Hi, I have set a 1200x900x25mm thick natural slate slab as a hearth which is going to be surrounded on 3 sides by 10mm thick x 600x600mm porcelain tiles. These are on a concrete floor and I previously made a 15-20mm recess in the concrete to allow the 25mm thick hearth to sit lower so that it will be flush with the tiles when they are laid. There will be a log burner sitting on the slate. My question is should I just allow for a 3-4mm grout joint around the slate where the tiles will meet it or should there be an expansion joint? Thanks