chimney and hearth construction


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Old 09-21-09, 06:38 PM
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chimney and hearth construction

I am in the process of constructing an old log house that I dis-assembled and reconstructed. I am currently building the chimney (solid brick inside and out)and the fireplace. The question I have concerns the hearth extension, during the chimney construction, when I reached the firebox height (which is approx. 6" not including the firebrick above floor level) I poured a solid 4" concrete slab with rebar from the back of the inside portion of the chimney to the outermost edge of the hearth extension. The main hearth and hearth extension is entirely one piece.The chimney and firebox is constructed completely on the exterior of the house with only the hearth extension going into the house. The hearth extension is cantilevered from the fireplace, This means the bottom of the hearth extension is 2" above the subfloor. I had the local inspector come out the other day to approve my work only for me to quickly become confused when he said, "I am not sure about the hearth extension and having the subfloor being under the extension". He and I read the code book, which states, "No combustible Matl. is to remain against the bottom of a hearth extension after construction is complete. It does not say anything about having sub flooring directly under the hearth extension. The distance from the front of the firebox hearth to the outermost edge of the hearth extension is 16" and distance from each side of the firebox to the outermost side edge of the extension is approx. 9". The dimensions are mandated because of the fireplace opening which is < 6. The inspector said he would have to ask his Boss if this would pass because of the liability. I do not want to have to cut off the extension I currently have in place and cut through the flooring if this is not necessary. I was also planning to build another fireplace on the 2nd floor using the same method as just mentioned for the hearth extension. Do you have any ideas or recommendations for a situation like this?
 
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Old 09-22-09, 07:10 AM
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"I am not sure about the hearth extension and having the subfloor being under the extension". He and I read the code book, which states, "No combustible Matl. is to remain against the bottom of a hearth extension after construction is complete. It does not say anything about having sub flooring directly under the hearth extension.

It's up to them, as subflooring is a combustible material. Not sure about your local entity, but there is a certain thickness that acts as a heat sink and will allow construction on subflooring. I usually build 6" thick hearths here, but they are mostly for gas fireplaces, so that's a different animal. It is easier to tile with 6" tile with minimal edge cutting.
 
 

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