The three classes
Interior wall and ceiling finish is classified by the tunnel test, ASTM E84 or UL 723. IBC Section 803.1.2 sets the ranges:
- Class A: flame spread index 0-25, smoke-developed index 0-450
- Class B: flame spread index 26-75, smoke-developed index 0-450
- Class C: flame spread index 76-200, smoke-developed index 0-450
A lower flame spread index is the better class. Class A, at 0-25, is the best and the most restrictive; Class C is the most lenient.
Smoke is the same for every class
The trap: the smoke-developed limit does not change by class. It is 450 for A, B and C alike. So the maximum smoke-developed index for Class C is 450, not 200. The 200 figure is the top of the Class C flame spread range, 75 is the top of Class B, and 25 is the top of Class A. None of those are smoke limits, and 0-450 is never a flame spread range.
Classifying a product: check both numbers
Read the flame spread index to pick the class, then confirm the smoke-developed index is 450 or less.
- A paneling product at flame spread 80 and smoke 300: 80 is above 75, so it is not Class B, but it falls within 76-200, and 300 is under 450. It is Class C. It is still classified, because 80 is well under the Class C ceiling of 200.
- A ceiling tile at flame spread 20 and smoke 500: the 20 looks like Class A, but 500 exceeds 450. It meets no class at all. A worse class does not buy more smoke allowance, so it cannot drop to B or C either.
NFPA 286 as an alternative
Under IBC Section 803.13, an interior wall finish tested to NFPA 286, the room corner test, that meets the acceptance criteria of Section 803.1.1.1 may be used wherever Class A is required. Class A is the most restrictive class, so this covers every location, exits and corridors included, not only Class B or C locations or rooms.