The vacuum tube in a CRT can implode if it is broken or punctured. The surrounding air will rush violently into the unsealed vacuum in the CRT, spraying broken glass in every direction. Color CRTs may contain mercury or other potentially toxic materials.
Instructions. Build a hearth for a fireplace that is 16 inches deep, according to This Old House writer Harry Sawyers. This is the standard requirement for municipalities and homeowner associations. The hearth should also extend a minimum of eight inches on either side of the fireplace.
Joy Fera/iStock/Getty Images. 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.