I'm remodeling a room that includes a fireplace. I tore down the old stonework and have stripped the fireplace down to the masonry. I would like to tile over the masonry, but there is gap at the top of the fireplace. This was previously covered by a large piece of angle iron and the stone work ran across the angle iron. I'm wondering if I can put a piece of hardibacker or concrete board across this gap and tile over it, but it will be subjected to intense heat from the fireplace. Any advice on how to deal with this?