How to Add a Hearth to a Chimney

Speak with workers at your location's building department to learn about regulations that need to be followed for the size of your hearth. Some communities have specific codes that must be followed.

Measure the width of the firebox, the firebox surround and mantel legs. Most hearths are either 4 inches wider than the firebox surround or even with the outside measurement of the mantel legs. Usually the width exceeds local requirements.

Place painter's tape on the floor at the width measurements you found. Mark the minimum depth requirements with the tape. Mark additional depth options up to 18 inches from the face of the firebox. Use the taped areas to select the hearth size that you like the best.

Cut the existing floor to the selected hearth size using a rotary tool. Remove the existing flooring you cut with a pry bar and pliers. Dry-fit the hearth material in the space. The hearth can be a single slab of stone, small tiles or a combination of both of those materials. It may be necessary to cut the legs of a mantel surround with the rotary tool if the hearth material needs to slide under the mantel legs. Allow for the hearth material, thin-set mortar and a moisture membrane when calculate the hearth's finished height.

Cut moisture membrane to the size of the hearth. Attach the membrane in place using thin-set mortar over the wood subfloor. Use a V-notched trowel to spread the thin-set mortar. The waffle-shaped moisture membrane is designed to compensate for the movement of a wood floor adjacent to a tiled floor surface to prevent cracks.

Apply thin-set mortar on top of the moisture membrane. Use the V-notched trowel to spread the mortar. Put thin-set mortar on the back of hearth material before placing it in position on the moisture membrane. Add spacers between tiles if necessary. Clean off the hearth material, and allow the mortar to dry overnight.

Mix grout with water. Apply the grout mixture over the hearth material's joints, and use it to fill the joints. Wipe off the excess grout mixture, and clean the hearth material immediately. Allow 48 hours before using the hearth.