help with rebuilding hearth

[A Stove in Philly]

We recently purchased a 1925 home in Philadelphia, and we're having a wood-burning insert professionally installed in early March (it was the earliest they could get to us). However, I've needed to do some demolition of the old mantle (a super ugly 2x4, drywall, and mirror monstrosity) and hearth to get it ready for the installers. We have a 2" piece of slate countertop we're going to use for the hearth extension, but the more I dug into the work around the hearth--literally and metaphorically--the more questions I had.

The hearth extension doesn't seem to be connected to the hearth itself (the floor of the firebox). It was layers of crumbling concrete poured into place covered over with a crumbling slate tile. It's supported underneath by the mold it was poured into, but that is supported by the floor joists. I have been reading that there should be no wood under the hearth, but I don't see how I'm going to accomplish that with the joists running the way they are. Am I missing something? I don't really want to install any supporting pillars.

Also, is pouring a new concrete pad and then laying the slate over that the best way to go? Are there other layers that should be involved? What about rebar or wire framing? I don't want to under or overbuild this thing.

And finally, should I be concerned at all about the floor or the walls of the fireplace itself, since it's going to be covered up by the insert? Any recommendations are welcome! Thanks!

 

[begreen]

It's not uncommon to find all sorts of errors in old homes. It's why modern building codes came into being. The 2" granite would need to satisfy the hearth requirements for the stove to qualify. It may need an insulation barrier of cement board or micore under it.

To make the hearth right, it would need to be cleared out, then the joist space boxed and reinforced. Then a temporary form bottom gets put in place, rebar is added tying the fireplace floor to the hearth extension. Then the concrete pour follows.

 

[A Stove in Philly]

It's not uncommon to find all sorts of errors in old homes. It's why modern building codes came into being. The 2" granite would need to satisfy the hearth requirements for the stove to qualify. It may need an insulation barrier of cement board or micore under it.

To make the hearth right, it would need to be cleared out, then the joist space boxed and reinforced. Then a temporary form bottom gets put in place, rebar is added tying the fireplace floor to the hearth extension. Then the concrete pour follows.

To clarify, do you mean the hearth extension would need to be cleared out? I don't need to clear out the entire floor of the firebox, right? And wouldn't there still be wood on the bottom of the heart, since the floor joists are there?

 

[bholler]

To clarify, do you mean the hearth extension would need to be cleared out? I don't need to clear out the entire floor of the firebox, right? And wouldn't there still be wood on the bottom of the heart, since the floor joists are there?

The floor joists need to go. There can be no wood under the hearth or hearth extension. There are also studs in the wall too close to the firebox.

 

[A Stove in Philly]

The floor joists need to go. There can be no wood under the hearth or hearth extension. There are also studs in the wall to close to the firebox

Do these codes apply to inserts? This isn't going to be an open fireplace but a wood-burning insert. It just seems like overkill to remove all the wood beneath if I'm going to have concrete AND slate between. The installation manual for the wood-burning insert only requires an R-value of 1.00.

 

[bholler]

Do these codes apply to inserts? This isn't going to be an open fireplace but a wood-burning insert. It just seems like overkill to remove all the wood beneath if I'm going to have concrete AND slate between. The installation manual for the wood-burning insert only requires an R-value of 1.00.

But doesn't the manual also call for a working code compliant woodburning fireplace?

 

[A Stove in Philly]

But doesn't the manual also call for a working code compliant woodburning fireplace?

It does. However, I'm seeing this in the code for the studs:

R1001.11 Fireplace Clearance: Wood beams, joists, studs and other combustible material shall have a clearance of not less than 2 inches (51 mm) from the front faces and sides of masonry fireplaces and not less than 4 inches (102 mm) from the back faces of masonry fireplaces. The airspace shall not be filled, except to provide fireblocking in accordance with Section R1001.12.​

So, that means that the studs are fine. They're nearly 3 inches.

I'm more concerned with the thickness of the hearth itself. I'm seeing this in the code:

R1001.9.1 Hearth Thickness: The minimum thickness of fireplace hearths shall be 4 inches (102 mm).​

and this:

R1001.9.2 Hearth Extension Thickness: The minimum thickness of hearth extensions shall be 2 inches (51 mm).​

So after this thickness is achieved, do I really need to remove floor joists?
Thanks for the help!

 

[bholler]

The problem is you don't have the required firebox wall thickness

 

[bholler]

And you are right about the hearth and hearth extension thickness but missed the part about combustible materials shall not remain against the underside of the hearth or hearth extension

 

[A Stove in Philly]

Thanks for the help. I hope to consult with the insert company today.

 

[bholler]

Thanks for the help. I hope to consult with the insert company today.

You have it open now just fix it right