Support for backer board above direct vent fireplace?
We are getting ready to install a direct vent fireplace in our home. We're planning on using tile as a surround between the fireplace and mantel instead of granite or marble. We'll install the tile over cement backer board. Our fireplace has standoffs on the top so there will be a 6"+- gap in the framing at the top similar to this:
It seems to me that if we simply install the backerboard over the wood framing, there would be too much give for tile (we're using 1/2" x 1/2" mosaics). Should we bridge the gap with a piece of steel or other non-combustible framing member or will it be strong enough without it?
Thanks for any help.