How to Install Slate Tiles Over Brick Fireplaces

Cover the bricks and the mortar lines between the bricks with a skim coat of mortar. Apply a thin layer of Portland cement-based mortar over the fireplace. Use the flat edge of the trowel to scrape the mortar until it fills any pits or indentations and creates a smooth, even surface. Let this mortar dry for at least 24 hours.

Lay out your slate tiles in an area near the fireplace. Take the slate from several boxes at once to ensure a pleasing blend of color and pattern. Some slate pieces will be thicker than others, and some pieces will be thin on one half and thick on the other. Arrange the tiles so thick ends meet and thin ends meet to make the installation smoother.

Mark any slate tiles to be cut to fit the edges of the installation with a grease pencil and straight edge. Cut the slate on a tile wet saw and double-check the fit with the other tiles. Use any particularly thin or thick tiles as cuts to minimize the surface area to even out.

Spread a second layer of mortar over the fireplace, beginning in the area directly above the center of the firebox. Rake this layer of mortar with the teeth of the trowel until it is even in thickness.

Apply mortar directly to the back of each tile. Place more mortar in the thinner areas by applying a scoop of mortar the size of a baseball to these sections, and less mortar to the thick areas by applying a scoop the size of a golf ball. Press each slate tile onto the fireplace surround that has had mortar combed over it. Begin directly above the firebox and work out and down evenly.

Observe every few tiles to ensure that the slate is even with no pieces sinking in deeper or coming out further than the others. Pry off tiles that sink too deep and add more mortar. Press hard on the tiles that stick out too far to sink them deeper into the mortar until all the tiles are installed and the surface is level. Let the slate dry in place on the fireplace for 48 hours.

Seal the slate to ensure the grout will release easily from its surface. Paint sealer on with a craft paintbrush, paying attention to any clefts or ridges in the stone. Wait 10 minutes and buff the slate dry with a lint-free cloth.

Direct the grout into the joints between the slate with a stiff grout float, pressing the grout in with the flat edge of the float. Wait 10 minutes for the grout to begin to dry and wash the excess grout off with a damp sponge. Pay attention to the clefts in the slate to ensure no grout is left behind.