24 CONTEXT 185 : SEPTEMBER 2025 the decisions are easier, and the work faster and cheaper. In the centre-nailed, double-lap system, the head lap is specified in relation to the roof pitch, driving rain exposure and slate size⁵, and hence the batten gauge is the slate length minus the head lap and divided by two. A further technique, tilting the slates, had been used to prevent leaks by directing water away from vulnerable areas. This is most relevant in the Welsh valley which was lined with slates rather than lead, which would have been unavailable or too costly. The valley slates are tilted up at each side by the valley board, turning water towards the centre line. On either side the slates overlap on to the valley slates, directing water away from the junction. The other valleys were all lead lined. Another tilting technique, doubling — laying two slates on top of each other — was applied to lift the slating to ensure that overlying slates sit tightly together, or to redirect water flow. On the existing roof, the masonry mortar had been washed out at the gutterless eaves, especially below the valleys. Here the overhang was extended in order to throw water away from the walls. Also, close to the foot of the Welsh The slates in a Welsh valley are tilted up at the sides by the valley boards, directing water to the centre. Far right: Slope 2. Slating tilted up by the valley slates Slope 2. Close to the foot of the Welsh valley a doubled slate tilted the slating to turn water away from the verge. Far right: Slating was raised on counter battens at the verges to direct water on to the roof slope. Slope 11. Slates laid at a specified head lap ⁵ See BS5534 Code of practice for slating and tiling for pitched roofs and vertical cladding.
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