To avoid failure, building materials are allowed to carry only a fraction of their ultimate strength. The ultimate strength of a material is the load it carries at the moment it fails. The amount of stress a material is allowed to carry is called its allowable design stress, which is usually is calculated by dividing its ultimate strength by a safety factor. A safety factor of 2 means the allowable stress is only 1/2 the ultimate strength of the material. Safety factors for stone have always been conservative compared with safety factors for ductile materials like steel. NEW PROPOSALS The desire to make natural stone more economical has encouraged-and required-the development of several newer, rationally determined design methods. Most of the new proposals for stone safety factors are controversial. Alex Gere proposed that stone safety factors be based on the variation in flexural strength test results as well as on the type of stone. Thus, the greater the variation in test results, the greater the safety factor should be. In 1988, Bernhard Wonneberger and Seymour Bortz argued that the effects of weathering on stone should be considered when calculating safety factors. Charles Clift and Jeffrey Bayer presented a new theory of stone cladding design based on probability of risk. This method is called the probability of stone failure theory (PSFT).