22 CONTEXT 178 : DECEMBER 2023 TERRY HUGHES Surveying vernacular roofs A roof survey should assess its condition, point to explanations for deteriorations and failures, and be alert to any special features or techniques the slater has adopted. Code of practice for roof slating (CP142) was prepared by the Council for Codes of Practice and published by the BSI in 1942. (Tiling was added in 1951.) It was clear from the start that the members of the drafting committee were aware that it would be impractical for the code’s recommendations to encompass many of the traditional techniques used on existing slate or stone-slate roofs, those we would today class as vernacular. The 1951 edition stated that ‘the code does not deal with stone slating or techniques peculiar to certain parts of the country’. This difficulty persisted in drafting the next four editions of CP142, its successor BS5534 1978–2018, and BS8000-6 Workmanship on Building Sites: Code of practice for slating and tiling of roofs and claddings 1990–2023. Nonetheless, the committee members tried to include aspects of the more widespread vernacular slating systems. But with innovations in roof design and construction and their consequences, vernacular techniques and materials gradually disappeared from the standards. Examples of the changes in the standard included: batten sizes increased from 38 x 19mm to 50 x 25mm for all slating, to prevent batten bounce on modern truss roofs, which made it difficult to drive slate nails and resulted in slates breaking; ventilation to cope with insulation at rafter level; and an aversion to the use of mortar and especially lime mortars, resulting in recommendations for mechanical fixing of ridge tiles and, in part, the adoption of plastic dry-verge systems. Many of these changes are unnecessary for vernacular slating which has been successful in resisting wind and rain over hundreds of years, and they may be incompatible with their details. Unfortunately, the repair or renewal of vernacular roofs often fails to recognise this, and specifications often simply include standard BS clauses or a cut-and-paste from a national building specification. BS5534s scope has recently been revised to draw attention to this. But who reads a standard’s scope? Probably not many. The code states: ‘This British Standard gives recommendations primarily intended for the design, performance and installation of newbuild pitched roofs, including vertical cladding, and for normal re-roofing work… The recommendations contained in this British Standard might not be appropriate for the re-slating or re-tiling of some old roofs, particularly where traditional and/or reclaimed materials are used. Users intending to adopt any of these recommendations for old roofs, and especially for Water penetration is prevented by adequate head, side and shoulder laps. Filming the work, such as on this shale valley at Edern in North Wales, is ideal for more complex jobs. The mortar on this Horsham stone roof in East Sussex is protected with clay tiles known as listings.
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