Hearth and Hearth Extension Question Regarding Construction Code and Clearances

Masonry Heater Construction
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Hello, I am new here and my 1st post – I have plans to build a Masonry Fireplace – I am unsure if anyone in this forum can help but I will ask anyway — because, if there are professional masons here, they may be able to help answer my question.

As I said, I will be building a masonry fireplace (a Rumford) 36" wide firebox. It will be built as part of a custom home build in Tennessee. In learning the code for fireplace construction, I find that the Code in Section R1001.9 - “combustible material shall not remain against the underside of the hearths and hearth extensions after construction”. This brings up a question about this underside clearance requirement.

I get it that the Hearth and Hearth “Extension” needs to be at least a 4 inch reinforced concrete slab and that the firebox hearth can be either at floor level or a raised hearth.

With the reinforced slab being a “cantilevered” member – My question is – can that slab be separated on the underside from any materials with an AIR GAP (like a space between the concrete underside and any materials) by some 2 inches or more of gap and non-contact?

As Code R1001.11 Fireplace Clearance states: “Wood beams, joists, studs and other combustible material shall have a clearance of not less than 2 inches (51 mm) from the front faces and sides of masonry fireplaces, and not less than 4 inches (102 mm) from the back faces of masonry fireplaces. The airspace shall not be filled, except to provide fire-blocking in accordance with [Section R1001.12].” I get that this is referring mainly to walls – However, it mentions “joists”.

My point in this – Is there an interpretation which would allow the HEARTH slab to be constructed OVER a floor joist IF AN AIR GAP between the slab and the floor joist is planned so that there is at least a 2-inch, or more, AIR SPACE/Gap provided?

I researched several places – In Scotland they allow a 50MM (2 inch) Gap from the slab underside to any combustible materials. I realize we are not in Scotland – But I still wonder if the gap has been a subject addressed by any masons who regularly build masonry wood burning fireplaces under Chapter 10 of the Code?

Again, I am asking here, as I cannot find other forums or info sites that may be able to provide comment or advise. Thank you i advance – Rich

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I interpret the code to be saying this is possible, as you are saying. I recommend making a section drawing for clarity if you would like further analysis.

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Max

THANK YOU SO VERY MUCH!

I do have a sectional drawing of my proposed heath construction – I will attach it here for your comments! Thanks again – RichInTN

Fireplace Hearth Over Truss Detail MODIFIED Version.jpg
Fireplace Hearth Over Truss Detail MODIFIED Version.jpg1092×870 164 KB

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As far as I understand the prescriptive fireplace code, there is not supposed to be any contact between floor framing and the fireplace foundation and the 2" clearance is meant to be respected.

Below is a drawing I made from the article on masonry heater foundations that shows one approach we take where we plan on them sharing a footing but not structure higher up:

816×1056 119 KB

Also, in general, you do not want to have enclosed air spaces that can build up static heat as part of your clearance. To meet the prescriptive code, you may want to extend the masonry bearing wall on the left to the edge of hearth extension.

Custom solutions could be addressed with an engineers stamp.

My understanding and experience is mostly specific to masonry heaters, as a subset and evolution of fireplaces. A lot of the fireplace code is trying to avoid fires being made at floor level heating up wooden structural members. Masonry heater fireboxes are generally elevated over an ash/air area and thus practically insulated from the wooden structure. Where channels approach the floor system they are usually a decent ways down stream from the firebox. The 2" clearance to the structural members and the allowance for the subfloor and flooring to bridge the gap have proven to be a safe combination.