Solid Hearth

[Berone]

I like the idea of a solid slab for a hearth. Maybe a piece of bluestone or some such 26" deep x 60" wide. Maybe 1/2" to 1" thick. Since my insert would sit on top of that, I would have to pad up inside the fireplace as well. Questions: Is there a reason not to do a solid hearth? Heat breakage or anything like that? Properly supported pressure breakage shouldn't be an issue. Do I need to put cement board under it? What would be best to pad in the fireplace? Can I just build up a couple of layers of cement board? Anything else I'm not thinking?

Thanks,
Anthony

 

[Wet1]

What does your stove manufacturer say is required? This is what the BI will look for. Overall I'm not a big fan of building a hearth to only meet amber protection, even if the manufacturer said that's all that's needed to meet their min. requirements.

Heat transfer is the biggest concern. If the hearth is built on a combustible surface, you ideally would want to create a thermal resistance barrier for safety... even if the stove or insert manufacturer says only amber protection is needed. I would shoot for an R2(+) to be on the safe side. Sometimes having the a higher R value hearth can contain the heat in the house better as well, such as a hearth built on an exterior concrete slab (such as a basement floor). This way you heat is not transfered to earth. I think dead air space and/or Micore are the ideal ways to build up the R value. Cement board is a very common substrate to lay your brick, tile, stone, etc. on.

BTW, the thermal resistance value per inch of thickness of concrete is between 0.1 and 0.2. So if you're building this hearth on top of combustibles, you'd need a LOT of concrete ($$$ and weight) to reach an R2.

 

[Berone]

The existing hearth meets code for the fireplace. The problem for the insert, codewise, is that the insert sticks out 7 1/2" further onto the hearth. So I guess I need an additional 7 1/2". The existing hearth is tile set flush to the floor. I've hated it since we bought the house and have planned to build up on top of it for some time. Now that I'm doing the insert it seems the right time to do it.

 

[firefighterjake]

I'm with Wet1 on this issue . . . follow manufacturer specs for both the floor protection requirement (including any R value if needed) and for hearth clearances . . . and to be safe you might want to check with your insurance company and/or person in your hometown who does the fire inspections for new installs.

Here where I work the Fire Inspector requires listed floor protection regardless of whether the manufacturer says only ember protection is needed since it it his belief that it is better to build in a little more R value protection if this can be done so relatively inexpensively . . . and so woodstoves, pelletstoves and inserts all must have some sort of listed floor protection. As I said . . . usually this is easily and cheaply accomplished with a $10 sheet of 1/2 inch Durock under the tile, stone, etc.

Going with just stone may not be allowed . . . I know it would not be allowed here . . . since our Fire Inspector's take on this would be that he does not know what the R value of the various stones and thicknesses and the stone technically is not listed. While he says commonsense would dictate that a very thick stone would probably provide adequate protection, he always errs on the side of safety and would require the Durock under the stone.

In the case of the existing hearth with tile . . . the question our Inspector would have would be what is underneath that tile . . . this is what would determine whether he would give his A-OK to the existing hearth or if he would require more protection.

If I were you and I could do it easily I would have no issues going with a stone slab . . . in fact I think it would look sharp and would help on clean-up . . . I think spalling or cracking would be minimized depending on the type of stone, thickness, how well it the stone is supported underneath (i.e. minimize any flexing of the floor underneath) and the type of insert (i.e. how hot it gets underneath the stove). I would try it if I were you . . . but I would encourage you to get some Durock for underneath the stone . . . this would do a few things . . . a) it would give you a raised hearth which I personally like, b) you would have the added safety of more R value from the Durock (although just as a reminder, the Durock alone may not be enough to meet the stove's particular R value requirement if there is any) and c) it would increase the rigidity of the "floor" helping to reduce any flexing of the stone.

 

[Berone]

Thanks! That gives me good information to work with. I'm going to do this (as I usually do) from a design perspective. Figure out what I want it to look like and then back into how to do it to meet code. I read a thread here about installing the same stove I'm getting (Hampton HI300) on a 4" raised hearth. That changes the specs for the r value. It also raises the door 4 inches which my back would appreciate. And I think it will look good. I better start drawing!

 

[Berone]

I went by the stone yard today to get an idea of what is available. They didn't have bluestone thicker than 1 1/2", and I wasn't crazy about the finished edges. Then the yard guy showed me a piece of what he called "colonial slab". It's a beautiful piece of stone about 38" deep and 66" wide, 3 1/2" thick. A piece 26" deep by 48" wide would satisfy all requirements. And as the guy suggested, the other piece could become a mantle. Since I have no love for the existing mantle that's a great idea. When I asked how much the slab was he said "how much cash you have in your pocket". He qualified that to be around $150, but I may do better when I show up with cash. $30 to deliver it, then I just have to cut it. Never done that before; should be fun!

Meanwhile, back at the ranch, I discovered that once I raised the insert up 3 1/2" it would no longer fit at the back of the fireplace, as the top slopes down quite a bit to the back. Well, that's why God invented the Hilti T60. I took out the glass door unit and, 45 minutes later the insert will fit just fine. While I was at it I took my sawzall and cut out enough of the damper that I won't have to "ovalize" the liner. I just need to get the chimney cleaned now and I'll be ready for the install. That is, as soon as the roofing guys get in the slate to do the repairs and get up on my roof.