The 2007 edition of NFPA 80 contains an important change regarding the clearance at the bottom of a fire rated door. In previous editions of this standard, there was a somewhat confusing table (Table 1-11.4) listing different allowable clearance dimensions depending on the flooring material. The 2007 edition simplifies this requirement, allowing 3/4″ clearance under the bottom of the door regardless of the flooring. The only exception is when the bottom of the door is more than 38″ above the floor, ie. dutch doors and counter shutters.
Unfortunately, there isn’t a definition of “clearance” to make the requirement crystal clear, but according to my research, the 3/4″ dimension is measured between the bottom of the door and the top of the flooring or threshold. In cases where a fire-rated door has more than 3/4″ between the bottom of the door and the flooring, a threshold can be added to help close that gap. In most cases the maximum threshold height is 1/2″ due to accessibility requirements.
Here is the text from the 2007 Edition:
4.8.4 Clearance. 18.104.22.168 The clearance under the bottom of a door shall be a maximum of 3⁄4 inch (19 mm). 22.214.171.124 Where the bottom of the door is more than 38 in. (965 mm) above the finished floor, the maximum clearance shall not exceed 3⁄8 in. (9.5 mm) or as specified by the manufacturer’s label service procedure.