Building An Indoor Masonry Fireplace

Building an indoor masonry fireplace can take a great deal of effort and time, but the finished product can be a highlight of your home and show off your creativity and workmanship. You can purchase fireplace masonry kits to make this project more manageable.

Step 1 – Building the Hearth and Foundation

The standard hearth base is approximately 4 inches thick and needs a masonry foundation to support it. The hearth base must be thick enough and deep enough to support not only the firebox and chimney, but also a hearth extension that goes18 to 20 inches beyond the fireplace opening.

The hearth base should be at least 3 inches below floor level to allow for a firebrick inner hearth and extension. Also, a gas starter or an ash-dump (if desired) can be incorporated into the inner hearth.

Step 2 – Fireplace Framing

Combustible materials should be at least 2 inches away from the outside of the fireplace or chimney. The rough opening in the combustible wall should be at least 4 inches wider than the masonry, and the header should be 3 inches above the top of the fireplace opening.

Step 3 – Inner Hearth

Use pre-mixed refractory mortar to lay firebrick for the hearth base. Make minimum 1/16-inch wide joints between the firebricks. The inner hearth should be large enough that the firebox can be placed on top of it.

Step 4 – Firebox

Assemble the steel frame for the glass doors that will be placed over the fireplace opening. Brace the door frame just inside the fireplace opening. Usually, this is flush with the inside surface of the wall.

Build the firebox with 9-inch refractory mortar and firebrick laid on edge. Use the glass door frame as a template. Lay the firebrick so that they form the front edge of the covings and meet the back inside edge of the door frame.

Step 5 – Backup Block

Back the firebrick with masonry that is 75 percent or more solid, making the firebox walls at least 8 inches thick.

Lay 4-by-8-by-16 solid concrete block on the concrete hearth base. This creates a backup for the firebox.

Leave an air space or bond break between the firebox backup masonry and the masonry outer shell. This will ensure that the finished masonry firebox will not crack as the box becomes hot and expands.

Step 6 – Set the Throat

Most fireplace kits come with a one piece throat. Set this in refractory mortar on top of the firebox. The throat frames the correct firebox depth and width of the fireback.

Step 7 – Platform

Place surrounding masonry up to the top of the throat and create a flat platform 13.5 inches above the top of the fireplace opening.

Step 8 – Set the Damper

Set the damper over the opening in the platform you just created. Set the damper so that the hinge pins do not hit the throat and the valve operates correctly. The valve should open forward.

Step 9 – Smoke Chamber

Place two pieces of vitrified clay in mortar over the damper to form the smoke chamber. Shift the smoke chamber so that the damper valve opens without striking the inside of it.

Use refractory mortar to fill the joints between the two pieces of smoke chamber. Lay surrounding masonry at least 4 inches thick around it, creating a total thickness through the smoke chamber wall of at least 6 inches.

Step 10 – Chimney

Interior chimneys must be at least 4 inches larger than the flue they surround in all directions. Combustible materials must be kept at least 2 inches away from the outside of the chimney walls.

Exterior chimneys normally extend 4 inches away from the exterior combustible wall of the house and the required 2 inches of air space for wall siding.

Chimneys must extend at least 3 feet above rooflines.