BCD 2019

78 T H E B U I L D I N G CO N S E R VAT I O N D I R E C TO R Y 2 0 1 9 C AT H E D R A L COMMU N I C AT I ON S and repair work. This should include a comprehensive photographic record with a scale in the photos where necessary. It should be noted that many buildings with Collyweston slate roofs are listed, and that protection extends to curtilage listed buildings within its grounds, such as barns and other ancillary buildings or structures. Any modifications that could alter the building’s original fabric or character will need listed building consent. The local planning authority should be enlisted to help decide if consent is necessary for the works proposed. Common failures in Collyweston slate roofs The most common reason for reroofing is the failure of the fixings – usually the nails or pegs, but lath or batten decay is also common. The slates themselves, by and large, do not fail, but the hole at the top can be a weak point depending on the thickness of the slate and how they have been dressed. If a slate does fracture at this point, it can be redressed and used as a smaller one higher up. Poor ventilation can prevent the roof from heating and cooling by effectively joining the slates together into one mass. Thermal expansion can lead to cracking of the slates and trapped moisture can greatly accelerate deterioration of the roof in general, including its timber supporting structure. Poor ventilation can be caused by the use of underlays with fully bedded systems, trapping moisture in the batten space. This is especially bad with vapour permeable underlays which encourage moisture into the batten space. If underlays are used there must be ventilation through the batten space, but this normally means raising the roof on counter battens, causing detailing problems at the verges and abutments. In the worst cases poor ventilation and other ill-considered alterations can cause extensive decay to the main structural timbers due to insect or fungus attack and leaks. Repair Localised repairs can extend the life of a roof with some slate slippage. However, regular maintenance is crucial as areas of slippage can rapidly worsen due to the weight of the slate. In order to understand whether simple localised repairs would be sufficient or if more comprehensive repairs are required, the roof would need to be inspected from the underside. According to guidance on the Collyweston Stone Slaters Trust website, more comprehensive repairs can include the replacement of small areas of failed battens and the re-laying of local areas of slate to the same specification as the rest of the roof. If the roof was torched it can be re-torched from the underside. Poor detailing and inappropriate materials can be very harmful to the slates themselves and result in failure of the roof. Some materials not only detract from the visual qualities of the roof but can also lead to poor ventilation and result in cracking and accelerated deterioration of the roof described above: these include spray-on sealing foams and bituminous products to stick the slates down. In addition any wholesale pointing up of the roof with a mortar which contains any cement will effectively end the life of the slates as it is almost impossible to clean and reuse them. Other improper practices include pushing slipped slates into position and securing them with mortar along with repointing whole slopes and/or large areas of slates by pushing mortar into all of the joints. Both methods have been used frequently to repair and prolong the life of the roof, although neither should be recommended. (Collyweston Stone Slaters Trust, n.d.) In conclusion, Collyweston slate is an important vernacular material that contributes to the local distinctiveness and character of towns and villages in the East Midlands. Traditional detailing is a vital part of this, particularly aspects that affect the roof’s visual qualities like valley treatments. Many lessons can be learnt from recording and replicating successful historic roofs. To ensure knowledge and detailing is continued along with conserving the historic environment, Collyweston slates roofs should always be re-laid according to their earlier layout and detailing, with the only exception being if unsuitable and unsympathetic methods have been used. Stone slate roofs can be repaired if they have minor slippage, but regular maintenance is needed to stop the damage worsening. Inadequate ventilation and cracking occurs when inappropriate materials are used for repairs to stone slate roofs. It is inadvisable to use mortar containing cement when installing a stone slate roof, not least because it makes the slates very difficult to reuse. (All photos on this page by Janine Dykes) Further reading J Burgess, Collyweston Slate and Its Use Today , unpublished, 1991 A Clifton-Taylor, The Pattern of English Building, London, 1987 The Collyweston Stone Slaters Trust , ‘Collyweston stone slate: a guide’, June 2001, www.collywestonstoneslaterstrust.org.uk/ C Wood (ed) and T Hughes, English Heritage Research Transactions; research and case studies in architectural conservation, Stone Roofing: conserving the materials and practice of traditional stone slate roofing in England , 9th ed. London: James & James Ltd, 2003 English Heritage, ‘Stone Slate Roofing: technical advice note’, London: English Heritage, 2005, (http://bc-url.com/ stone-slate) T Hughes and R Jordan, ‘Vernacular Slating in the East Midlands’, SPAB: Regional Advice Note 3, 2018 Stone Roofing Association, ‘Glossary of Stone Slate Roofing’, 2010, www.stoneroof.org . uk/historic/Historic_Roofs/Collyweston_ project.html JANINE DYKES is an inspector of historic buildings and areas for Historic England in the East Midlands. She joined HE in 2013 after working as a conservation officer in several local authorities. Collyweston slating has been a lifelong interest which has carried on into her professional life.