How to Lay Uneven Slate Tile

Prepare the area for installation just as you would any other tile installation. Once you have the floor prepped and laid out, you can get the slate ready for installation. Sort through the slate in an out-of-the-way area, and stack similar-sized pieces together. Carry those stacks to your installation area for ready access.

Spread the thinset mortar onto the floor as you would normally for other tile installations. Comb the thinset into the floor with the notched side of the trowel. Take an individual piece of slate, and give the back side of it a quick scan for evenness. Wherever the stone is thinner, you must apply extra thinset by smearing it onto the back side with the tip of your notched trowel.

Install the slate tile into the bed of mortar. Press it down firmly into the bed of thinset, and check it for overall flatness and movement. Repeat this process with the surrounding pieces, and check them all against their neighbors for height variations. Levelness is not important (as some floors slope toward outer perimeters), but flatness is. You can add or remove more thinset as necessary to make up for abnormally thick or thin pieces and thus create a flat surface, where there are no tiles higher or lower than others.