I love it when I’m able to solve one of life’s great mysteries. Today I was asked whether a 90-minute fire rated door required a threshold. The short answer is “no” but my coworker Greg chimed in to ask about fire rated openings with combustible floor covering running through. There’s a paragraph in NFPA 80 that I’ve wondered about in the past (for about 5 seconds), which says that combustible floor coverings can extend under doors that are rated up to 90 minutes as long as the minimum critical radiant flux is .22 W/cm2. What that means, I have no clue, although Zeke tried to put it in terms of a lightbulb for me one time.
I’m not a carpet expert but it’s amazing how much information you can find online. It turns out that the IBC and NFPA 101 both require floor coverings that comply with NFPA 253 Class I or II (Class I is less flammable than Class II) in locations where fire rated openings would be found, so a threshold would not be required for openings up to 90-minute rated. NFPA 80 goes on to say that combustible floor coverings shall not extend through 3-hour rated openings, so this could be a location for a threshold, although it is not required per se.
NFPA 80 – 2007, 2010:
4.8.5 Floor Coverings.
188.8.131.52 Combustible floor coverings shall be permitted to extend through openings required to be protected by 11⁄2-hour,
1-hour, or 3⁄4-hour rated fire protection fire door assemblies without a sill where they have a minimum critical radiant flux
of 0.22 W/cm2 in accordance with NFPA 253, Standard Method of Test for Critical Radiant Flux of Floor Covering Systems Using a Radiant Heat Energy Source.
184.108.40.206 Combustible floor coverings shall not extend through openings protected by 3-hour rated fire protection door
You can read all about the development of fire tests for carpet on page 24 of this online book by Andrew J. Fowell, called Fire and Flammability of Furnishings and Contents of Buildings.