CONTEXT 178 : DECEMBER 2023 23 BUILDINGS IN THE COUNTRYSIDE historically or architecturally important buildings, are advised to consult with the local planning authority or an appropriate conservation organization to check their suitability.’ The gradual changes in the standards reflected technological changes in the slate quarries: the evolution from small and shallow to large, deep quarries. In the former, production was so small that they had to sell a mixture of sizes (random slates sold by weight) mainly to a local market. The large quarries were able to segregate their slates and sell them as single sizes (tally slates sold by count) and, with the development of transport systems, to a wider market. Ultimately, tally slates came to dominate the market as vernacular, random slates gradually fell out of production and the small quarries closed. The consequence for roofs is that vernacular slating is now mainly seen in remote areas and to a large extent on neglected buildings. Happily, the situation for stone-slating was not so bad, and many small delves are still producing, albeit often with long lead times. To aggravate an unhelpful specification and tendering process, appointed slaters are often unfamiliar with the specific vernacular techniques involved. And all too often the existing roof is stripped without investigating the details of the slating. Consequently, the wrong detailing is adopted for the repair, either by mistake or on the mistaken assumption that the existing detail is unsuitable or defective, and may only need repair for an unrelated reason, such as nails rusting. The consequence is many historic roofs are spoiled or lost every year. For example, a vernacular valley on a building of high conservation importance could not be installed because the wrong slates had been specified. They were the standard modern production, much larger than the original vernacular slates and too wide to fit to a curved valley. On another roof the wrong valley was installed because the wrong name was used in the specification, despite the slater pointing this out. The Pevsner now describes the valley as beautiful. It may be, but it is wrong, and it should not be where it is. Look at the roof before pulling it to pieces. Think about what is evident before reslating, and especially before making any changes. It will be money well spent if serious errors are avoided. The objectives of a roof survey are to describe what is there, assess its condition, and point to explanations for deteriorations and failures. It should also be alert to any special features or techniques the slater has adopted. The oldest slating did not use lead to weather vulnerable junctions or rely on underlay to carry away occasional leaks. Instead, the slater relied on experience and an understanding of how water moves across and between slates and how the flow direction can be influenced. The first level of defence is the lapping between slates, the second is tilt. These are the factors to describe: not simply what is seen now, because the slating may have moved or partially collapsed. Rather, the objective is to understand the original detailing and the slater’s intentions. Besides ensuring that the roof is weather resistant, these determine the roof’s appearance, and are essential for authentic repairs or reslating. Water penetrates slating unless it is prevented by adequate head, side and shoulder laps. The way the roof has been set out (gauged) may be fundamental to some of the detailing. The slating should be measured to record the slate lengths, the lath or batten gauges, head laps, minimum side lap for each slate length and margins for as many courses as necessary. Slate listings here in Pembrokeshire have been covered with mortar slurry over the whole roof, a local technique to extend the life of failing slates. Where purposemade ridge stones or tiles were not readily available, the roofing stones or slates were used, as here with Cornish slate.
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