A few months ago, a friend and I removed the ceramic tile and mantle around my fireplace. The sheetrock underneath the tile came off when we did the demolition, so I had to reinstall the sheetrock. I have a metal "firebox" lined with brick. My question is, do I need to leave some clearance between the sheet rock and the firebox? I will place tile over the sheetrock, but the cut edge of the rock behind the tile will either contact the firebox edge or else I can leave a gap. If I leave a gap, what do I fill it with so that I can have backing there for re-tiling or the use a type of veneerstone product?