Hearth Is crumbling

[lostDuck]

As I prep to install my new Brolet 1800 insert I was removing the slate the was on top of the hearth so I could extend it. When I pulled the slate off I found a mess. The cement below it is cracked and it rises and falls by 1/2" +/-. So obviously I have to level it out.

So
1) I have a fireplace like in the picture below with the hearth extension over the basement floor joist.
2) From the basement the hearth extension is solid cement.
3) Pulling away one of the cracked cement blocks I find nothing but rocks between the top of the cement and the bottom cement layer.

Now with the question.
1) Is this normal to have a hearth extension that is cement - gravel - cement? If so can i just breakout the top part of cement an re-pour it level?
2) Do I need to do something else?
3) The insert instructions require a 4" pad to extend 16" out from fireplace hearth.

Thanks,
LostDuck... truly

 

[begreen]

It's called rubble fill and yes, it's common, right back to Roman times and earlier. Clean it out, mortar or thinset a sheet of Durock on top for a flat level surface.

 

[lostDuck]

Begreen you are awesome! I so appreciate you comments as I fumble my way though this install.

Since I have some extra Micore 300 should I put that down first and then Durock to have a level service for the slate?

 

[BrotherBart]

The extension under it in the basement should be cantilevered concrete with rebar in it. Is it wedge shaped in the basement under the extension?

 

[lostDuck]

BB - Yes the bottom layer looks wedge shaped with rebar going longitudinally from the fireplace to the first floor joist.

 

[BrotherBart]

Yeah they just fill the stuff you ain't gonna see with scrap. I just wouldn't do anything that adds weight to the support beneath. The guy that built your hearth was the lowest bidder.

I often glance up at the concrete in the basement and hope that I don't wake up some morning with a burning stove on the floor of the basement. Hasn't happened in 28 years here. But I pay attention to the weight I put on top of it. Not what is filling space under the hearth extension.

 

[BobUrban]

Yea - with about 275 10lb. each pavers I used to build my hearth + the stove, durarock, mortar, etc.. I get to thinking I need to get that baby beam and floor post installed this summer every time I add a couple arm loads of wood to my storage space. Not happening now because there is a cord of ash stacked down there. I have put my plumb stick on it a couple times from the basement just to check and no detectable movement yet. I was thinking my house might tip over if I added any more hearth.

 

[lostDuck]

Well the hearth is rebuilt. I ripped out all of the old grout that was cracked and re-did it. Plus 1" of Micore 300 (from eFireplace), 1/2" of Durock, and slate.

Stove is going in this week...