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.
Standard is different from typical or average. Average mantel height is about 54 above the hearth extension. Standard depends on the fireplace size, required clearances and the aesthetic desired, which can vary widely, as shown below. This one's about 48 high. This one's about 96 high: