Building codes typically require that a structure's walls, floors and roof be fire resistant in accordance with standards set forth by the International Building Code (IBC). Fire resistant walls are designed to contain a fire to a room or building for a designated time period. The wall's rating indicates how long it can keep a fire from spreading.
Fire resistant walls provide sufficient time to discover a fire, control it and evacuate the building if necessary. Ratings are determined using procedures developed by the American Society for Testing and Materials that simulate actual fire conditions. They can also be calculated using tables developed by the IBC.
Fire resistance ratings are expressed in the number of minutes or hours a structure can withstand a fire simulation test. According to the Engineered Wood Association, a one-hour rating indicates that a wall constructed in a manner similar to the one tested will contain flames and high temperatures, and support its full load, for at least one hour after the fire begins.
According to the book "Commercial Drafting and Detailing," a typical one-hour fire rated wall consists of 2x4 studs spaced 16 inches from the center of one stud to the center of the next, covered by 5/8-inch, type X gypsum board. Type X gypsum board is wallboard to which non-combustible fibers have been added.