There are several categories of fire rating classifications, including exterior wall fire-resistive rating, a flame spread rating, and the combustibility of a given material. Each rating has its own standards.
Exterior wall fire-resistive rating is measured by the amount of time that a given exterior wall can withstand fire exposure before the structure fails. The classifications are: 4-hour, 3-hour, 2-hour, 1-hour and non-rated construction.
A flame speed rating measures how far and fast flames spread across the surface of a given material. These classifications are either Class I, II or III, or Class A, B, or C.
Material combustibility is classified by combustible, noncombustible, fire-resistant or ignition-resistant. These classifications range from materials not able to be ignited under normal circumstances to those materials that are ready to ignite and burn.