For slate tiles with said edge, the same applies as to ceramic tiles. In principle, there is no difference in their installation. A joint width of approx. 5 mm is often used. When gluing slate slabs with a rustic cut edge, a larger joint width is required. As a rule of thumb, it should be half the thickness of the slate slab itself. E.g. a cut slab with a thickness of 30 mm should have a joint width of min. 15 mm.