We are installing for a hearth 1 piece of slate 6' x 18". We were told to use White Flexbond modified polymer. For the Hearth it is made of a frame of 2x4's covered with 2 pieces of 3/4" plywood, ember strip, then 1/2" Durock, then the slate. We needed the frame and plywood to raise the hearth economically. The front of frame will also be covered as well with same materials except no plywood just the Durock and slate. Question: we have been told various things as to the depth of the cement to use for the slate. Slate place says 3/8" -1/2" flexbond others say 1/4". I would think more cushion is better for more even support is better especially on top, but I am obviously no expert. Do I also need that much flexbond for the front edges that will be under the top slate. There will be a planned (?) +/-1/2" overhang of the top piece over the edges of the front and side. My brother who has put slate down on his floor will help with cementing and mostly strength. Any help or suggestions is much apperciated?? Photo is of base unit of hearth without the plywood,Durock or slate.