A Class 1, or Class A, fire rating means that the building material is highly resistant to fire and does not spread flames quickly. In the flame-spread rating test, a score of 0 to 25 constitutes a Class 1 rating. The score is relative and balanced between asbestos cement sheet, with a score of 0, and pure red oak, with a score of 100.
Building materials with a Class 1 fire rating are often man-made or nonorganic substances. The most extreme example is asbestos cement sheet, which is extremely resistant to fire but hazardous to humans in other ways. True AC sheets are banned in many countries, though it is possible to find a safer version that uses cellulose instead of asbestos. Other Class 1 building materials include brick, tile and cement. Some carpets also have a Class 1 fire rating.
In personal dwellings, experts recommend using materials with a Class 1 rating, but it isn't mandatory. In the United States, public buildings such as nursing homes, prisons, schools and hospitals must conform to the highest fire-safety standards and are often required to use Class 1 materials.
Pure wood used straight from the tree has different ratings depending on the type of tree. Some trees have a Class 2, or Class B, fire rating. The wood of most trees has a Class 3, or Class C, rating. Composite materials such as plywood and particle board also fall into the Class 3 category.