[Jim Frye]

I did a "floating mantel shelf" some years ago on the face of a marble rock fireplace. The ledger board was bolted to the fireplace with two threaded inserts set in mortar joints and the shelf was screwed to the ledger board with two screws on the underside of the shelf. The shelf itself was a "torque box" that slid over the ledger board, thus hiding it completely. Very strong setup. French cleats are meant to support the weight in a downward, or vertical force. The mantle shelf might put more outward strain on the joinery. By the way, my shelf was carved to fit tightly up against the rock face and there was no chiseling of the marble rocks to get things mounted. I used Bondo on the back side of the ledger board to fit it snugly up against the rocks. I put plastic wrap on the rocks and pressed the ledger board up against the rocks while it was still soft to get the fit without laborious carving of the Bondo. The Bondo was done in four 6 inch "lifts", two on each end. The top and bottom of the mantle shelf was 3/4" oak plywood with the core framing of 1x3 pine. The ledger board was made from two layers of 1x3 strips of 3/4" plywood glued and screwed together. The shelf was 4 1/2" thick, nearly 10" deep, and about 6 feet long. Here's a couple of pics of things.
http://www.woodworkingtalk.com/memb...orking-projects/38841-fireplace-mantle-mount/
http://www.woodworkingtalk.com/memb...orking-projects/38833-fireplace-mantle-shelf/

 

[Jim Frye]

Just caught my technical error in my post from yesterday. The mantle shelf was a "torsion box", not a "torque box". The torsion box has all sides enclosed, while a torque box has one or two sides open. Sorry for any confusion.