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.
The NFPA states that mantel depth is directly related to the required height clearance between the top of the fire box and the bottom of the mantel. For combustibles, such as wood or particle board, surrounding a wood-burning fireplace: * A 2-inch depth must have a minimum height clearance of 11 inches.