As the title says, I am building on to my hearth to have it meet code, as it is now it extends out 12.5", I just.bought.some 19x20 hearth stones I will be setting for the new hearth. I knocked the clay tile off of it last weekend and theres a pretty level bed of mortar there now that the tiles were on (hearth is brick). My question is can I frame in the extention butting cement backer board flush with the exisiting mortar, tape the seam and be able to lay the stone on tppe without having to worry about cracking of the mortar or do I need to cover the entire hearth with the backer board? Reason for.asking is if I have to do the latter I will have to make everything set higher and I habe minimal clearance in the Firefox for my insert, thanks.