A standard mantel shelf may range from 8 to 12 inches or so deep, depending on the fireplace style, but safety comes before aesthetics. A combustible wood mantel must be an approved depth so that the only wood getting burned is the firewood.
Clear on Clearance
Depending on local building regulations, minimum mantel clearances may differ. According to the International Residential Code, for example, no combustible material can be within 6 inches of the firebox or fireplace opening, but projecting elements, such as a wood mantel, require even more space. The IRC states that a combustible mantel cannot project more than 1/8 inch for every 1 inch of clearance within 12 inches of the opening.
Aesthetically, mantel depth is determined by fireplace style. A sleek, modern unit, for example, may suit a narrow, 7- or 8-inch shelf or even a flush mantel-surround unit. An ornate Renaissance, colonial or Gothic fireplace, conversely, can visually support a high, beefy 10-, 11-, 12-inch or deeper mantel. A wood-burning firebox that sits forward from the wall affords more mantel depth.