How to Put River Rocks on Existing Fireplaces

Prepare the surface of the fireplace to receive the stones. The face of the fireplace surround should be completely smooth; if the existing fireplace surround is covered in bricks or field stone, spread on thin set with the flat end of the trowel to create an even surface. Allow this thin set to cure for 24 hours before tiling.

Lay the sheets of river stones out in an area of equal size to the fireplace in a dry layout to determine placement, cuts and grout joints. Lay the interlocking mesh sheets of rocks so that they appear to be one, even surface. For the edges of the surround, you will need to pull individual rocks from the mesh sheet. Use a razor blade to trim the mesh. Set the loose stones aside until you are ready to place them, individually, on the edges of the surround.

Trowel a small amount of thin set over the area to be tiled. Comb the thin set until the ridges are uniform in height. Press each sheet of stones into the thin set so the mortar comes up slightly around each stone. Hold each sheet in place for 30 seconds to ensure a good bond. Wiggle the sheets together in the thin set to get an even appearance between the sheets.

Edge the surround by applying a small amount of thin set to the back of each loose stone and pressing it into place. Twist the stone in different directions and distances from the other stones until it creates a smooth edge. Step back frequently to double-check placement.

Allow the thin set to cure for 24 hours and then seal the stones. Apply an impregnating sealer using a foam paint brush. Fully cover the stones, allow the sealer to penetrate for about an hour, and then wipe the stones with a lint-free cloth.

Grout the stones by packing the joints with grout using a grout float. River stones use a great deal of grout; you may need to pack it into the joints with the grout float more than once. Avoid trying to pack too much grout into them at one time; the stones should round up out of the grout slightly, not be flush, so go slowly and make two or three passes with the grout and float if required.

Allow the grout to set for 20 minutes and wipe up the excess with a damp sponge. Be careful not to over-wash the grout, removing it or lightening its color. Allow the grout to cure for an additional 24 hours.