Advice on designing hearth flush to floor

[skinanbones]

I'm not sure what the r value on granite is as here in canada we only need ember protection. We had a client go to a supplier of granite counter tops and picked out a 3/4 slab and had that cut to the size required for his jotul castine. That granite was the same thickness of the hardwood flooring that was used and when finished look amazing.

 

[begreen]

In Canada I would think that the stove would need at least meet the mfg. tested standard. I have never read about Canada having more lax hearth standards. Nor have I seen ember protection only required if installing in Canada in any manual. If anything hearth reqs. are a bit more strict, requiring 18" in front of the door.

 

[elmoleaf]

I agree with everything but the steel plate I don't see any reason for it.

As I described it, he wouldn't have the 3/4" plywood he described. Thus the steel is required for structural support of hearth and stove....to eliminate flexing of micore and cement board, especially if his stove legs don't align with the floor joists. Tile and 1" of micore/cement board alone have insufficent structural strength for span between floor joists if load is applied.
Construction adhesive, in modest amounts at the perimeter areas, will help the sandwich of materials to act as a monolithic element (similar to reason why when you build a built up beam from 2x material, you don't just locate them parallel...they get fastened together). And as noted, micore and cement board really can't be screwed down to a steel plate.
I don't think the micore 300 gives sufficient R value?
However, as described at end of my first post, he could eliminate supporting steel by using his original cleat/recessed 3/4" plywood system but with added solid perpendicular blocking. I agree the all wood solution is seemingly "easier" given that materials are readily available. It just requires a lot more fussing with cleats and getting them all dead level so the inset plywood is dead level with tops of joists, some of which may crown up or down. One sheet of steel over joist tops can eliminate most of that time and effort.

It'd be interesting to know how the OP proceeds and see some pictures.

 

[bholler]

yes but he was going to put ply between the joists on top of cleats so I still se no need for the steel. That steel will cost a whole lot more and ad a un needed complexity to it.

 

[begreen]

They require an R=1.1 hearth for the F3CB. 1/2" of Micore 300 = 1.03 + 1/2" Durock NexGen (@ .39) total R = 1.42

 

[bholler]

I would just like to point out one thing that I have been told I don't know how much of a concern it is but I have been told not to use screws in this situation and instead just use thinset between the layers because of heat transfer through the screws. I honestly don't know if it is really an issue but I know I remember someone telling me that and it does make some sense.

 

[begreen]

I think that is a non issue. The durock should be screwed down. Thinset won't bond well to micore, it's a fibrous material.

 

[bholler]

Ok I just though id throw that out there. I think the easiest option here would be to simply buy a manufactured stove board and sit it in the recess. I think that is what I would do. I am sure it would cost a little more but not if you are paying to have the work done.

 

[Grisu]

I would just like to point out one thing that I have been told I don't know how much of a concern it is but I have been told not to use screws in this situation and instead just use thinset between the layers because of heat transfer through the screws. I honestly don't know if it is really an issue but I know I remember someone telling me that and it does make some sense.

If the screws are just used to connect non-combustible materials like Micore and Durock I don't see how any heat transfer could be worrisome. It may be different if they end up in a wood support. To avoid that I would screw the first layer to the wood support and then use screws at an offset to attach additional layers. That should avoid any thermal bridging.

 

[jkranes]

As I have heard it, thinset is always used between sandwich layers. The purpose is not to act as adhesive but to fill in any potential air gaps. Otherwise the upper layer can flex down into the gap and crack the tile job.

 

[fire_man]

I'm not sure it's a good idea to put thinset on the micore, the moisture might not be a good. Also, that stuff is like styrofoam and I don't see how it would adhere in the least.

 

[fire_man]

I would just like to point out one thing that I have been told I don't know how much of a concern it is but I have been told not to use screws in this situation and instead just use thinset between the layers because of heat transfer through the screws. I honestly don't know if it is really an issue but I know I remember someone telling me that and it does make some sense.

I have a sheet of micore waiting to install. No way would I trust thinset to captivate that stuff. It easily crumbles and I cannot see how the thinset would adhere. Maybe some kind of adhesive would stick, but I would not use adhesive in a hearth.

 

[geo91324]

I had exactly the same situation as the OP when I switched out my Encore (which sat mostly inside my kitchen fireplace) with my Heritage (which sits completely outside the fireplace). The small existing stone hearth was removed and I built a slate hearth deeper into the room, butting up to the hardwood. I wanted an R value of over 1.0 (if you've ever stuck your hand under (not in front of, under) a stove with a raging fire going you know the flooring can get hot. Stone, cement, granite, etc. have virtually no R value, so that heat would be transferred directly to the subfloor.
I did exactly as the OP suggested by cutting the subfloor plywood flush with the joists and blocked them appropriately underneath. Then, as Begreen suggested, I used micore and durock with, in our case, the slate on top of that. Of course, it's still a good half inch or so taller than the wood floor. (A project for this spring: bevel the edges.)

 

[skinanbones]

In Canada I would think that the stove would need at least meet the mfg. tested standard. I have never read about Canada having more lax hearth standards. Nor have I seen ember protection only required if installing in Canada in any manual. If anything hearth reqs. are a bit more strict, requiring 18" in front of the door.

