Hearth extension, woodburning fireplace, adjoining structure

[GeneDavis]

When the fireplace hearth is 12+ inches above finished floor, may the all-masonry hearth extension sit and bear on the woodframed floor?

 

Proposed fireplace sits on its own foundation that goes to basement floor 9 feet below finished floor.

 

If the hearth floor level was to be at finished floor level and not raised, we know we would need to build a cantilevered slab on the fireplace foundation for the hearth extension finish.

 

But with it raised as proposed, does the code permit the extension to be built separate from the fireplace, adjoining the hearth closely, but built atop the floor?

[javatom]

No.  A masonry fireplace has a section that cantilevers out for the hearth.  The floor structure is framed around it.

[Gawdzira]

It seems like if the combustible material is 12" below it would meet the code criteria for separation. I have an Isokern lightweight masonry fireplace on a current project that sits directly on wood framing but that is what they specialize in.

https://earthcore.co/

[javatom]

isokern or similar system is a good way to go.  Very few people install actual masonry fireplaces any more.  Even these lightweight units need a footing system though.  Bad builders try to eliminate it but it is just too much weight to bear on a normal floor system.  Especially if you start adding the weight of a faux stone surround.

[parkwest]

ISOkern... now that brought memories.  Had a homebuyer spec one for a house we built for him... in 1990.  I did pour a foundation under it... even though the homebuyer insisted the fireplace was "lightweight" and didn't need it.  From what I was told, the man really enjoyed his new fireplace.

 

I have not had a request for an isokern since then... all prefabs.

[javatom]

Most people now want the simplicity of gas.  Very light units, put them any where and a lot of them are direct vent.  I built a real masonry fireplace at my own house but it can be quite a financial commitment.  even though its the real thing and I can burn wood in it.  I installed a gas line igniter so I can always install a gas insert unit if I get tired of the wood thing.

[DRAWZILLA]

I used to put many masonry fireplaces in my projects because that was the norm back then. Now you can't find any mason's to do them anymore and if you were doing a second story addition , around here cities won't let you extend the chimney's to the existing one.  We now tear down masonry fireplaces and replace with metal., it's less expensive than rebuilding masonry and having the engineer calc. it, if you can even find someone to do it. Most of the older chimney's fell during one of the many earthquakes we have had.