Building an Indoor Masonry Fireplace with a Kit

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 far more manageable for the average DIYer.

Step 1 - Build the Hearth and Foundation

The standard hearth base is approximately four inches thick and needs a masonry foundation to support it. This base must be thick enough and deep enough to support not only the firebox and chimney, but also a hearth extension that goes 18 to 20 inches beyond the fireplace opening. It should also be at least three inches below floor level to allow for a firebrick inner hearth and extension. A gas starter or an ash-dump (if desired) can be incorporated into the inner hearth.

Step 2 - Make Fireplace Framing

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

Step 3 - Construct Inner Hearth

Use pre-mixed, refractory mortar to lay firebrick for the hearth base. Make minimum 1/16-inch wide joints between the firebricks as you go, using a spacer to keep it consistent if you can. The inner hearth should be large enough that the firebox can be placed on top of it.

Step 4 - Build 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, which is usually flush with the inside surface of the wall.

Build the firebox itself with nine-inch refractory mortar and fire brick laid on edge. Use the glass door frame as a template for the sizing. Lay the firebrick so they form the front edge of the covings and meet the back inside edge of the door frame.

Step 5 - Lay Backup Block

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

Next, lay 4x8x16 solid concrete blocks on the concrete hearth base to create 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 the finished firebox will not crack as it 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 depth and width of the fireback.

Step 7 - Make a Flat Platform

Place surrounding masonry up to the top of the throat and create a flat platform 13 1/2 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 so the hinge pins do not hit the throat and the valve operates correctly. The valve should open forward.

Step 9 - Attach Smoke Chamber

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

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

Step 10 - Make a Chimney

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

Exterior chimneys normally extend four inches away from the combustible wall of the house and require two inches of air space for wall siding. Chimneys must extend at least three feet above rooflines.

Since constructing a chimney takes far more skill and experience than kit building a fireplace, it is recommended that you hire a contractor for this part of the job before you even start. This way they can potentially advise you on the progress of your fireplace kit and so they know what they’re dealing with before they try to install a chimney.