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Hearth Extension

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A fireplace was built well after the original construction of the house. In the crawlspace, the base of the block chimney was built around the original wood girder. The wood girder is directly under the fireplace hearth extension.

I'm second guessing myself since the hearth is at least 8 inches thick. (trying to interpret the code). Tell me this is not allowed no way, no how.

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According to older standards, the requirement was 8" solid masonry and a 2" air space between combustibles and the inner surface of the firebox or flue. The 2" air space got thrown into the mix in the early 80's.

No doubt it's funky and probably some form of violation.

I suspect reactions will be all over the place on this one, but could I sleep at night with embers in the firebox?.. Like a baby.

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Back when I was doing brick work, full masonry fireplaces got concrete or masonry support for the hearth extension with the floor joists properly headered around it, for inserts the facade and hearth were built directly on the wood subfloor.

It doesn't look right, but if the hearth is properly sized and the rest of it appear to be well built I wouldn't be too concerned about it. If the whole thing looks DIY I'd call it.


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NFPA 211

Hearth Extensions

8-3.1 ...with no combustible material against the underside thereof.

IRC '06

R1001.9 No combustible material shall remain against the underside of hearths and hearth extensions after construction.

There is a problem with those provisions in practice. The mason I worked for built a fireplace fast. The day before, two guys would erect the scaffold and load a hundred or so brick on it, stage all the block in the basement and most/all the interior brick inside, and stage the mixer and the mortar components in just a couple of hours. The building started at seven the next morning with two brickies and one laborer, and we would have the whole thing done to the cap, the scaffold loaded on the trucks, and be on our way home by four. Every one of the hearth extension forms was left in place, if they weren't the hearths would end up in the basement before they cured. That was back in the 80's, and unless the framers pulled those forms they're still there. We never went back to pop 'em off.

I only ever saw one mason pull a hearth extension form, he took 5 weeks to build a split boulder interior/hammer head exterior stone work of art. His poor laborer split and shifted enough boulders to build three such fireplaces. Never seen another crew around long enough to pull those forms.


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