tradergordo said:
What is it? And how do you know durock wouldn't have had the same result given whatever conditions that material were under?
As best I can tell from the picture, the peice that is tilted up is the tile or whatever is being used for the floor treatment. The underlayment looks like some sort of cement board, covered with whatever adhesive was used to fasten down the tiles, can't tell if it's Durock brand or not. The discoloration (I'm not sure it's enough to rate being considered "charring") I wouldn't say is necessarily a problem with the cement board, as it could be a stain from the thinset or other material.
What I can say is that Durock is "UL LISTED for use with UL Listed solid fuel room heaters and fireplace stoves" and that Durock's data sheets specify the ANSI 118.4 or 118.1 standard for the mortar. When I was planning out my hearth extension, and doing some research for the Hearth Construction article in the WIKI, I spent considerable time on the phone calling the tech support lines for Durock, Hardibacker, Wonderboard, and Permabase. Durock was the ONLY one that said flat out that hearth pad construction was a reccomended application. Hardibacker said they were approved for making clearance reduction walls, but NOT for hearth pads, Wonderboard didn't really answer beyond saying that it shouldn't be exposed to more than 350*F, and Permabase said they were "not non-combustible"
When I asked for clarification on the mortar specs, Durock's tech support folks said that the standards basically translated to "Latex modified" thinset, and specifically suggested the Flexbond brand. (Which isn't one of their products BTW) I have subsequently heard from another poster that Versabond also meets the ANSI standards, however NONE of the "readymix in a bucket" products I've checked do. Both Flexbond and Versabond are dry powder products that get mixed with water at the time of use (like concrete) - the reason given for the use of the latex fortified mortar is that it is better able to handle the thermal cycling that a hearth undergoes.
BTW, my Encore cat (w/ required bottom and ash pan door heat shields) requires only a "non-combustible surface" such as '1/4" non-asbestos mineral board or 24g sheet metal' - Durock plus tile far exceeds this requirement.
What I've observed is that my raised brick hearth (that barely met the old NFPA spec as it turns out) gets hot enough to be on the edge of what is comfortable to stand on for any length of time. The 18" Durock and slate tile extension gets decidedly warm, but not uncomfortable.
Gooserider