"The interpretation and application of the model building codes for roofing fire performance is complicated by subtle differences in code language and testing criteria. A roof assembly is all of the components above the roof's structural frame (think of it as the 'skeleton') and includes the roof deck, vapor barrier, insulation or thermal barrier, roof covering, coatings or toppings, or any combination of these components. A roof covering, on the other hand, is the outer membrane that covers the roof deck for weather, fire or impact resistance. One might imagine it as the 'skin' that covers the outside of the roof. Given the wide variety of climates under which buildings and roofs must be constructed to withstand the elements, it is no surprise that test standards vary."
Coffee Break Training - Fire Protection Series No. FP-2013-33
Federal Emergency Management Agency, United States Fire Administration: http://www.usfa.fema.gov/