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.

Fireplace
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Detail of a fireplace mantel.

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 Speaking

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.