A concrete slab is the best subfloor for any type of tile installation. Installing slate over concrete is a project anyone can complete, but preparation is important. Make sure you have at least 10 percent extra tile for cuts and waste. It is usually a good idea to have 20 percent extra so you will have enough tile on hand in the future to fix and replace any tiles that break.
Check the concrete surface to check if it is level, using either a 4-foot or an 8-foot level. If it is not level, use a self-leveling compound, like LevelQuik, to make the concrete surface level. Allow the leveling compound to dry completely.
Mix a batch of thinset (such as FlexBond) in a 5-gallon bucket. Spread the thinset over the concrete with a tiling trowel that has notches that are appropriately sized for the tile you are installing (see Tips).
Set the tile into the thinset. Do not spread more thinset onto the concrete than you can cover in less than 5 minutes. Space the tiles evenly by using tile spacers. Repeat this process over the entire surface of the concrete. Cut tiles as needed with a wet tile saw. Allow the thinset to dry for at least 24 hours.
Mix up a batch of grout in a 5-gallon bucket to the consistency of peanut butter. Place the grout into a grouting bag. Squeeze the grout out of the grouting bag into the grout lines. Completely fill the grout lines with grout, leaving no voids, gaps or holes. Allow the grout to cure for 72 hours.
After 72 hours, apply a high-quality grout and tile sealer over the entire surface of the grout and the slate tile. Apply two to four coats of sealer. If you like the wet, glossy look, apply two coats of glossy sealer over the penetrating sealer coats. Allow the sealer to dry before allowing traffic on the tiled surface.