"The stockings were hung on the chimney with care..."
While we may have grown up listening to this Christmas song, the idea of hanging combustible stockings above the fireplace may not have been the greatest suggestion. There are National Fire Codes that require all flammable materials to be at least 6" from the fireplace opening, and depending on the overhang and material of the mantel, that distance could be more.
These codes state that for every 1/8 inch of protruding combustible material, you must add an additional inch of clearance. All operational fireplaces are to be constructed in accordance with safety regulations that position flammable material, such as a wood mantel, far enough away from the flames and heat so that it does not ignite.
The building codes will typically determine the height to place the mantel from the hearth, and the distance from the firebox. As a general rule, the mantel is placed about 12 inches above the fireplace opening. Then adding 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.
Installing a fireplace mantle without adequate clearance from the fireplace opening could be a serious fire hazard and it may lead to damage and injury.
Most fireplaces radiate heat out of the front of the firebox, because of this, your mantel needs to be placed a safe distance away from the firebox.
A non-combustible material such as marble, stone, or tile is also required to be installed between the fireplace opening and your mantel.
Use a mantel heat shield when you have small clearances between the top of your fireplace and the bottom of your mantel. A mantel heat shield will prevent your mantel from becoming too warm. It will also prevent any discoloration to your mantel due to heat.
If you have a fireplace insert, or are looking to install one to improve the efficiency of your home heating, the manufacturer clearly dictates mantel clearances in their installation instructions.