Coffee Break Training: Fire Protection Series: Building Construction: Fire Wall Parapet Alternatives [open pdf - 483KB]
"Fire walls are constructed to contain a fire within a specific area that is manageable by either the built-in fire protection features or fire suppression forces. Fire walls typically are rated as two- or four-hour fire-rated assemblies. They must run continuously from the lowest point of a building's foundation, through the roof, and to a parapet at least 30 inches (76.2 centimeters) above the roof plane. By extending the wall above the roof plane, a parapet wall prevents flames from immediately igniting a combustible roofing membrane on the opposite side of the wall. The parapet in the illustration is a classic example of how these elements are employed."
Coffee Break Training - Fire Protection Series No. FP-2013-12
United States Fire Administration: http://www.usfa.fema.gov/