What standard is the flame spread index measured to?

The flame spread index is a critical measurement used to assess the fire performance of building materials. It indicates how quickly flame spreads across the surface of a material. The appropriate standard for measuring the flame spread index is ASTM E 84, also known as the "Standard Test Method for Surface Burning Characteristics of Building Materials." This test is often referred to in conjunction with UL 723, as Underwriters Laboratories operates under this standard as well.

In this test, materials are subjected to a controlled flame exposure, and the resulting flame spread is compared to that of a reference material. This indexed value provides vital information for safety codes and regulations regarding materials used in construction and can significantly impact design decisions in both residential and commercial settings.

The other options pertain to different standards that do not specifically measure flame spread index. ANSI Z 21.10 relates to gas water heaters, ISO 9705 focuses on the room corner test for building materials, and NFPA 701 tests the flammability of textiles and film materials. Thus, these options, while relevant in the context of fire safety and materials testing, do not address the flame spread index directly, making the referenced standard ASTM E 84 or UL 723 the correct choice for this measurement.