Interior finishes affect how fast fire and smoke spread, so IBC Chapter 8 limits them by class. Wall and ceiling finishes are rated Class A, B, or C based on flame-spread index and smoke-developed index (tested per ASTM E84), and Table 803 sets the minimum class allowed by occupancy and by location — the most restrictive requirement falls on vertical exit enclosures and exit passageways, then exit-access corridors, then rooms. Floor finishes have their own critical-radiant-flux requirements.
The rule of thumb: the closer a finish is to the egress path, the stricter the class it must meet.
The common mistake is specifying a decorative wall finish in a corridor or stair that doesn't meet the required class for that location, or missing the floor-finish requirement in exit enclosures. Reviewers check finish schedules against Table 803 by location — this is a frequent comment because designers often pick finishes for appearance and only later confirm the flame-spread class the code demands.
This guide describes the model code for general understanding and is not a substitute for the adopted code and amendments enforced by your local authority having jurisdiction. Verify all figures against your jurisdiction's codes of record.