another zero-clearance installation wall surround question

[stangds]

I'm putting in a Napoleon NZ-26 zero clearance fireplace, as I've mentioned in a few other posts here.

I have another question about my installation before I get it all wrapped up:

I have the fireplace framed into meet the "non-combustable" clearances, but what do I use to get from the framing to the fireplace?

Can I use durock to fill the gap, then glue my surround to that (I'm planning on using Granite)?

i've attached a sketch to try to make it clearer.

Also, my instructions were extemely vague on the hearth (like no mention of insulative properties or anything). I was going to triple layer durock and then put the granite sheet on top of that. Is this a good idea, or is there a better way to do it?

 

[DiscoInferno]

I just have 1/2" standard sheetrock around my ZC, which I will eventually glue a slate surround to. That's how the prefab fireplace I replaced was setup also; slate glued to drywall. The Ultima instructions say that combustable materials are fine around the unit as long as they are flush with the face, i.e. don't project out. The outer edges of the Ultima never get past slightly warm.

My instructions were like yours quite vague on the hearth; simply "noncombustable". The previous hearth was slate glued to subfloor; the previous owners pulled it up and laid wood flooring. When I used the fireplace I laid 3 18" square slate tiles on top, but when I replaced it with the Ultima I cut out the floor and laid the tiles flush. They're just mortared to the subfloor now. I think that's safe, although the middle tile does get pretty warm from downward radiation.

 

[spot]

I used durock on mine exactly like you are planning to do.
Just leave a small 1/8-inch gap between the durock and the metal of the fireplace.
Fill this gap with special high-heat fireplace caulking.
You can then use your slate tiles to bridge this caulk-filled-gap slightly which will create a neat clean edge.
You need to have durock under your hearth, too.
In my opinion the triple-layer of durock is overkill unless you want it to be that high off the floor anyway.
This is just in case the mortar between the granite pieces (or the granite itself) cracks and an ember finds its way into the crack.

Hearth Installation
A hearth extension is required to protect a combustible floor in front of the fireplace.
Note:Hearth Extension must not cover the air inlet opening of a fireplace.
The hearth extension must be a durable non-combustible material with a minimum
(total)Rt value of 1.09.
Secure the hearth extension to the floor to prevent shifting,using trim molding or other similar means at three (3)outer edges.Seal crack between the fireplace hearth and hearth extension with a non-combustible material.

Minimum Hearth Extension Dimensions
47 1/4 inches wide
18 inches deep
8 inches beyond firebox opening to the sides
R value = 1.09