12 inches
Aesthetic Considerations. Depending on the size of your fireplace, the size of the mantel and the height of your wall, you may want to hang it significantly higher than the required 6- or 12-inch safety requirement. For a mantel that is designed to be a focal point, you will want it to be at eye level, about 5 to 6 feet above the ground. Most codes specify mantel distance from the firebox itself. As a general rule, a mantel is placed about 12 inches above the fireplace opening. Add an inch to the distance for every inch that the mantel protrudes. So, a mantel 6 inches deep would be attached 18 inches above the firebox opening. For safety reasons, any mantel shelves must be placed properly. If your shelf projects 1 1/2 inches or less, there must be at least six inches between the top of the fireplace and the mantel. Any parts of the mantel that project more than 1 1/2 inches must be at least 12 inches from the top of the fireplace. Of course, these guidelines are subject to change based on several factors. Safety. For safety reasons, any mantel shelves must be placed properly. If your shelf projects 1 1/2 inches or less, there must be at least six inches between the top of the fireplace and the mantel. Any parts of the mantel that project more than 1 1/2 inches must be at least 12 inches from the top of the fireplace. Of course, these guidelines are subject to change based on several factors.
http://www.ehow.com/decision_6877237_high-should-fireplace-mantel-be_.html http://homeguides.sfgate.com/recommended-distance-hearth-mantel-102956.html http://www.doityourself.com/stry/how-high-should-a-mantel-shelf-be-above-a-fireplace http://www.doityourself.com/stry/how-high-should-a-mantel-shelf-be-above-a-fireplace
how far above the firebox should a mantle be