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Originally published in the July 2018 issue of Sweeping Magazine

Question: For the depth of a masonry fireplace, where does the hearth or firebox end, and the hearth extension begin, and how or where do I measure them?


by Mike Segerstrom 


This may seem like a simple question, but with some fireplaces, that's not always the case. For the purpose of this discussion we will focus on a fireplace system where the firebox floor is flush or even with the top of the hearth extension. Both of these may be flush with the room floor, or as we often see, both the hearth extension and firebox will be raised typically anywhere from 8 to 12 inches above the room floor.


First let's look at what the minimum required depths are, and then we'll take a look at how and where to measure.


The IRC requires that a masonry hearth or firebox floor has a minimum required depth of 20 in. (Exception: Rumford fireboxes shall have a minimum required hearth depth of not less than 12 inches)


The NFPA 211 requires the same hearth depth of 20 inches, with the same 12-inch minimum requirement exception for Rumfords.


The IRC requires the hearth extension depth to be a minimum of 16" where the fireplace opening is less than 6 square feet, and a minimum hearth extension depth of 20 inches where the fireplace opening is equal to or greater than 6 square feet. The NFPA 211 has the same hearth extension depth requirements.


A quick note about raised fireboxes: The IRC has an exception for hearth extension thickness, when the bottom of the fireplace opening is raised a minimum of 8 inches above the top of the hearth extension surface. When the Firebox is raised the minimum of 8 inches, the hearth extension may be constructed of a minimum 3/8" thick approved non-combustible material. But even with this thickness exemption, the hearth extension still must extend the minimum 16 or 20 inches in front of the firebox. The NFPA 211 describes raised fire boxes and requires that the hearth extension extend in front of it, but does not have an exemption for thickness.


The challenge we sometimes face in measuring, relates to the facing material installed on the fireplace. Facing material is often brick, whether it extends to the ceiling, just to a mantel shelf, or is limited to around the perimeter of the fireplace opening. In some cases this brick may be covered by marble, tile worry stone veneer. There are also cases where we see the original brick veneer has been removed, and has been replaced with marble, tile or a stone veneer attached directly to the front of the firebox walls.


When measuring firebox depth, out of habit, many of us will commonly start our measurement up against the back wall of the firebox on the floor, and measure to a point right where the facing material begins, regardless of the facing material thickness. The IRC however, has 2 diagrams that clearly show the hearth or firebox depth being measured to the outside surface of the facing material. In both of those diagrams, the facing material on the fireplace is brick. 


It's important to note that the IRC diagrams only show a single course of brick as the facing material. There are fireplaces we see that will have an 8in thick brick face, and sometimes a stone face that's even thicker before the firebox wall begins. In those cases, if we measured to the outside of the face, the floor maybe 20 in of fire brick, but the side walls of the firebox may be significantly shallower, potentially only 12 in deep. This may create a potential concern if the fireplace is built into a combustible wall and framing is concealed somewhere behind the face adjacent to the firebox. We will want to really scrutinize these types of installations.


The NFPA 211 does not have a diagram depicting where firebox depth is measured, and does not offer any additional language or text other than the minimum 20-inch depth requirement.


For hearth extension depth, neither the NFPA 211 or the IRC have additional language or text on measuring, but both have diagrams. And the diagrams in both clearly show with arrows that hearth extension depth measurements are taken from the outside of the facing material.


So while neither offer great detail in their narrative or text, both have diagrams that are helpful in clearing this up. Both the IRC and the NFPA 211 require hearth extensions to be a minimum of 16 or 20 inches depending on fireplace opening size, with both measuring from outside the fireplace face.


And both require a minimum hearth or firebox depth of 20in, with the IRC measuring to the outside of the face of the fireplace.


One difference in requirements between a Hearth & Hearth extension is the specifications for thickness. Both the IRC and NFPA 211 require that hearths are a minimum of 4 inches thick, and hearth extensions are a minimum of 2 inches thick. It's not common to see a hearth extension only 2 inches thick, but they are out there. If the transition between the hearth and hearth extension is not clear, but we know the hearth extension is only 2 inches thick, we need to try to make sure that 4in is present where we measure our 20-inch firebox depth.


And lastly, there is a simple way to at least get started on our measurements. If we know based on the fireplace opening size that our hearth extension needs to be 20 inches deep, we can take an overall measurement from the back of the firebox to the outside edge of our hearth extension. If it's not 40 inches or more, we know either the firebox isn't deep enough, the hearth extension isn't deep enough, or a combination of both. Taking that measurement first is often helpful in calculating the other dimensions.

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