The mantle serves as the decorative shelf space above a fireplace. While the shelf is largely decorative, it is part of the assembly and subject to safety codes. Builders need to follow the requirements of the National Fire Protection Association standards current at the time of construction. The standards detail the distance between the mantel and the fireplace combustion chamber or fire box.
The further the mantel projects from the wall the further it must be placed from the combustion chamber. As the mantel extends away from the wall, the risk of combustion increases. A mantel of 10 inches, for example, must be placed at least 19 inches above the top of the firebox or combustion chamber.
A 2-inch wide mantel must be placed at least 11 inches above the combustion chamber. Place wider mantels of 4 inches at least 13 inches above the combustion chamber. The rules apply even if the mantel is covered with plaster, cement or other fireproof material.
Support for the mantel, sometimes known as mantel legs, extend down the side of the fireplace and hold up the mantel. The further the mantel legs extend from the wall the further the legs must be placed from the combustion chamber. A 10 inch deep leg must be at least 11 1/2 inches from the combustion chamber while a leg extending 2 inches from the wall can be placed within 3 1/2 inches from the chamber.
Trim boards--boards placed flat against the fireplace wall and extending less than 1 1/2 inches from wall--can be placed as close as 6 inches to the combustion chamber. If the trim extends more than 1 1/2 inches from the wall, it must be no closer than 12 inches of the combustion chamber.