Rebuilding a hearth

[diceymatters]

I've got an Avalon Ranier 45 coming (which will sit 10 inches in front of the fireplace) and so I needed to extend my hearth from 18 inches to 26+ inches.

My existing hearth was bricks that were raised about 1/4 inch from the floor. I've removed the bricks and four courses of oak floor in front of the hearth and my plan is to set one single piece of 60 inch by 27 inch bluestone. It will rest partly on the sub floor and partly where the bricks used to be. At this height, the stone will be 1/4 inch raised from the adjacent floor which is how I want it to be.

In order to keep the bluestone from cracking I realize that I need to make the subsurface extremely level. Also, since this is an insert that is sitting 10 inches onto the hearth, I realize that whatever I put under the bluestone (where the bricks used to be) must be able to resist the heat of the stove.

The Avalon manual seems to be silent on the issue of materials to use under it. The only requirement is that the hearth , "Must be non-combustible 1/4" thick insulating board with a thermal conductivity of K = 0.72" but I'm assuming that this is referring to the part of the hearth that is in front of the stove - not directly under it.

Any thoughts on what materials I should put under the bluestone that will 1) enable me to make a level surface with the subfloor, and b) will provide enough R-value to be safe?

Also, I was thinking that once the subsurface is level, I wouldn't even use mortar or adhesive to set the bluestone. Do I need to use some adhesive/mortar?

Thanks!

 

[begreen]

Before going to far down this path I would contact Travis and give them the same details. I believe you are correct in the hearth requirements, in front of the stove. But the insert itself is expected to be in the fireplace with full masonry below it. That puts the area where the old hearth was under scrutiny. What is currently underneath the hearth that was torn out?

For the hearth extension, a sheet of 1/2" cement board like Durock or Wonderboard should work. Try to get the surface below the cement board (old hearth) as smooth as possible first.

 

[diceymatters]

The material under the former hearth is concrete - probably a couple inches of it, which is supported by wood framing which is visible in the basement. I've contacted the dealer who has contacted Travis and supposedly my situation will be fine and there is no specific defined R value that I need to achieve under the unit. The firebox on the Ranier 45 as an insert is raised up an inch from the floor, so the air provides some R value. My plan for the hearth is to pour concrete into the cavity so it is level with the adjacent subfloor, and then set the stone with thinset without using cement board. Does this sound crazy?