The Flame Spread Index (FSI) of APA Performance-Rated Wood Structural Panels is determined by tests conducted under ASTM E84, Standard Test Method for Surface Burning Characteristics of Building Materials.
Tests conducted by Underwriters Laboratories in 1974 demonstrated that APA-trademarked plywood conforming to U.S. Product Standard PS 1 has a flame spread index in the range of 76 to 200. This range is referred to as Class C.
ASTM E84 tests conducted in 1985 and 1989 for APA by an approved independent fire test laboratory demonstrated that nonveneer APA Rated Sheathing and APA Rated Sturd-I-Floor panels, such as oriented strand board (OSB), also have a flame spread index within the Class C range.
Also, nonveneer APA Rated Siding panels with a medium-density phenolic paper overlaid face (pre-primed) and grooved surface pattern have a flame spread index within the Class C range. The flame spread index for most panels was 150 or less, with an occasional panel up to approximately 170 and some panels less than 100.
The smoke-developed index for most panels was 200 or less, with occasional panels up to approximately 270. Thus, these APA-trademarked products can be used as interior finishes in rooms or other areas in most building occupancies.