How to Build a Fireplace With Stone That Lets the Heat Into the Room

Design the fireplace’s proportions to be efficient: The height and width at the back of the firebox should both be equal to the depth; the area of the fireplace’s opening should not be more than 10 times the size of the flue opening; the height of the fireplace opening should not exceed the width; the centerline of the flue must line up with the middle of the hearth.

Locate or create a footing or foundation for the fireplace, sufficiently strong to take the weight of the firebox, concrete-block sleeve and stone veneer. Many houses that are built without fireplaces have the footings put in place; if a suitable base is not available, float a slab using regular “Type M” mortar available premixed from any home improvement warehouse. Use the same mortar mix throughout the project. The footing must, in combination with the base of the firebox, raise the combustion area a safe distance above the floor; 4 to 6 inches is generally sufficient

Build a concrete block shell. The internal dimensions of the shell must be sufficient to accommodate the firebox, and the outside must allow for the veneer to be applied; the external free space necessary is determined by the thickness of the intended veneer. If the stones are to be laid vertically, a 2-inch space is all that is typically required; if the stones are to be laid horizontally, a much larger space is required. Follow the directions supplied with the firebox to create a good, tight fit and to ensure the lintel is accurately located. Cross-hatch the blocks for strength and insert metal wall ties -- often called butterfly ties -- every course for tying in the veneer.

Install the firebox. Some are better installed when the concrete block shell reaches a height similar to the exterior height of the firebox, while others can slide into the aperture once the shell is complete. Check the manufacturer’s instructions. Fit the flue liner in accordance with both the liner manufacturer’s and the firebox manufacturer’s directions.

Allow the mortar bonding the concrete block shell to dry, then attach the stone veneer to the outside of the shell. Drop plumb lines at all corners of the project, sited away from the corners of the shell by the distance necessary to accommodate the veneer; if the veneer is 7 inches thick, drop the plumb lines 7 inches out from the corners. Work to the lines as closely as possible, but the nature of the stone allows for a margin of error. Only work a few courses at a time -- if the wet work becomes too heavy, it will slump -- then allow the mortar to dry before proceeding. Keep trimming and shaping of the stone to a minimum, using a proprietary wet cutter, a concrete wheel on an angle-grinder, or a hammer and chisel.

Attach a mantel above the fireplace if desired.