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Before securing the tiles to the concrete slab, make sure they will fit and decide on a pattern that works well with the design. Place the tiles and figure out where they will sit on the slab and which ones need to be cut.
Using a wet saw or tile saw, cut the tiles to size so they fit on the slab. Slate is extremely delicate, so buy more than you think you'll need. Some will break in the process.
The mortar, or thinset, will secure the slate to the concrete slab. Mix a small amount of the dry mortar with water to the right consistency, following instructions on the bag.
Starting from the center of the slab and working to the outside edges, begin to apply the concrete thinset. The best tool for the job is a trowel with ½" grooves. Apply the mortar tile by tile so it doesn't dry out.
Slowly lay each piece in the chosen pattern and then place plastic "X" spacers in each corner of the tile, making sure each tile is equal distant apart and leaving spaces for the grout. The spacers should be 1/4" or 3/8". If you don't want to see the grout, jam pieces together.
Remove the plastic spacers. Mix the tile grout as specified on the package. With a sponge and bucket of water within reach, begin applying grout inside of the cracks. Wipe away the excess grout with a damp sponge. Let it dry. Give the patio overnight before walking on the surface so the tiles won't shift.