There is an older thread with similar question, but I thought a new topic could be opened since my question is how to ensure pressure is distributed evenly underneath a fairly large granite slab.
My project is refacing a masonry fireplace. Existing hearth is brick+mortar topped with ceramic tile. I removed the tile and what's left is brick hearth with mortar infill.
The mortar filling the space between brick is not perfectly even. I will try pouring self-leveling concrete, but even then I'm afraid the concentration of the load will end up being in only a few spots (3? 4?) it's impossible to tell.
People are telling me that construction PL glue bead underneath will be just fine, nothing else is needed, but since the dimensions of the hearth are large, I am still concerned that cured PL may have much higher elasticity than granite and I would end up with (again) only few rigid contact spots risking cracks if someone e.g. steps on the hearth.
Dimensions of the granite slab are: 84"x19"x1 1/4". Worth mentioning that out of 19", 14 1/2" is supported and there is an overhang of 4 1/2" on the front, left and right.
My current thought is: if I can ensure there is no contact between granite and underlying concrete, then I would be confident that with uniformly applied bead, PL glue would work fine since it would gradually spread the pressure under the slab.
I'm not sure how to ensure that the slab is "floating" on the glue. If I put spacers all around, will the span be too much? Along the width, the span would be 78". (I would, of course, remove the spacers once the glue has cured).
Any other ideas? Thinset? In my mind, that's a perfect solution, but seems hard to apply due to the weight of the slab, I can not "plop" it as I would do with a tile.