There is no hard and fast rule only that you have the splash zone covered (vertically) and that you weather the heads of the tiles horizontally. Standard exposed verticals require a 150mm lead upstand, although the peculiarity of a dormer means that side abutting soakers can be weathered by the finishing material. Conc' interlocking tiles would need the standard head-lap weathering i.e. 75mm - 100mm (horizontally) down the tile or 100mm for plains.
Good point. As with any profiled tile, it is important that the lead travels across enough of the tile so that the water is kept out by the profile and the cover.
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