Building a hearth platform - calculating R value...

[mmcc]

Hi All - I'm going to be installing a Woodstock Soapstone Keystone in front of an existing masonry fireplace. Unfortunately the hearth isn't deep enough - only 18", and I need it to be more like 20.

So, my plan is to build a platform in front of the hearth that is flush with the surface of the hearth. The front legs of the stove will sit on the platform, rear ones on the hearth. Additionally, the platform will provide the necessary side and front clearances.

I'm thinking that I'll make the platform out of 2x10s, tile the sides to make it look nice and set a slab of 2" thick sandstone on top (I found someone on craigslist selling old sidewalk slabs),

The manual says I need an R value of .59 for a hearth pad. The info I found about sandstone online gives it an R value of .08 for a 1" thickness. So...

- I'm not sure how R value math works. Does that mean a 2" thickness would have an R value of .16?
- Presumably that means I need something between it and anything combustible. Any recommendations? Hardie backer? Cement board?

 

[yooper08]

Right. R value is additive, meaning that as you add more layers/material, you can add them together to get the total R value. The other value you might see, the K value, is not additive.

As for building a hearth, haven't done it, so I'll let others speak towards that piece.

 

[firefighterjake]

I'm pretty bad at math, but even at .16 it looks like you have a ways to go to build up to the required .59 value you mention.

I didn't need it, but Micore is pretty popular for use when a higher R value is required. It apparently is soft so most folks sandwich it between materials.

I used Durock on my hearth . . . basically a cement board . . . I cannot remember the R value though since it was quite some time ago and honestly my stove only required ember protection.

 

[mmcc]

Right. R value is additive, meaning that as you add more layers/material, you can add them together to get the total R value.

Great! I found a link to a bunch of R values for different materials...

https://chimneysweeponline.com/horvalue.htm

Looks like Durock wouldn't quite do it, unless I doubled up (which would certainly be doable).

But regular ol' 1/2" "Gypsum or Plaster Board" would be adequate - could I just set the sandstone on a layer of drywall?

 

[bfitz3]

I made a platform with 2x4 laid flat, scrap 3/4 ply, durock, micor, durock, tile. I believe I have R value to spare nd could have lost one layer of Durock. I doubled up for the stiffness and aesthetic as it gave me the height I thought looked good, gave me clearance from the floor, and didn't require ripping boards. I want to say that the sheet of durock I bought was large enough for both layers too, so it was "free" to double up.

If you have reasonable DIY skill, it's an easy project. Making your platform blend nicely with an existing hearth is likely a more involved project.

Something I'd recommend for everyone building something like this... Spend a little extra to get a finished surface that you like and goes well with your stove. It'll be a focal point in your house and you'll be looking at it a LOT! I bought tile that was triple the cost of what I would have spent at HD. The increase was only about $40... The cost of a movie with the family.

 

[mmcc]

Spend a little extra to get a finished surface that you like and goes well with your stove. It'll be a focal point in your house and you'll be looking at it a LOT!

Good point. I'm not going to be able to match the brick of the hearth exactly, so that's why I'm going with sandstone. One of my design rules is "if you can't hide it, point at it". I can't hide the fact that the platform isn't part of the hearth, but I can use a nice contrasting material that will provide pleasant focal point.

 

[bholler]

Hi All - I'm going to be installing a Woodstock Soapstone Keystone in front of an existing masonry fireplace. Unfortunately the hearth isn't deep enough - only 18", and I need it to be more like 20.

So, my plan is to build a platform in front of the hearth that is flush with the surface of the hearth. The front legs of the stove will sit on the platform, rear ones on the hearth. Additionally, the platform will provide the necessary side and front clearances.

I'm thinking that I'll make the platform out of 2x10s, tile the sides to make it look nice and set a slab of 2" thick sandstone on top (I found someone on craigslist selling old sidewalk slabs),

The manual says I need an R value of .59 for a hearth pad. The info I found about sandstone online gives it an R value of .08 for a 1" thickness. So...

- I'm not sure how R value math works. Does that mean a 2" thickness would have an R value of .16?
- Presumably that means I need something between it and anything combustible. Any recommendations? Hardie backer? Cement board?

If it is being attached to the hearth extension of an open fireplace it needs to meet code for a hearth extension not just for the stove. That means it needs to be made completely of non combustibles.

 

[mmcc]

If it is being attached to the hearth extension of an open fireplace it needs to meet code for a hearth extension not just for the stove. That means it needs to be made completely of non combustibles.

I wasn't planning on attaching it, just having it adjacent. Like a very tall hearth pad.

Is that problematic?

 

[bholler]

I wasn't planning on attaching it, just having it adjacent. Like a very tall hearth pad.

Is that problematic?

Well I am not positive but I think if it is the same height and touching it is considered part of the hearth extension. If it was lower it would be fine. Functionally I dont see a problem with your plan as long as you have the required r-value of the stove. But I believe it would not be to code. It would not be hard at all to just do it all out of masonry.

 

[Xtrl9]

I had to build a hearth extension as well that was 2" short so I made the extension ~5" by ~5" the chart on the web link there has higher values that the chart in the manual for my stove. I used the one for my stove to plan the extension and it was mostly based off a scrap piece of the safe n sound roxul mineral wool insulation in a box of durock. Has an R value of 9, which is more than my 4" of mortar and stone hearth.

Just used some thinset and the seam tape for durock to hold the box together. I'm not done, done with it as I have the stone tiles to put on the outside but I learned a valuable lesson about the durock screws during this project. That lesson being that the screws probably work great going into subfloor but they don't do anything but bore a hole in other durock. Inspector said it was fine when he came and we both had a laugh at my stupidity and told stories about bats getting in the house.