The slab roller is a tool used for rolling out even thickness slabs of clay. The boards underneath the large roller control the thickness of the slab, so put what you think are the right amount of boards in there and roll a small test slab. If it is too thin, remove a board. If it is too thick, add a board.
ALWAYS have a piece of canvas underneath your clay! Make it is big enough and make sure it doesn’t lay over the guide rails of the slab roller!!
ALWAYS make sure the roller is not rolling over any canvas! The metal from the roller should always be directly in contact with the metal guide rails.
When you are rolling, leave the thick canvas stretched over the roller. If you are rolling a lot of slabs or very thin slabs, you’ll find it easier to add another piece of canvas between the top of the clay and the canvas stretch over the roller.