Your fireplace book will have a chart showing how far above the fireplace you have to put the mantel. It also says how far out the mantel can go, depending on the height above the fireplace. What sucks, is that height is always a lot more then people want. It looks odd to have it so far up there, but it's a fire hazard if it's where most people want it.
I've had several jobs where the mantel was too low, and the wood was coming apart. In one case, she bought a concrete mantel that looked just like wood. Since it wasn't combustible, I felt comfortable mounting it where she wanted it.
Is that a real log that you are using for a mantel?
