Slate tiles have a beauty and rustic elegance unmatched by other natural stones or by other types of tile. Along with this rustic elegance brought out by the material's cleft face and uneven surface is an uneven thickness as well. Slate tiles vary in thickness by as much as 1/4 inch between two tiles or within one tile. To even out this thickness during installation, you must back butter the tiles, or apply additional mortar to the tiles as well as to the wall to help make up for the thickness difference and keep your wall tile installation smooth.
Measure the wall where you would like to install the slate tiles. Mark off a flat area of equal size on a nearby work table or floor. Slate tiles require a dry-fit or dry-layout before installation to ensure an even blend of colour and to work out the placement of the tiles, which will involve laying the tiles out without mortar in the same pattern you will install them on the wall.
Lay out the slate tiles with the first tile laid in what will become the bottom, centre of the wall. Lay the next tiles evenly to both sides and up for a balanced installation. Take slate tiles from multiple boxes at a time to blend the colours of the tiles together.
Set tiles that are extremely uneven in thickness to the sides to use as cut tiles at the edges of the installation. Make a note of which tiles are thinner and which are thicker so you can even them out during installation.
Cut the tiles to fit on a tile saw, and return them to the installation to double check their fit and placement before installation.
Spread a small amount of thin set mortar over the area to tile with a trowel. Rake this thin set with the notches on the trowel to produce even ridges. Scoop up some additional thin set, and apply four scoops of thin set to the back of each piece of slate tile. Place more thin set on the backs of thin tiles and less on the backs of thick tiles.
Press the tiles onto the wall in the same pattern you laid them out in the dry-fit. Beat each tile into the thin set with a wooden mallet until the tiles are all even in surface, with no lips or depressed tiles. If a tile sinks too far into the mortar, pull it out and apply more thin set to the back of it before beating it in again.
Allow the thin set to dry for 24 hours before sealing your slate tiles. Paint the slate with an impregnating sealer using a foam paint brush, and wipe up the excess sealer with a lint-free cloth.
Grout the slate tiles with standard grey grout. Press the grout between the tiles with a grout float, and wipe up any excess grout from the wall with a damp sponge. Allow the grout to dry for 24 hours.
If you wish to brighten and enhance the colour of your slate tiles, seal them with a topical, colour enhancing sealer after grouting. Take care not to get the enhancer on the grout as this will lead to a sheen on the grout. The enhancer will not change the finish of the tiles; it will simply deepen the colour.