The opening in the front of the fireplace where the fire itself is contained is known as the combustion chamber or firebox. Mantels mounted too close to the firebox create a potential fire hazard, especially when the mantel is made of wood or other combustible material. Make sure you comply with both local building codes and the manufacturer's guidelines when installing a new mantel.
Fireplace mantels should be a minimum of 12 inches away from the top of the fireplace opening. However, mantel depth -- the distance from the wall to the front edge of the mantel -- affects placement. The National Fire Protection Agency recommends that mantels 6 inches deep be mounted 15 inches above the combustion chamber, with the height increasing to 17 inches above the chamber for 8-inch depth, and to 19 inches for 10-inch depth.
Getting a Leg Up
Wooden support legs, whether decorative or structural, mounted under the mantelshelf also have a minimum distance based on their depth. "Log Home Living" says that legs must be at least 6 inches from the firebox, plus a distance equal to the depth of the leg. For example, a leg that is 3 inches deep from wall to front should be at least 9 inches from the firebox so it won't be struck by sparks.
If you plan to place trim material within 12 inches of the fireplace opening, the International Building Code dictates that the trim can project from the fireplace front only by 1/8 inch for every 1 inch above the firebox. Under no circumstances may combustible trim pieces be placed less than 6 inches from the fireplace opening. Following this ruling, a combustible trim board can be no more than 3/4 inch thick when placed 6 inches from the fireplace opening: 1/8 x 6 = 6/8, or 3/4.