Begreen yes it is true that our dimension are larger 18" in front of the door and 8" on the other 3 sides, but our only requirement is ember protection. There is no mention of building hearths with an r factor for a certified stove.
This is why certian jotul stoves have bottem heat shields installed from the factory and why all vermount castings stove required a bottem heatsheild when installed in canada.

 

[Moorboy3]

I'm confused. An architect is a licensed professional who should be able to research the codes and design this for you. What services are they providing? Or is this person a designer without a professional stamp?

The architect and contractor are both excellent but none of us have had much experience in this area. We didn't want to reinvent a complicate, code compliant solution if people in the wood burning community had experience of this problem they would be willing to share. And share they have - thank you all so much!

Reading through the great responses posted it looks like our proposed solution - with a few mods - should work.

 

[Moorboy3]

As I described it, he wouldn't have the 3/4" plywood he described. Thus the steel is required for structural support of hearth and stove....to eliminate flexing of micore and cement board, especially if his stove legs don't align with the floor joists. Tile and 1" of micore/cement board alone have insufficent structural strength for span between floor joists if load is applied.
Construction adhesive, in modest amounts at the perimeter areas, will help the sandwich of materials to act as a monolithic element (similar to reason why when you build a built up beam from 2x material, you don't just locate them parallel...they get fastened together). And as noted, micore and cement board really can't be screwed down to a steel plate.
I don't think the micore 300 gives sufficient R value?
However, as described at end of my first post, he could eliminate supporting steel by using his original cleat/recessed 3/4" plywood system but with added solid perpendicular blocking. I agree the all wood solution is seemingly "easier" given that materials are readily available. It just requires a lot more fussing with cleats and getting them all dead level so the inset plywood is dead level with tops of joists, some of which may crown up or down. One sheet of steel over joist tops can eliminate most of that time and effort.

It'd be interesting to know how the OP proceeds and see some pictures.

This is great. We hadn't thought of sheet steel as an option but I'll look into this. Unfortunately the location of the stove is directly above a load of water pipes coming from the basement so access to the space is pretty much only from above by cutting thru the floor. It may be time saving to use a slab of steel.

Also, you're right about the micore 300. From my reading only the 160 has enough R value at the thicknesses we were looking for.

I'll post pics once we start the work!

 

[Moorboy3]

They require an R=1.1 hearth for the F3CB. 1/2" of Micore 300 = 1.03 + 1/2" Durock NexGen (@ .39) total R = 1.42

That leaves me with 1/2" to play with for thinset and some kind of tile or stone. This might just work!
Thanks for your input.

 

[Moorboy3]

If the screws are just used to connect non-combustible materials like Micore and Durock I don't see how any heat transfer could be worrisome. It may be different if they end up in a wood support. To avoid that I would screw the first layer to the wood support and then use screws at an offset to attach additional layers. That should avoid any thermal bridging.

I had not thought of this issue, but ill look into this further as I'm basically paranoid.

Am I not right in saying that when using a rear heat shield with air gap you would ceramic spacers but steel screws? This seems like a similar situation but on below the stove. I think you are not supposed to use screws directly behind the stove. I wonder if this is why.

 

[begreen]

Begreen yes it is true that our dimension are larger 18" in front of the door and 8" on the other 3 sides, but our only requirement is ember protection. There is no mention of building hearths with an r factor for a certified stove.
This is why certian jotul stoves have bottem heat shields installed from the factory and why all vermount castings stove required a bottem heatsheild when installed in canada.

What happens when an Englander 13 NC or a Hearthstone Homestead is installed in Canada?

 

[skinanbones]

not sure as neither company have much of a canadian presence. The few hearthstone we ran across all had bottem heatsheilds, i never seen an englander here in ontario

 

[M1sterM]

If you look at his drawing he has kleets running along the length of the joists ....the plywood should be glued and screwed to that...essentially that will form a solid box....

Cleats don't form a box, unless they are bridged along the bottom, too.

 

[begreen]

not sure as neither company have much of a canadian presence. The few hearthstone we ran across all had bottem heatsheilds, i never seen an englander here in ontario

Englanders are sold by Canadian Tire. I suspect there are a lot of them in Canada. Can you show me any code statement that says wood stoves in Canada will only require ember protection hearths? I think this is a misstatement but am willing to be proven incorrect.

 

[fire_man]

Yea I would like to see that too - about the ember protection. I can't imagine the manufacturer's specs for Hearth R value requirements would not apply.

 

[fire_man]

I called USG to ask about Durock Next Gen. as a Hearth underlayment. Their response was: "we have no recommendation". He said his literature specified a max continuous rated temp of 175F, which he personally did not think was adequate for a hearth. He mentioned it was non-combustible, but might not hold up over time under high temp. I think this jives with what others have been told.

 

[goofa]

Begreen where did you find the r value for this particular stove I have searched hotel's online manuals for the r value for the coastline but cannot seem to locate it nor the Oslo.... because I am in the process of my livingroom remodel and never even gave the "r value" a thought for the raised hearth. I'm glad I found this thread pheeeeeeew. I would have had to get out my favorite tool...... the cats paw lol ha