How to build a masonry fireplace

Building a masonry fireplace isn’t an easy job and requires skills and patience, but in the end, this fireplace will look great in your home.

Building the hearth and foundation

The hearth will need a masonry foundation for support. It is usually 4 inches thick but it has to be strong enough to support the fireplace and chimney.

Usually the hearth is built 3 inches below the floor level and it can house a firebrick inner hearth. You can also build a gas starter and an ash-dump.

Fireplace framing

Be careful with the combustible materials as they need to be at least 2 inches away from the outside of the fireplace. The opening in the combustible wall needs to be 4 inches wider than the masonry, and also the header needs to be 3 inches over the top of the fireplace opening.

Inner hearth

For the hearth base, use refractory mortar to assemble the firebricks. The inner hearth needs to be big enough so that the firebox can be built on top of it.


The firebox needs to have 9 inches of refractory mortar and firebricks laid on the edge. Install the steel frame for the fireplace opening and also mount the door. Use the door as a template to lay the bricks.

Backup block

The firebox needs to have walls of at least 8 inches thickness. You can create a backup of the firebox with solid concrete blocks. Leave a little air space between the backup and the masonry outer shell to prevent any cracking when the box becomes hot.

Setting the throat

The throat is usually made of one big block. You need to position it on top of the firebox and then fix it with refractory mortar.


Create a platform 13 inches above the firebox opening by placing some masonry up to the top of the throat.

Setting the damper

The damper needs to be set over the opening in the platform. The hinge pins of the damper need not to hit the throat. The valve needs to open forward in order to operate correctly.

Smoke chamber

To build the smoke chamber you need to fix with some mortar two pieces of vitrified clay. The smoke chamber needs to be placed as to allow the damper valve to open freely.

The walls of the smoke chamber need to be at least 4 inches thick and fixed with refractory mortar.


Also the chimney walls needs to be 4 inches thick. The combustible materials need to be at least 2 inches away from the outdoor of the chimney walls.

Make sure that the chimney extends at least 3 feet above the roof.

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