Design conventions evolve over time because they work either in terms of physical convenience or safety. Eventually, they end up in standards or building codes and are used, not just because convention has become law, but because these design elements just look right due to familiarity. Fireplace mantel and mantel shelf height provides an example of such a convention.
Open fires came indoors to cook and warm occupants. Chimneys were early safety features, invented to draw away smoke and embers from stone fireplaces with openings high and wide enough to allow cooks to lean in to move large pots and roasting game. The mantel was a decorative feature, added to great fireplaces in cold stone castles. Carved full mantels and simple mantel shelves have adorned plain fireplaces for hundreds of years, bearing bric-a-brac and family heirlooms. The mantel must not be too high for its decorative function.
The National Standard Building Code (NSBC) provides guidance for state and local building codes, and the National Fire Prevention Association (NFPA) publishes a national fire code. Both groups recommend a minimum of 6 inches separation for flammable materials from the limits of the fireplace opening. The NSBC further dictates that the distance be expanded for mantel sides that extend 1.5 inches or more beyond the face of the firebox and that mantel tops that extend more than 1.5 inches beyond the plane of the opening be placed at least 12 inches above the lintel, or the steel bar that runs atop the opening. The NFPA is more conservative, suggesting an additional 1 inch of separation for every 1/8 inch of extension.
What It Means
National standards are written as guides for state and local governments, but many adopt the 6-inch rule and adopt some version of added separation for extensions. A 12-inch separation is not uncommon. You must check your local building code for its interpretation. Using the national standard, if your fireplace has a 3-by 3-foot opening and your mantel shelf extends six inches beyond the plane of the firebox opening, the base of your mantel should stand at least 4 feet -- plus the depth of the hearth stones -- high.
Given a 1.5-inch hearth stone and a simple 2-inch mantel shelf, you might place your mantel 4 feet 1.5 inches above floor level, making the final height of the shelf 4 feet 3.5 inches. If your ceilings are 7 feet or higher, the shelf might look a bit cramped, so you might set it higher. A decorative marble or stone surround also provides a way to add separation to deep mantels and mantel shelves. The mantel itself might also add height to the top of the shelf; a 4- to 5-inch-thick mantel shelf or 1-foot-wide full mantel would raise the mantel top to between 4.5 and 5 feet, a comfortable viewing height.