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DHTiling
Depending on the quality of the slate you can get warped pieces and that can cause lipping that is removed as already mentioned and also by giving a slightly wider joint and not washing to much grout out as hollow joints will make it look worse..
Just remove the worst bits with a chisel and it will sheer the top of another layer...then seal and it will look fine...that is the best thing with riven natural split and it doesn't show..
Just remove the worst bits with a chisel and it will sheer the top of another layer...then seal and it will look fine...that is the best thing with riven natural split and it doesn't show..