Fireplace Install

We have a fireplace coming up and (http://maplered.com) I was wondering how you all would install this one. The header is the only piece on it and (http://maplered.com) she wants me to stick it directly to the metal, which is not very sturdy. 2cm Marble (http://www.lightemprador.com) , I'm worried about the piece coming down in years to come if we don't install any supports. I'd rather find a good adhesive that I can trust, she's going to sign a liability waiver, which is good. Should I screw durrock down b4 I install the Marble (http://www.lightemprador.com) ? Then the question is how?

Matt W.