We're putting together our own fireplace mantel from fire-resistant materials because we can't use wood at a reasonable height over the EPA-approved unit we installed (very hot) and can't afford a stone fireplace surround.

So the plan is a marble shelf supported on corbels across the 5-foot fireplace wall (which sticks out 5 inches from the wall on each side), with everything below that tiled, probably matching marble mosaic (creamy white on white in a similar room). I'm looking forward to being able to decorate it with the seasons and have really enjoyed seeing what you guys here do.

So, the question. I'm mocking up a 7" depth, but really should go shallower. What is the least depth you'd want in a mantel? Any comments on what can be accomplished and what not with different depths?

Thanks very much for your thoughts.