Flame-spread, used to describe the surface burning characteristics of building materials. The best known test for developing this rating is the American Society for Testing and Materials (ASTM) Test Method E-84, commonly known as the tunnel test.
The tunnel test measures how far and how fast flames spread across the surface of the test sample. In this test, a sample of the material 20 inches wide and 25 feet long, is installed as ceiling of a test chamber, and exposed to a gas flame at one end. The resulting flame spread rating (FSR) is expressed as a number on a continuous scale where inorganic reinforced cement board is 0 and red oak is 100. The scale is divided into three classes.
Class A or I – Flame spread 0-25
Class B or II – Flame spread 26-75
Class C or III – Flame spread 76-200