Flame Spread Index (FSI). The normal output of the NFPA
255/ASTM E84 test is a flame spread index (FSI). This is a relative
number based on the area under the flame spread distance
versus time curve of the test, with an FSI of 0 assigned to cement
board and an FSI of 100 (approximately) assigned to red oak
flooring. Building codes use the test results as “classes,” each
class representing a range of flame spread indices, as shown in
Table 6.3.3. The test method also assesses smoke obscuration
and assigns a smoke developed index (SDI) based on a similar
concept to the FSI, except that in this case the area assigned to
red oak flooring is exactly 100 by definition. To be classified
as Class A, B, or C, a material must have an SDI not exceeding
450, as shown in Table 6.3.3.