Cementitious or other fireproof foam

[Easy Livin’ 3000]

I will be expanding my hearth outward, when I do I need it to be level with the existing floor. This will take it out over the framed cellar space, where there are currently floor joists. I'll frame a box where the joists are removed, and would like to fill the space with something that is fireproof, then cover the space with hardiboard, then thin stone tile. I'd like to frame out the box with something fireproof and easy to work with as well.

What material can I use for the frame and fill? I saw cementitios foam on a building program, but I'll only need a couple of square feet so hiring a contractor that specializes in the material is probably out. Regular concrete would be too heavy in this application.

 

[bholler]

I will be expanding my hearth outward, when I do I need it to be level with the existing floor. This will take it out over the framed cellar space, where there are currently floor joists. I'll frame a box where the joists are removed, and would like to fill the space with something that is fireproof, then cover the space with hardiboard, then thin stone tile. I'd like to frame out the box with something fireproof and easy to work with as well.

What material can I use for the frame and fill? I saw cementitios foam on a building program, but I'll only need a couple of square feet so hiring a contractor that specializes in the material is probably out. Regular concrete would be too heavy in this application.

Well to be to code for a hearth extension for a fireplace it needs to be concrete with no combustibles under it. We typically bend a metal pan to act as the form then drill long expanding anchors into the slab of the hearth. tie it all together with rebar and pour it. We usually add a post in each corner for extra insurance but it would probably be ok.

 

[begreen]

Is this for a regular fireplace or a ZC unit? It's not clear why will the joists be removed? What is to support the cement board? It needs a rigid backing?

 

[bholler]

Is this for a regular fireplace or a ZC unit? It's not clear why will the joists be removed? What is to support the cement board? It needs a rigid backing?

If he is extending a fireplace hearth that is what you have to do to meet code if it will ever be a fireplace again. Otherwise I would just cut out the finish floor and sub floor block between the joists and then build up from there.

 

[Easy Livin’ 3000]

Well to be to code for a hearth extension for a fireplace it needs to be concrete with no combustibles under it. We typically bend a metal pan to act as the form then drill long expanding anchors into the slab of the hearth. tie it all together with rebar and pour it. We usually add a post in each corner for extra insurance but it would probably be ok.

It is for an insert in the fireplace. I don't understand what you mean by a post in each corner? If you are referring to posts that go down to the floor in the cellar, that is a no-go, as I need the space to remain open. This is why I was asking about cementitious foam, as it might meet code while not weighing as much as regular concrete. Another idea that I have is to use perlite as the aggregate in a home brew mix to save weight. The metal plan will work, and I can drill into the existing slab that was installed for the fireplace. Would you just frame out the box with lumber on the sides? I was thinking of fabricating some laminate out of hardibacker cut to dimension to build the box.

 

[bholler]

It is for an insert in the fireplace. I don't understand what you mean by a post in each corner? If you are referring to posts that go down to the floor in the cellar, that is a no-go, as I need the space to remain open. This is why I was asking about cementitious foam, as it might meet code while not weighing as much as regular concrete. Another idea that I have is to use perlite as the aggregate in a home brew mix to save weight. The metal plan will work, and I can drill into the existing slab that was installed for the fireplace. Would you just frame out the box with lumber on the sides? I was thinking of fabricating some laminate out of hardibacker cut to dimension to build the box.

How big is your framing and at what span? It can absolutely be done without posts if you know how to properly structure it.