North’s patent incorporated ‘a fillet of slate, lead or other metal attached to the underside of the slate or slab; which fillet is scalloped on its underside, more or less as may be required, so as to admit air from without to the timbers of the roof and to let steam, heat and damp, escape from within.’ It also included what we would today call an interlock at the lateral butt joints. (From The Repertory of Patent Inventions and Other Discoveries and Improvements in Arts, Manufacture and Agriculture vol III Jan ) Fig 8: Colley House, Reigate. The slates were butt jointed vertically as well as horizontally. The vertical joints were bedded onto slate noggins rebated into the rafters. It may be an example of William North’s system. Fig 9: A recent example of under-sealed patent slating in Warsaw seen from the ridge looking down. It uses metal channels to weather the joints. (Photo: Valerijus Shlikas, Skaluneris UAB) Fig 10: Patent slating can usually be repaired and conserved. This under-sealed roof in Stromness, Orkney was renewed in 2005. Fig 11: An original 1860s roof which was repaired using new and reused slates in the 1980s. The main difficulty when re-laying these roofs is removing the slates and cover strips without breaking the fixing holes. To deal with all these defects the gutters were lined with stainless steel and, to increase the shallow upstand of the roof over the gutter, the slating was raised with a new timber structure. This also provided an opportunity to support the slates at closer centres to reduce the chances of them being broken if they were ever walked on. A FUTURE FOR THE SYSTEM Classic patent slating seems to have fallen out of favour by the middle of the 19th century but the urge to devise methods of single-lap slating was undiminished. Many patents for similar single-lap systems were registered in Europe and the USA in the 19th and 20th centuries and some are still being registered today. Modern variations tend to use slate, metal or various sorts of damp-course materials or roofing membranes to weather the perpendicular joints. The latter two are especially ill-conceived as the open butt joints leave the membranes exposed to UV light degradation. One of the latest incarnations uses metal channels rather than cover strips to under-seal the joints (Fig 9).