CONTEXT 185 : SEPTEMBER 2025 23 ROOFING modern range of widths, ± 5 mm. To replicate this, the new tally slates were trimmed on site to a wider width variation. On the roof this can be seen in how the perpendicular lines wander slightly. On Slope 10, slates from Slopes 1, 8 and 10 were reused, centre nailed at an adequate head lap. This involved mixing the two colours, but this was the least visible slope. Driving rain Three slating systems had been used to resist driving rain: gauged at three-and-a-half pin⁴ on the oldest Slopes 2–4 and 6, which is triple lapping and gauged in thirds on Slopes 1 and 8. On Slopes 5 and 9–11 double lapping to a specified head lap had been used, which is how slates are normally fixed for new roofs today. Thirds slating is a method of gauging the roof (setting out the laths) at one third of the slate A&B random slates are no longer available, so block was purchased from the same quarries and handmade on site by WJC. On Slope 1, new slates were redressed on site to various widths to replicate the early modern slates which had large dimensional variation. This shows how the perpendicular joints wander slightly. Left: Slope 3. New slates laid at threeand-a-half pin Right: Slating in thirds length and hence a large head lap. Three-anda-half pin gauges the laths at two sevenths of the length by dividing by 3.5, providing an even larger head lap and an extra layer of slates. This is essential to prevent leaks with very narrow slates with small side laps. Besides preventing leaks, these systems help the slater, who has to select and place each slate using experience and judgement, to have adequate side lap over the slates below. Because side laps can be smaller, ⁴ ‘Three-and-a-half pin’ is the name used in Cornwall. Other local names are not known.
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