What is the recommended grout line width when installing slate tiles?
Are you supposed to have a wide grout space when laying slate tiles.
Someone recommended to have them butted together.
This depends on your particular preference and the max/min grout width recommendations of the grout manufacturer. Other than this, there are no hard and fast rules when laying slate floor or wall tiles, on average you would use a 5-10mm grout joint depending on the tile size.
We would suggest that if it is a Chinese slate tile then you use a 10mm grout joint as they have a tendency to not be very square & we would not recommend you butt these slate tiles either, thoroughly check them before going along this path.
If it is a Brazilian slate then you could get away with a 3 or 4mm grout joint as these slate tiles are cut more precisely, but to be honest whatever slate tile you use it will be hard to butt them completely as all slate tiles have a +/- from 1-2mm on size.
You also have to consider that your subfloor can move in any direction due to heat fluctuations in the form of expansion and contraction, especially on new concrete floors. Butting your tiles up can create problems further down the line but fixing slate tiles with a grout line and using a flexible grout will compensate for this type of movement.