The interior finish requirements for walls and ceilings in an atrium are regulated under the 2021 International Building Code (IBC), Section 404 – Atriums, and Table 803.11 – Interior Wall and Ceiling Finish Requirements.

IBC Section 404.8 – Interior Finish in Atriums:

"The interior finish of walls and ceilings in an atrium shall be not less than Class A."

Class A interior finishes have the highest level of fire resistance and low flame spread to minimize fire hazards in large open spaces like atriums.

Class A Interior Finish Ratings (Per ASTM E84 and NFPA 286):

Flame Spread Index: 0 – 25

Smoke Developed Index: 0 – 450

Why Other Options Are Incorrect?

B. Class B – Less fire-resistant than Class A, not permitted for atriums.

C. Class C – Provides even lower fire protection, making it unsuitable.

D. Class D – Not recognized as an acceptable interior finish classification for atriums.